While off-target dicamba injury to soybean has dominated the news the past year, it is important to recognize that dicamba is not the only Group 4 herbicide (HG4) capable of injuring soybean. These herbicides mimic the activity of indole acetic acid (IAA), a hormone that regulates the activity of numerous genes involved in plant growth. IAA also is referred to as auxin. HG4 products can induce plant responses at lower doses than most other herbicide groups, thus off-target injury has been a problem since their introduction in the 1940’s. This article will discuss some of the problems observed this growing season. All HG4 cause malformed leaves, and distinguishing symptoms between products is difficult (Figure 1). Timing of symptom development and patterns of injury are important in identifying the source of injury.
Carryover following use in corn
Figure 1. Typical dicamba symptoms on soybean.
Clopyralid is sold individually as Stinger, but in corn is more commonly used in the premixes Hornet and SureStart. While clopyralid has a much longer half-life than 2,4-D and dicamba, the rates used in corn typically do not carryover at toxic concentrations into the following season. However, many areas of the state were rain deficient during the 2017 growing season, leading to increased persistence. When clopyralid residues are responsible for damage to soybean, symptoms typically appear by the V1 stage (first trifoliate). Damage is usually associated with soil type or in streaks related to spray overlaps, rather than field wide injury.Spread of contaminated hay or planting of soybeans into former pastures
HG4 are commonly used in pastures and hay fields for broadleaf control. Grazon P+D and GrazonNext are popular products that contain picloram and aminopyralid, respectively. Like clopyralid, these two herbicides are relatively persistent. These herbicides can persist at phytotoxic concentrations in the soil, in forage harvested from treated fields, and in manure of animals consuming forage. Problems may develop when bales of hay from treated fields are placed or spread in fields to be planted to soybean (Figure 2). Concentrations can be high enough to severely damage, or even kill, emerging soybean. Several instances of this type of injury have been observed this year.
Drift from adjacent fields
Figure 2. HG4 injury from spreading hay from field treated with picloram on field planted to soybean.
Undoubtedly, movement of HG4 from treated areas is the most common source of off-target injury. Off-target injury from use of dicamba in Xtend soybean was a significant problem in Iowa in 2017. Last year we estimated that 150,000 acres of soybean in Iowa were damaged. However, it is important to recognize that HG4 are used in other areas than soybean. We are aware of numerous situations this season of dicamba used in corn damaging adjacent soybean, and each year HG4 use in roadsides results in damage to sensitive plants.
Dicamba use in Xtend soybean poses a greater risk than when used in corn since it is applied later in the season. Numerous label changes and required training for applicators using dicamba on soybean should help reduce the frequency of off-target damage, but it is too early in the season to determine the impact of these changes. In 2017, reports of dicamba injury in soybean did not begin until after July 4 in north central Iowa. Numerous reports of injury have been reported in 2018, at this time mostly in southern Iowa.Mistaken sources of injury
The widespread occurrence of herbicide resistant weeds has resulted in an increase in both the quantity and frequency of herbicide applications in soybean. This increases the likelihood of adverse crop responses – sometimes dicamba gets blamed for damage that it isn’t responsible for, and sometimes dicamba injury is blamed on other factors.
The distinct symptom of dicamba is cupping of leaves that emerge after exposure; this injury is typically not noticed for 7-14 days after application due to the time it takes for new leaves to emerge following exposure. The number of leaves affected is determined by the rate of exposure. All HG4 cause distorted leaf growth, but there can be differences in the type of leaf malformations caused by different herbicides. Frequently 2,4-D results in elongated, strapped leaves in contrast to the cupping normally observed with dicamba (Figure 3). However, dicamba-type cupping occasionally may be caused by other HG4, and elongated leaflets may develop following dicamba exposure. Thus, it is always critical to identify the source of problem rather than assigning blame based solely on the symptomology observed.
Figure 3. Elongated leaflet associated with preplant 2,4-D application with sprayer malfunction resulting in excessive rate.
There has been discussion that AMS used with Liberty and other postemergence products may be the source of leaf cupping. AMS has a long history of use as a spray additive, and leaf cupping is not a plant response associated with AMS. The use of group 15 herbicides (Dual, Warrant, Zidua, etc.) has greatly increased in soybean to improve waterhemp control. These herbicides may cause abnormal development of leaves, but the symptoms do not involve the veinal distortion typical of dicamba or other HG4. The classic symptom of preemergence applications of HG15 is shortening of the midrib on leaflets, resulting in heart-shaped leaflets. Post applications can also cause this symptomology, or they can cause other distortion of leaflets, resulting in irregular margins of leaflets (Figure 4). Sometimes, distortion of developing leaves can happen with POST applications of the HG14 contact products as well. This distortion is generally accompanied by some contact burn.
Figure 4. Leaf malformations following postemergence application of HG 15 product. Note lack of symmetry in symptoms on leaflets.
Group 4 herbicides are important tools for managing weeds in a variety of situations. While effective tools, their ability to induce plant responses at fractions of label rates requires a higher level of management than other herbicides. When symptoms of HG4 appear it is important to identify the source of the herbicide, and take steps to avoid repeat occurrences in the future.Crop: SoybeanCategory: WeedsTags: dicambagrowth regulatorSoybeanherbicide injur
With most corn in Iowa at the V7-V12 range, it’s important to be aware of potential corn diseases at this particular time. Given the wet growing conditions over the last month, corn in parts of Iowa will be very susceptible to Physoderma brown spot and node rot, caused by the fungus Physoderma maydis, and gray leaf spot, caused by the fungus Cercospora zeae-maydis.Physoderma brown spot
Physoderma brown spot and node rot risk increases when warm (75-85 degrees Fahrenheit) and excessively wet conditions result in water pooling in the whorl and occurs during the early vegetative stages (V3-V9) of corn growth. The causal fungus produces zoospores, that swim through water in the whorl and infect the meristematic tissue. Given the recent large amounts of rain, coupled with the warm temperatures, it is likely that Physoderma brown spot and node rot may be observed in some fields.Symptoms
Physoderma brown spot symptoms include very small (approximately ¼” in diameter) round-to-oval lesions that are yellowish-brown in color and occur in high numbers and in broad bands across the leaves. In addition, dark-purple to black spots occur on the midrib. These midrib lesions help to distinguish this particular disease from other diseases such as eyespot and southern rust. Because infection requires a combination of light, free water and warm temperatures, alternating bands of infected and non-infected tissues commonly develop on the plant. Symptoms may also appear on the stalk, leaf sheath and husk.
A more severe case of browning along the midrib.
Photo by Brandon Kleinke
Middle stages of Physoderma brown spot. Take note of the spots developing in bands across the leaf, as well as the developing brown markings along the mid rib. Photo by Adam Sisson
Physoderma node rot symptoms are recognized as snapping of the corn stalk at one of the lower nodes (usually 6th, 7th or 8th) during the mid-reproductive stages (R3-R5). The node is often rotted, but the pith is not. Orange sporangia of P. maydis may be easily rubbed off the rotted node or leaf sheath attached to the rotted node.
Younger plants are more susceptible to this disease and become more resistant with age. The causal fungus overwinters in infected host tissue or infested soil for several years.Management
The best time to scout for Physoderma brown spot is during the V12 through R1 stages of growth, and R3-R5 for Physoderma node rot. The disease may be more prevalent in fields with infested corn residue or those with a history of the disease. Hybrid susceptibility to Physoderma brown spot and node rot varies.
There are no in-season management options for Physoderma brown spot and node rot. Although some fungicides are labeled for Physoderma brown spot, field trials at Iowa State University have not shown a reduction in disease or yield protection.Gray Leaf Spot
Warm temperatures (75-85 F) and relative humidity greater than 90 percent favor gray leaf spot development. Symptoms of the disease are most likely to appear following long periods of heavy dew and overcast conditions, and in bottomlands and fields adjacent to woods where humidity can be very high. In Iowa, we typically see gray leaf spot start to develop around tasseling. Because of weather conditions this growing season, however, it is likely that gray leaf spot may start to develop prior to VT.
Gray leaf spot can be more severe when corn follows corn in the same field, and in reduced or no-till systems. The fungus survives in corn residue and spores are spread by wind and splashing rain. Hybrid susceptibility and weather conditions strongly influence disease development. This means that gray leaf spot can be locally severe but not cause widespread damage throughout a region. For corn that was planted late, there is usually an increased risk for disease that could result in higher levels of infection and potential yield loss.
A corn leaf with gray leaf spot developing. Take note of the gray, rectangular lesions across the band of the leaf. Photo by Alison Robertson
Gray leaf spot lesions begin as small, oval or jagged light-tan spots that expand to become long, narrow and rectangular. The lesions are always confined by and expand parallel to the leaf veins. Later infections may turn gray. Depending on the hybrid, the lesions may be surrounded by yellow or orange halos. Gray leaf spot always begins in the lower canopy and progresses up the canopy. Yield loss will depend on disease severity, and much of the upper plant canopy is affected.Management
Management of gray leaf spot begins with selection of resistant hybrids for fields where the disease commonly occurs. Inoculum levels may be reduced through rotating crops and reducing surface residue through residue management. Fungicides are usually effective at managing the disease. Time of application is important, and applications made in the very early stages of disease development (few lesions in the lower canopy) are more effective at slowing disease development and protecting yield.Crop: CornCategory: Plant DiseasesTags: physoderma brown spotPhysoderma stalk rotgray leaf spotCorn diseasesCorncrop scouting
High rainfall in some areas the past couple of weeks has produced another wet spring in Iowa. This leads to questions about nitrogen (N) loss and need for supplemental N application to corn. Unfortunately, this question has become almost the norm - I have written approximately 20 articles on the subject since 2007.
The discussion of N loss should include losses from both the soil N supply and residual nitrate-N. There is usually tile drainage every spring and occasionally in the late fall, leading to N losses. Also, losses can be rapid if soils become saturated, soils are warm, and nitrate is present; these conditions lead to denitrification (biological conversion of nitrate to N gas). Some N loss from soils is typical, the magnitude depends on many factors. Prediction of the effect of these losses on N supply to corn, additional N fertilization need, etc. in wet periods is difficult. There are several approaches in making estimates of N status or loss.
Remember that guidelines for N application rates for corn in Iowa take into account “normal” N losses as N rate research trials are conducted in the field. This is especially important as those N rate trials incorporate supply and loss of soil derived-N, not just applied N. This means that the accumulation of N rate research trials, like used in the Corn Nitrogen Rate Calculator, builds in some variation of soil N supply and climatic conditions.Approaches to estimating N loss
Use the Late Spring Soil Nitrate Test (LSNT)
Use of the LSNT test in Iowa corn production in described in an Iowa State University Extension and Outreach publication (CROP 3140). At this time in most fields, we are past the calendar time and growth stage calibrated for that test and it should no longer be used.
Use of computer models is relatively new for production agriculture. There are several models currently in the market place, including the ISU Extension and Outreach FACTS website that does supply information on nitrate-N in the soil profile (the site was recently updated). You can pick a research site near you and look at the increase and decrease in soil nitrate-N concentration between soil depths. Remember that corn is rapidly taking up N at this time, so there is expectation of decreasing overall nitrate in the profile due to uptake. However, very rapid changes would indicate leaching movement or denitrification, besides crop uptake.
Estimates of nitrate-N production and loss
An example of this was discussed in a 2014 ICM News article (Estimating Nitrogen Loss in Wet Corn Fields). Important components are the estimation of how much nitrate-N has formed from applied N by the time of wet conditions, and the length of soil saturation (which can vary greatly across fields, ex. ponded vs. not ponded areas, and runoff vs. infiltration). When soils are warm, loss can be rapid and large, but slow when soils are cool or there is little nitrate. Recent sidedress fertilizer N applications would be fully or partially “protected” from loss if the application included ammonium which is retained on the soil cation exchange complex and not subject to leaching or denitrification (such as urea, ammonium, and anhydrous ammonia). Anhydrous ammonia is all ammonium, urea rapidly converts to ammonium, and urea-ammonium nitrate solution (UAN 28 or 32%) is one-quarter nitrate, one-quarter ammonium, and one-half urea. Early sidedress applications were likely converted to nitrate before the recent high rainfall events.
Details of this approach were discussed in a 2016 ICM News article (Precipitation and Nitrogen This Spring). The amount of spring rainfall to trigger the need for additional N application was updated with research data from 2016. Those rainfall totals are now 17.8 inches from March 1 to June 30 for Southeast Iowa, and 15.5 inches from April 1 to June 30 for the majority of Iowa. These rainfall totals have about a 76% chance for estimating correctly (adequate N or deficit N) if N loss is sufficient enough to consider additional N application. One does not need to wait until the end of June to calculate total rainfall. That can be done on an on-going basis and if the total begins to approach those values, then be thinking about plans for applying additional N. According to precipitation maps, we have exceeded those rainfall totals in the central to northern part of Iowa. Local rainfall measurements would provide more specific rainfall information. The more rainfall there is above those trigger totals, the more likely supplemental N would be needed. A caveat to use of the rainfall totals is if there are heavy, short duration, rainfall events. If water runs off the field, and does not get into the soil profile, then there should be a discounting off the total. Also, if the rainfall reaches those totals in the early spring, there should also be some discounting off the total due to less nitrate buildup and less denitrification with cool soils. For example, total rainfall amounts in just an individual month, like April or May, do not provide the same level of success as when June rainfall is included. The rainfall triggers are related to use of suggested economical N rates (MRTN) from the Corn Nitrogen Rate Calculator. If higher or lower N rates were applied to fields, then the odds of needing additional N go up or down.
Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production (CROP 3140)
Corn Nitrogen Rate Calculator
ISU Extension and Outreach Soil Fertility Web Site
Nitrogen Use in Iowa Corn Production (CROP 3073)
Japanese beetle is an invasive insect that feeds on corn and soybean plus many other plants. This pest has been in Iowa since 1994 but its distribution in field crops is considered sporadic around the state. Statewide populations in field crops have been variable since 2014 and it is unclear if pressure will be significant this year. Several reports around Iowa indicated high numbers of grubs within fields, but it is not clear if they were Japanese beetle or another closely-related species. Adult emergence well before corn silking is noteworthy.
Japanese beetle adults need about 1,030 growing degree days (base 50°F) to complete development and will continue emergence until around 2,150 degree days. Based on accumulating degree-day temperatures in 2018, Japanese beetle adults should be active in some areas of southern Iowa this week (Figure 1).
Figure 1. Growing degree days accumulated (base 50°F) for Japanese beetle adults in Iowa (as of June 12, 2018). Adults begin emergence around 1,030 degree days. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.
Japanese beetles have one generation per year in Iowa (Photo 1). Adults emerge from grass in late June and immediately begin to feed on low-lying plants. Adults eventually move up on trees and field crop foliage to feed and mate. Mated females move back to grass in August and September to lay egg masses in soil cavities. The eggs hatch into small grubs that feed on roots until late September when the temperature cools. The almost fully-grown grubs burrow down in the soil and remain inactive all winter. In the early spring, grubs become active again and feed until turning into resting pupae. The pupae hatch into adults and emerge from the soil.
Plant Injury and Management
Photo 1. Adults are metallic bronze and green with white tufts along the side of the abdomen. Photo by David Cappaert, www.ipmimages.org.
Japanese beetles have a wide host range that includes many species of fruit and vegetable crops, ornamentals and field crops. On soybean, adults prefer to feed between the leaf veins and can ultimately consume most of the leaf (Photo 2). The treatment threshold for Japanese beetle in soybean is 30% defoliation before bloom and 20% defoliation after bloom. It is important to note most people tend to overestimate plant defoliation. Use Photo 3 to help calibrate defoliation estimates.
Photo 2. Japanese beetles skeletonize soybean leaves. Photo by Mark Licht, ISU.
Photo 3. Approximate percent defoliation on soybean. Photo by Marlin E. Rice.
In corn, Japanese beetles can feed on leaves, but the most significant injury comes from clipping silks during pollination (Photo 4). Consider a foliar insecticide during tasseling and silking if: there are three or more beetles per ear, silks have been clipped to less than 1/2 inch, AND pollination is less than 50% complete.
Photo 4. Japanese beetles are strongly attracted to silking corn. Photo by Erin Hodgson, ISU.
Crops: CornSoybeanCategory: Crop ProductionInsects and MitesTags: pestscoutingbeetledefoliation
The soybean cyst nematode (SCN) is a major pathogen of soybean in Iowa and throughout the Midwest. Damage often goes unrecognized until lower-than-expected yields are harvested because above-ground symptoms may not be obvious, especially when adequate or excess rainfall occurs during the season. A key way to check fields for this pest is to dig soybean roots and look for the small, round, white adult SCN females. It typically takes 5 to 6 weeks or more after planting for the first SCN females of the growing season to develop and appear on roots.SCN Females Already Apparent on Roots
Soybean plants from two fields in central Iowa were brought to Iowa State University on June 5. Numerous adult SCN females were observed on the roots (see figure). The soybeans were planted on May 10.
The appearance of SCN females on soybean roots just 26 days after planting is as early as I have seen. The rapid development of SCN females likely is due to the very warm temperatures that occurred in Iowa in May.
Rapid Development May Lead to Large Increases in SCN Numbers
Adult soybean cyst nematode (SCN) females on soybean roots collected on June 5, 2018, from a field in central Iowa that was planted on May 10, 2018. There are more than a dozen SCN females (cream-colored objects) on the roots shown in this image.
Discovery of SCN females on roots in early June doesn't bode well for the remainder of the season. The appearance of the white, adult SCN females on roots indicates that the first generation of the nematode is being completed. The nematode will continue to reproduce in successive generations throughout the growing season, into the fall. And if above-average temperatures persist, there may be more generations of SCN produced in the 2018 growing season than in seasons with cooler temperatures. Each SCN female produces 250 or more eggs and when numerous generations occur in a single growing season, SCN population densities (numbers) can “blow up” to disastrous proportions in a single year.Soybean Resistance to SCN is Fading
The females observed on roots described above were on two different SCN-resistant soybean varieties. This point illustrates that many SCN populations in Iowa have built up the ability to reproduce on SCN-resistant soybean varieties.
It is not common to find a few SCN females on the roots of SCN-resistant soybean varieties, but it is troubling to see so many SCN females on the roots of resistant soybean varieties so early in the season. There are more than a dozen SCN females on the few roots shown in the image above.Managing SCN
Iowa State University recommends growing resistant varieties that provide good SCN control in rotation with the nonhost crop corn and consider using nematode-protectant seed treatments when growing soybeans.
Information on how well resistant soybean varieties control SCN numbers is available at www.isuscntrials.info. Also, more information about the biology and management of SCN is available at www.soybeancyst.info, www.soybeanresearchinfo.com/diseases/scn.html, and www.TheSCNCoalition.comCrop: SoybeanCategory: Plant DiseasesTags: SCNchecking fields for SCN
Building soil health is important to sustain soil resiliency and productivity. Many conservation practices can maintain and enhance physical, chemical and biological soil properties that contribute to overall soil biological functions as fundamental drivers that support plant growth and productivity. However, these properties are complex and interrelated as each function is influenced by a central building block, soil organic matter (SOM). Soil health is one of the co-benefits of improving SOM through soil carbon sequestration or storage.
Therefore, when we think about soil health, we need to keep in mind the role of SOM as a central property in choosing management practices that rejuvenate a soil biological system and function. There is a tendency to oversimplify and divide these properties into discreet parameters with no linkage to SOM in assessing soil health indicators. The approach to the assessment of soil health by focusing on SOM can reduce the commercialization of soil health. Overall, it was documented by research and is well accepted that there is a relationship between the improvement of SOM and soil physical, chemical, and biological properties. In addition, there is a consensus that SOM plays an essential role in a biologically diverse and well-balanced soil system. A good example is the highly productive Mollisol soil in the Midwest, which is high in organic matter compared to other soil that has low organic matter. It is true that a gradient in SOM is influenced by the soil forming factors of climate, organisms, time, topography and parent materials, which collectively define soil properties. However, among these factors, climate and organisms are defined as active factors compared to time, topography and parent materials, which are passive factors because their effects are not immediately observed.
The interactions between factors involved in soil formation requires a careful approach of assessing soil health in order to create a matrix that is easy to understand, reflects management effects and is economically affordable. There are two approaches to determining soil health parameters:
- The basic research approach to understand the fundamental processes and relationships between soil parameters and to shed light on the mechanisms governing those relationships (i.e., ecosystem services).
- The applied approach which determines the influence of management practices (tillage, crop rotation, cover crops, etc.) on SOM as the main driver of soil health indicators (physical, chemical and biological).
The applied approach to determining soil health indicators in relation to soil management focuses on SOM as an essential property along with biological indicators, that we need to focus on, in order to achieve the soil health benefits as an outcome of SOM improvement. Improving the SOM is a slow and long-term process in detecting significant changes as in any conservation system. This long-term process is due to the instability and potential susceptibility of SOM to loss through soil tillage and weather conditions (loss as CO2, leaching, sediment loss, etc.). Thus, the focus on soil biological system assessment as an indicator for the short-term SOM improvement is important to consider.
To improve soil health we need to keep in mind that it is a long-term commitment that requires using a system approach of stacked conservation practices that includes diverse crop rotations, less soil disturbance (i.e., no-till), integration of perennial grasses on marginal land and a growing plant during the off-season (i.e., cover crop). The benefits of using a system with multiple practices over a singular practice will improve SOM, soil resiliency, and ecosystem services.
Change in soil system under no-till and cover crop.
Crops: CornSoybeanCategory: SoilsTags: soil healthsoil organic matter
Change in soil system under conventional tillage.
Tracking degree days is a useful tool to estimate when common stalk borer larvae begin moving into cornfields from their overwintering hosts. Foliar insecticide applications, if needed, are only effective when larvae are migrating and exposed. Start scouting corn for larvae when 1,300-1,400 degree days (base 41°F) have accumulated. Counties south of I-80 in Iowa reached this important benchmark this week (Figure 1), and therefore scouting for migrating larvae should begin now to make timely treatment decisions. Stalk borer larvae in northern counties will migrate later in June.
Figure 1. Degree days accumulated (base 41°F) for stalk borer in Iowa (January 1 – June 5, 2018). Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.
Female moths prefer to lay eggs in weedy areas in August and September, so minimizing weeds (especially giant ragweed) in and around corn during that time will make those fields less attractive. Long-term management requires controlling grassy edges around corn so that females will not lay eggs in that area during the fall. To prevent stand loss, scout and determine the percent of infested plants. Consider applications at peak larval movement, or 1,400-1,700 degree days (base 41°F). The use of an economic threshold (Table 1), first developed by Iowa State University entomologist Larry Pedigo, will help determine justifiable insecticide treatments based on market value and plant stage. Young plants have a lower threshold because they are more easily killed by stalk borer larvae.
Table 1. Economic thresholds (expressed as percent of infested plants with larvae in the whorl) for stalk borer in corn, based on market value, expected yield, and leaf stage.
Stalk borers tend to re-infest the same fields, so prioritize those for scouting first with extra attention to the field edges. Applying insecticides to larvae that have entered the stalk is not effective. Instead, target foliar applications to larvae as they migrate from grasses to corn. Look for larvae inside the whorls to determine the number of plants infested. The larvae are not highly mobile and typically only move into the first four to six rows of corn. Look for new leaves with irregular feeding holes or for small larvae resting inside the corn whorls. Larvae will excrete a considerable amount of frass pellets in the whorl or at the entry hole in the stalk. Young corn is particularly vulnerable to severe injury, but plants are unlikely to be killed once reaching V7.
Using burndown herbicides before corn planting can force stalk borers to move and infest emerging corn. If an insecticide is warranted based on stalk borer densities, the application must be well-timed to reach exposed larvae before they burrow into the stalk. Border treatments should be considered particularly because the infestations are localized. Make sure to read the label and follow directions, especially if tank-mixing with herbicide, for optimal stalk borer control.Description
Stalk borer larvae have three pairs of true legs and four pairs of fleshy prolegs. The body is creamy white and dark purple with brown stripes. Often there is a creamy white stripe running down the back of the thorax and abdomen. A distinctive feature of stalk borer larvae is an orange head with two dark lateral stripes (Photo 1). The adults are dark grey and brown colored moths, with jagged white lines and two to three clusters of white spots (Photo 2).
Photo 1. Common stalk borer larva. Photo credit Adam Varenhorst.
Photo 2. Common stalk borer adult. Photo credit Adam Sisson.
Stalk borers have one generation annually in Iowa. Stalk borer eggs are laid on grasses and weeds in the fall and overwinter in this cold-hardy stage. Egg hatch typically occurs around April 19 – June 5, and about 50 percent of egg hatch happens at 494 degree days. Young larvae will feed on grasses and weeds until they outgrow the stem of the host plant. The number of larval molts is variable, depending on food quality, and ranges from seven to nine instars. Migration to larger hosts begins around 1,300-1,400 degree days. Fully developed larvae drop to the soil to pupate. Approximately 50 percent of pupation happens at 2,746 degree days, with 50 percent adult emergence at 3,537 degree days. Peak adult flight occurs during the first two weeks of September.
Corn adjacent to grassy and weedy areas becomes a suitable food source for migrating larvae. The most susceptible corn growth stages for infestation are V1-V5 or about 2-24 inches in plant height. Larvae can defoliate leaves and create non-economic injury. More often, larvae kill corn plants by entering the stalk and destroying the growing point (i.e., flagging or dead heart). A dead heart plant will have outer leaves that appear healthy, but the newest whorl leaves die and can cause barren plants.
Photo 3. Stalk borer larvae can shred corn leaves and destroy the growing point.
For more information on stalk borer biology and management, read a Journal of Integrated Pest Management article by Rice and Davis (2010), called “Stalk borer ecology and IPM in corn.”
Crop: CornCategory: Crop ProductionInsects and MitesTags: pestscoutingdegree daysCorn
Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average peak hatching date of June 6 in central Iowa. In 2018, the average hatching date will be ahead of the average, despite having cool April temperatures. Development is driven by soil temperature and measured by growing degree days. Research suggests about 50 percent of egg hatch occurs between 684-767 accumulated degree days (base 52°F, soil). Most areas in Iowa have reached peak corn rootworm egg hatch (Fig. 1). Larvae will start feeding on corn roots if available.
Figure 1. Accumulated soil degree days in Iowa as of May 29, 2018. Expect 50 percent corn rootworm egg hatch between 684-767 degree days. Map data courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy.
To generate degree day accumulation on corn rootworm egg hatch for your area, use the ISU Agronomy Mesonet website. To create an accurate map, make sure to set the start date to January 1 of the current year and the end date to today, and set the plot parameter to “soil growing degree days (base = 52).” Be aware that some locations are having some technical difficulties with the soil temperature probes this year.
A severe corn rootworm larval infestation can destroy nodes 4-6; each node has approximately 10 nodal roots. Root pruning can interfere with water and nutrient uptake and make the plant unstable (Photo 1). A recent meta-analysis showed a 15 percent yield loss for every node pruned. Regardless of agronomic practices to suppress corn rootworm (e.g., crop rotation, Bt rootworm corn, or soil-applied insecticides), every field should be scouted for corn rootworm root injury. Fields with continuous corn and areas with Bt performance issues are the highest priority for inspection. Looking at corn roots 10-14 days after peak egg hatch is encouraged because the feeding injury will be fresh. Assess corn rootworm feeding and adjust management strategies if the average injury is above 0.5 on a 0-3 rating scale. Also, consider monitoring for adult corn rootworm to supplement root injury assessments. Aaron Gassmann, Iowa State University corn entomologist, has a webpage for additional corn rootworm management information including an interactive node-injury scale demonstration and efficacy evaluations.
Photo 1. Severe root pruning by corn rootworm larvae can dramatically impact yield. Photo by Aaron Gassmann, Iowa State University.
Crop: CornCategory: Crop ProductionInsects and MitesTags: rootwormCornpestscouting
Isolates of Cercospora sojina showing reduced sensitivity to quinone outside inhibitor (QoI, strobilurin) fungicides were recently recovered from soybean in Iowa. This pathogen causes frogeye leaf spot, an important foliar soybean disease that can be managed with fungicides. With the confirmation of fungicide-resistant isolates, it becomes especially important to understand when to spray and what products to use for long-term control of this disease.
Frogeye leaf spot symptoms on a soybean leaf. Image: Daren Mueller
Cercospora sojina survives in infected soybean seed and plant residue. Spores are produced on infested residue in spring and carried short distances by wind or rain splashes. Warm, humid conditions that are overcast and heavy dews are required for spores to germinate. This pathogen commonly infects newer leaves towards the top of the soybean plant.Fungicide mode of action and site of action
Like herbicides, fungicides have a specific mode of action (MOA) that interferes with or inhibits specific cellular processes of fungi. Each MOA has a specific site of action (target site). These are usually specific enzymes required for a cellular process that a fungicide binds to. As with herbicides, the continued use of a single MOA places selection pressure on a pathogen, which can result in a population less sensitive or resistant to the chemical being used. Common active ingredients (AIs) farmers across the Midwest use include the QoI (FRAC group 11), DMI (FRAC group 3), and the SDHI fungicide (FRAC group 7).
Qols are considered a “high risk” class of fungicide for resistance development because resistant development is often the result of a single gene/single site mutation, most commonly the G143A mutation that occurs at the fungal cytochrome b gene. The Qo (Quinone outside) inhibiter fungicides act on the outer (Qo) binding site of the cytochrome bc1 complex. By blocking complex III in the electron transport chain, the pathogen is unable to produce energy
DMI fungicides are considered a “medium risk” class of fungicides for resistance development because they typically have polygenic resistance. This means for resistance to occur, several mutations at the target site are required. Each mutation leads to a small reduction in fungal sensitivity. Consequently, multiple mutations usually need to accumulate in a fungal isolate before the reduced sensitivity is large enough to impact the efficacy of a DMI fungicide under field conditions (a slow step-wise erosion of efficacy).
SDHI fungicides are considered a “medium to high risk” class of fungicides for resistance development and a single mutation in the fungus confers resistance. Similar to the group 11 QoI fungicides, the group 7 SDHI fungicides inhibit respiration in fungi. SDHI fungicides target the enzyme succinate dehydrogenase, the so-called complex II, which is a part of the tricarboxylic cycle and linked to the mitochondrial transport chain. Although both groups (QoI’s and SDHI’s) of fungicides inhibit respiration, they target different sites with no cross resistance between the two fungicides shown. This means that if a pathogen develops reduced sensitivity/resistance to a QoI fungicide, it will not automatically have resistance to a SDHI.Generic fungicides
Some fungicide AIs are now off patent, allowing generic versions of these fungicides to be sold for use on corn and soybean. Some generic fungicides contain only one active ingredient, such as QoIs (FRAC group 11). It is important to know what active ingredients/or FRAC groups are in the fungicide you choose to spray. Spraying a fungicide with only one site of action comes at a risk.Knowing the pathogen
Pathogen isolates with reduced sensitivity to fungicides may be present in a population due to natural mutations and are not necessarily the result of fungicide application. Applying a fungicide will select for these mutants and they will proportionally increase as part of the population that is still sensitive to the fungicide. This will consequently lead to a pathogen population that has reduced sensitivity or is resistant to the fungicide.
Understanding the lifecycle of the pathogen of concern is as important as knowing the MOA and/or the site of action of the fungicide being used. Pathogens may be monocyclic (one lifecycle per season) or polycyclic (multiple lifecycles per season). They can produce millions of spores within a single growing season. This can affect the chances of a pathogen becoming resistant to a particular fungicide. The more life cycles in a season, and/or the more spores produced, the greater the genetic variation within the pathogen, and the higher the chance of a genetic mutation that could lead to reduced sensitivity to a fungicide.Other management strategies
The QoI-resistant strains can still be managed effectively with other fungicides groups. Alternative disease management practices such as planting frogeye leaf spot-resistant cultivars, crop rotation with non-host crops are recommended for the successful management of the disease.Crop: SoybeanCategory: Plant DiseasesTags: cercosporafungicide-resistancefoliar diseasefungicidesprayQoI
ISU Extension and Outreach Field Agronomists continue to receive calls regarding fomesafen carryover injury to rotational corn. There are several factors resulting in this injury: 1) continued problems with waterhemp result in late-season applications, 2) fomesafen is relatively persistent, and 3) many areas of Iowa received less than average late-season rainfall in 2017. In most cases, this carryover injury has been limited to relatively small sprayer overlap areas, though some fields are showing injury on a more widespread area.
Characteristics of the primary products used postemergence to control waterhemp in Iowa are listed in Table 1. For an herbicide to pose carryover risks, it must persist at toxic concentrations into the following growing season and be biologically available. Both fomesafen and glyphosate are relatively persistent compared to the other herbicides. The KOC is a measure of how tightly a chemical is held to soil colloids. As the KOC value increases, less herbicide is available to be absorbed by plants. Whereas fomesafen can be readily absorbed by plants due to low adsorption to soil colloids, glyphosate is unavailable to plants due to tight binding to soil particles.
Table 1. Herbicide characteristics influencing carryover potential. Herbicide Handbook, 10th Edition.
Weed Science Society of America.
The relatively long half-life of fomesafen, combined with below average rainfall late-season rain in central Iowa, is the reason for the increased issues with carryover in 2018. Much of central Iowa received between 2-5 inches less rain than normal during July and August of 2017 (Figure 1).
Figure 1. Deviation from normal rainfall during July and August of 2017
The primary symptom of fomesafen injury is striped leaves due to chlorotic or necrotic veins on the leaves (Figures 2 and 3). Other factors can cause striping on leaves, but fomesafen is unique in that the veins are affected rather than interveinal tissue. Some of the leaves may fold over midway due to loss of integrity of the leaf midvein. Frequently only two or three leaves are affected and injured plants recover quickly. However, at times there can be stand loss and the only way to determine the potential impact is to determine the percentage of plants affected and closely monitor the rate of recovery.
Figure 2. Veinal chlorosis typical of fomesafen carryover on corn
Figure 3. Fomesafen injury due to sprayer malfunction in 2017
Rather than focusing on the problem of corn injury due to fomesafen, I think it is more important to consider why late-season applications of postemergence herbicides are required. An integrated program relying on full rates of effective preemergence herbicides and early postemergence applications, along with an increased emphasis on driving down the size of the weed seed bank, should minimize the need for the late-season applications.Crop: CornCategory: WeedsTags: fomesafencarryoverherbicide injuryherbicide persistence
The black cutworm (BCW) is a migratory pest that cuts and feeds on early vegetative-stage corn. Black cutworm moths arrive in Iowa and other northern states with spring storms each year. These moths lay eggs in and around crop fields and emerging BCW larvae cut seedling corn. This pest is sporadic, making it essential to scout fields to determine if management is needed. Scouting for BCW larvae helps to determine if an insecticide application will be cost effective.
When to scout for BCW caterpillars is based on the “peak flight” of moths and accumulating degree days after the peak flight. Degree days are a measure of temperature used to gauge insect development. A peak flight for BCW is defined as capturing eight or more moths over two nights in a wing style trap baited with a pheromone lure.
To find out when moths arrive in Iowa, cooperators around the state monitor pheromone traps and report moth captures. Cooperators started checking traps in the beginning of April and captures of BCW moths did not occur until mid-April. Moth captures picked up during the last part of April and early May, with several peak flights recorded. The peak flights observed during this time period were in line with captures in surrounding states.
The map (Figure 1) shows predicted BCW cutting dates for the nine Iowa climate divisions, based on actual and historical degree day data and peak flights during late April and early May. We may continue to see peak flights occur in Iowa. Adult moth trap captures do not necessarily mean there will be economically significant BCW infestations in a particular location. Field scouting is essential to determine if an economically damaging infestation exists. Also, as you are out in fields assessing stands, be on the lookout now for early season insect injury in corn – BCW or otherwise.
Figure 1. Estimated black cutworm cutting dates for each Iowa climate division based on peak flights of moths occurring in 2018.
Several states bordering Iowa also track black cutworm flights and make estimates about cutting dates, especially to the north and east. Several of the predictions in these states are in close proximity to the Iowa border, and some are for counties directly adjacent to Iowa. Data from these out-of-state locations may be informative for Iowans living nearby. Extension resources on black cutworm can be found at the University of Minnesota, Wisconsin Pest Bulletin, and University of Illinois’s The Bulletin.
Scouting. Poorly drained, low lying, or weedy fields, as well as those next to natural vegetation or with reduced tillage, may have higher risk of BCW injury. Those cornfields with poorly terminated cover crops may also be attractive to egg-laying females. Late-planted corn can be smaller and more vulnerable to larval feeding. Some Bt hybrids provide suppression of BCW (e.g., Vip3A, Cry1A.105, Cry2Ab2, and Cry1F proteins), but larvae can still cut young plants.
Scouts are encouraged to start looking for any activity during early season stand assessment, or at least several days before the estimated cutting dates. Early scouting is important because local larval development may be different due to weather variation within a climate division. Fields should be scouted for larvae weekly until corn reaches V5. Examine 50 corn plants in five areas in each field for wilting, leaf discoloration and damage, or those that are missing or cut (Figure 2). Flag areas with suspected feeding and return later to assess further injury. Larvae can be found by carefully excavating the soil around a damaged plant.
Figure 2. Black cutworm larval injury usually begins above the soil surface. Leaf feeding (left) may be observed. As larvae mature, they can severely damage or kill plants (right). Photo on left copyright Marlin Rice; photo on right courtesy Jon Kiel.
Identification. BCW larvae have grainy, light grey to black skin and four pairs of fleshy prolegs on the end of the abdomen (Figure 3). There are pairs of dark tubercles, or bumps, along the side of the body. The pair of tubercles nearest the head is approximately 1/3 to 1/2 the size of the pair closest to the abdomen (Figure 4). BCW larvae can be confused with other cutworms and armyworms.
Figure 3. Black cutworm larvae have grainy and light grey to black skin. Photo by Adam Sisson.
Figure 4. Black cutworms (left) can be distinguished from other larvae, like the dingy cutworm (right), by the dark tubercles on the middle of the back. On each segment, the tubercle closest to the head is about 1/3 the size of the tubercle closest to the rear for black cutworm. Corresponding dingy cutworm tubercles on each segment are roughly the same size. Photos by Adam Sisson.
Thresholds. Common thresholds for seedling, V2, V3, and V4 stage corn plants are 2, 3, 5, and 7 plants cut out of 100, respectively. A dynamic threshold for BCW may be useful with corn price and input fluctuations. An Excel spreadsheet with calculations built in can be downloaded here and can be used to help with black cutworm management decisions.
Preventive BCW insecticide treatments applied as a tank-mix with herbicides are a questionable practice. BCW is a sporadic pest and every field should be scouted to determine insect presence before spraying insecticides.
If you see any fields with BCW larvae while scouting, please let us know by sending a message to email@example.com. This information could help us to refine future predictions.Crop: CornCategory: Crop ProductionInsects and MitesTags: cutwormscorn insectspest scouting
Iowa’s most significant soybean insect pest, soybean aphid, has host-alternating biology. This species has multiple, overlapping generations on soybean in the summer and moves to buckthorn in the winter. Fall migration to buckthorn is based on senescing soybean, and decreasing temperatures and photoperiod. For the majority of the year, soybean aphids are cold-hardy eggs near buckthorn buds (Photo 1). As spring temperatures warm up, soybean aphid eggs hatch and produce a few generations on buckthorn before moving to soybean (Photo 2). Tilmon et al. (2011) goes into more detail about the life cycle and biology of soybean aphid.
Photo 1. Sexual females deposit eggs near buckthorn buds in the fall. Photo by David Voegtlin.
Photo 2. There are a few wingless generations produced on buckthorn before the spring migration to soybean every year. Photo by Chris DiFonzo (Bugwood.org).
For many aphids that overwinter as an egg, hatching often happens when the host resumes spring growth. This makes biological sense because the aphids feed on phloem from actively-growing tissue. If egg hatch happens too soon, they can suffer mortality from starvation. Research has confirmed soybean aphid egg hatch happens around buckthorn bud swell. Bahlai et al. (2007) developed a model to predict soybean aphid egg hatch based on accumulating degree days. They adjusted the model to include ambient air temperatures and solar radiation. Soybean aphid egg hatch occurs between 147-154 degree days (base 50°F) and buckthorn bud swell happens shortly after that (165-171 degree days). Based on air temperatures in 2018, we expect egg hatch is occurring in northern Iowa, where most of the buckthorn in Iowa is located (Figure 1).
Figure 1. Accumulated growing degree days (base 50°F) in Iowa from January 1 – May 7, 2018. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.
Bahlai, C. A., J. A. Welsman, A. W. Schaafsma, and M. K. Sears. 2007. Development of soybean aphid on its primary overwintering host. Environmental Entomology 36: 998-1006.
Tilmon, K. J., E. W. Hodgson, M. E. O’Neal, and D. W. Ragsdale. 2011. Biology of the soybean aphid in the United States. Journal of Integrated Pest Management 2: A1-A7.Crop: SoybeanCategory: Crop ProductionInsects and MitesTags: SoybeanpestIPMscouting
Adult alfalfa weevils become active and start laying eggs as soon as temperatures exceed 48°F. Alfalfa weevil eggs develop based on temperature, or accumulating degree days, and hatching can start around 200-300 degree days. Start scouting alfalfa fields south of Interstate 80 at 200 degree days and fields north of Interstate 80 at 250 degree days. Based on accumulated temperatures since January, weevils could be active throughout southern Iowa this weekend (Fig. 1).
Figure 1. Accumulated growing degree days (base 48°F) in Iowa from January 1 – April 26, 2018. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.
Biology. Alfalfa weevil is an important defoliating pest in alfalfa. Heavy infestations can reduce tonnage and forage quality. Adults feed on plants, but typically the larvae cause the majority of plant injury. Female alfalfa weevils can lay 800-4,000 eggs in a lifetime and insert 5-20 at a time into alfalfa stems. Newly hatched larvae can be found feeding on terminal leaves, leaving newly expanded leaves skeletonized. Maturing larvae (Photo 1) move down the plant and begin feeding between leaf veins. Peak larval activity occurs around 575 degree days. Often silken pupal cases are attached to leaves in the lower canopy or in leaf litter. The time it takes to reach the adult stage is dependent on temperature, but can take about eight weeks. Adults (Photo 2) eat along the leaf margin, leaving irregular notches. A heavily infested field will look frosted or silver (Photo 3).
Photo 1. Alfalfa weevil larvae have a dark head and pale green body with a white stripe down the back. Fully-grown larvae are about 5/16 inches long. Photo by Clemson Cooperative Extension Slide Series, www.ipmimages.org.
Photo 2. Alfalfa weevil adults have an elongated snout and elbowed antennae. Their wings and body are mottled or brown in color. Photo by Clemson University, www.ipmimages.org.
Photo 3. Heavily-defoliated alfalfa fields appear frosted from a distance. Photo by Whitney Cranshaw, Colorado State University, www.ipmimages.org.
Management. After reaching benchmark degree days (200 in southern Iowa and 250 in northern Iowa), use a sweep net to sample for adults and larvae. South-facing slopes warm up faster and may be a place to start sampling. After larvae are first collected in sweep nets, collect six alfalfa stems from five locations throughout the field. Take each stem and vigorously shake into a bucket to dislodge larvae from the plant. Small larvae can be difficult to separate from the plant and therefore careful plant inspection is also needed. Average the number of larvae per 30 stems and plant height to determine if the economic threshold is approaching (Table 1). Remember, cutting alfalfa is an effective management tool for alfalfa weevil larvae, and an insecticide application may be avoided if harvesting within a few days of reaching the economic threshold. For more information on how to interpret the table, click here.
Crop: Biomass and ForageCategory: Crop ProductionInsects and MitesTags: weevilscoutingIPMforage
Table 1. Economic threshold of alfalfa weevil, based on the average number of larvae in a 30-stem sample (Originally published by John Tooker, Penn State Extension).
Bean leaf beetle adults (Photo 1) are susceptible to cold weather and most will die when air temperatures fall below 14°F (-10°C). However, they have adapted to winter by protecting themselves under plant debris and loose soil. Each spring, adult beetles emerge from overwintering habitat and migrate to available hosts, such as alfalfa, tick trefoil, and various clovers. As the season progresses, bean leaf beetles move to preferred hosts, like soybean. While initial adult activity can begin before soybean emergence, peak abundance often coincides with early-vegetative soybean.
Photo 1. Adult bean leaf beetle. Photo by Winston Beck.
An overwintering survival model developed by Lam and Pedigo from Iowa State University in 2000 is helpful for predicting winter mortality based on accumulated subfreezing temperatures. Predicted mortality rates in Iowa for the 2017-2018 winter range from 64-97 percent (Figure 1). Northern Iowa experienced colder temperatures and most bean leaf beetle adults are not expected to survive (mortality ranging from 89-97 percent).
Figure 1. Predicted overwintering mortality of bean leaf beetle based on accumulated subfreezing temperatures during the winter (1 October 2017 – April 2018).
The statewide-predicted mortality from the 2013-2014 winter was the highest since Marlin Rice started tracking these data in 1989. The 2015-2016 and 2016-2017 winters were milder compared to the 2017-2018 winter. Last winter, the predicted mortality of bean leaf beetle in central Iowa was 75 percent, which is slightly higher than the 29-year average of 71 percent (Figure 2). It is important to remember insulating snow cover and crop residue can help protect bean leaf beetle from harsh air temperatures. Fluctuating temperatures can negatively influence spring populations.
Figure 2. Predicted bean leaf beetle mortality by year for central Iowa; the red line indicates the average mortality rate (71 percent).
Overwintering beetles moving to crops are expected to be low this year; however, consider scouting soybean fields, especially in southern Iowa, if:
- Soybean is planted near alfalfa fields or if the field has the first-emerging soybean in the area. Overwintering adults are strongly attracted to soybean and will move into fields with emerging plants.
- Fields have a history of bean pod mottle virus.
- Food-grade or seed fields where reductions in seed quality from bean pod mottle virus can be significant.
Bean leaf beetles are easily disturbed and will drop from plants and seek shelter in soil cracks or under debris. Sampling early in the season requires you to be “sneaky” to estimate actual densities. Although overwintering beetles rarely cause economic damage, their presence may be an indicator of building first and second generations later in the season. More details information about bean leaf beetle and bean pod mottle virus are available.
Crops: SoybeanBiomass and ForageCategory: Crop ProductionInsects and MitesTags: pestscoutingmortalityIPM
With delayed spring weather and low or uncertain grain prices, farmers and crop consultants are asking questions about starter fertilizer for corn this spring. The placement of small amounts of plant nutrients in bands offset to the side and below the seed row or in the seed furrow increases the concentration of nutrients near seedling roots. Common starter fertilizers have nitrogen (N), phosphorus (P), and potassium (K) and sometimes sulfur (S) or micronutrients. Research in Iowa and the north central region has shown that early plant growth increases from starter fertilizer are common and can be large in corn but are uncommon and small in soybean.Phosphorus and nitrogen are the key nutrients for starter application
The early growth responses to starter fertilizer usually are more frequent in low-testing soils or when conditions are colder than usual. With cold soil, root growth is slowed, the capacity to absorb nutrients is reduced, and the diffusion of nutrients through soil towards the root surface is slowed. These effects are more likely to happen with reduced tillage and high residue cover because the residue keeps soils cooler and wetter for a longer time compared with soils with little cover. However, starter may also help with late planting even if soils are warm. Wisconsin research showed that starter P application was likely to increase yield and reduce grain moisture with very late planting and full-season hybrids. Iowa results were not as consistent, probably because the research did not include many years with late planting dates, and many of the Wisconsin research sites were farther north.
Phosphorus is less mobile in soil than K, and much less than mineral N forms (especially nitrate), thus the P concentration in the soil solution near roots at a particular time can be low. Also, P is very critical for plants during very early growth, and an early P deficiency seldom can be fully corrected after crop emergence. Therefore, it is not surprising that early-season corn growth responses to starter mixtures often is explained by P, sometimes even in soils with optimum soil-test P levels. Starter K seldom increases corn early growth, except with less than optimum soil-test K. In corn, early growth responses to starter N occur less often than for P, and occur mainly when the primary N rate is not applied pre-plant in spring, with no-tillage and in continuous corn. Recent research with starter N applied to the sides and below corn seeds after a cereal rye cover crop showed inconsistent results.Expectations of yield response to starter application
The effects of starter fertilizer on corn grain yield aren’t as consistent as effects on early growth. Yield responses to starter are more likely in northern Iowa. When the recommended pre-plant P rate is broadcast, yield responses to starter are more likely with cool, wet soils and reduced tillage since high residue cover keeps soils cooler and wetter in spring. However, a yield response to starter P is seldom observed when the two-year P rate for the corn-soybean rotation is broadcast before corn. A response to starter N is unlikely in corn after soybean when the primary N rate is applied pre-plant in spring and when N solutions are used as herbicide carriers. Starter is a good way for applying micronutrients, but extensive research has shown no corn or soybean response to micronutrients except for occasional corn responses to zinc and inconsistent soybean response to iron in highly calcareous soils (with pH greater than about 7.2 or 7.3). ISU Extension and Outreach publication PM 1688 (A General Guide for Crop Nutrient and Limestone Recommendations in Iowa) suggests starter applications for corn under conditions of poor soil drainage, cool soil, crop residues on the soil surface, or late planting dates with full season hybrids .Be careful with high starter rates with in-furrow application
The rates of starter N and K applied in the seed furrow can’t be too high because salt can damage seedlings. The traditional rule of thumb for in-furrow starter application is to apply less than 10 or 12 lb of N plus K2O/acre, mainly with fertilizers containing ammonium, potash, potassium chloride (KCl), or potassium nitrate. Application to the furrow of urea, ammonium sulfate, and either ammonium or potassium thiosulfate is not recommended. South Dakota State University developed a tool that helps make decisions for in-furrow starter application (Seed-placed Fertilizer Decision Aid). In spite of studies over the years and this tool, no research can fully answer the question of how "safe" higher in-furrow starter rates can be because of several unpredictable factors. Definitely, higher in-furrow rates are not recommended when the soil is dry and lower than normal rainfall is forecast.In summary, when is a corn yield response to starter fertilizer likely?
- With lower than recommended P and K broadcast application rates
- Without primary N application before planting
- Cooler than normal soil temperatures
- No-till with high residue cover with low pre-plant application rates
- Continuous corn, especially in no-till with low or no pre-plant application rates
- Northern Iowa soils with moderate to poor drainage
- Late planting dates
Seedcorn maggot is a seed and seedling pest of corn and soybean. Plant injury is especially prevalent during cool and wet springs. The larvae, or maggots, feed on germinating corn and soybean seeds or seedlings (Photo 1). They can feed on the embryo, delay development or kill the plant. Infestations tend to be field-wide instead of grouped together like many other pests. To confirm seedcorn maggot injury, check field areas with stand loss and look for maggots, pupae and damaged seeds (e.g., hollowed out seeds or poorly developing seedlings).
Photo 1. Typical seedcorn maggot injury in soybean (Photo by Marlin Rice) and corn (Photo by Purdue Extension).
Biology. Seedcorn maggots overwinter in Iowa as a pupa in the soil. Adult flies emerge and mate in April and May, and females lay eggs in soil. Maggot densities will be higher in soils with high organic matter. Land that is heavily manured may be especially attractive to early-laying females. Recent soil tillage, regardless of residue type, is attractive to egg-laying females. This fly species has a lower developmental threshold of 39°F and upper threshold of 84°F. Peak adult emergence for the first generation is at 360 accumulated degree days. There are 4-5 generations per year in our area.
Figure 1. Degree days accumulated (base 39°F) for seedcorn maggot in Iowa (January 1 – April 23, 2018). Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.
Identification. Seedcorn maggots are white, legless and 1/4 inches long with a tapered body (Photo 2). The maggots have a black mouth with hook-like mouthparts to feed. The pupa is brown and looks like a “wheat seed” (Photo 1). The adult fly is grey to brown in color with red eyes. Adult seedcorn maggots are 1/20th inches long and look like a small house fly (Photo 3).
Photo 2. Seedcorn maggot (left) and pupae. Photo by Brian Lang, Iowa State University.
Photo 3. Seedcorn maggot adult is a small, grey fly. Photo by www.ipmimages.org.
Management. There are no rescue treatments for seedcorn maggot. No-till fields are less attractive to egg-laying females. Target planting when soil and moisture conditions are conducive to quick germination and vigorous growth to reduce seed and seedling pest problems. Farmers with persistent seedcorn maggot infestations should consider a later planting date, shallow planting, higher seeding rates, and terminating cover crops early (Bessin 2004). Waiting two weeks (or 450 growing degree days) after tillage or manure applications to plant corn or soybean should provide enough time for the seedcorn maggots to complete development and move to another host (Gessell and Calvin 2000).
Insecticidal seed treatments are also an option for persistent seedcorn maggot pressure. If significant stand loss occurs, replanting the field is an option. A replant decision should be based on percent stand loss and cost of additional seed. Corn and soybean resources to aid in replant decisions are available at https://store.extension.iastate.edu/Product/Soybean-Replant-Decisions.
Bessin, R. 2003. Seedcorn maggots. ENTFACT 309, University of Kentucky Cooperative Extension Service.
Gesell, S., and D. Calvin. 2000. Seed corn maggot as a pest of field corn. Entomological notes, Department of Entomology, Penn State University.
Holm, K. and E. Cullen. 2012. Insect IPM in organic field crops: seedcorn maggot. A3972-01. University of Wisconsin Extension.Crops: CornSoybeanCategory: Crop ProductionInsects and MitesTags: pestIPMscoutingseed
Before making pesticide applications this year check the FieldWatch® online registry so you are aware of sensitive crops and beehives in the area. In 2017 the Iowa Department of Agriculture and Land Stewardship Pesticide Bureau received a record number of complaints regarding pesticide applications. Taking the time to review what is near a field prior to applications can help mitigate future problems.
FieldWatch® features a voluntary mapping tool through Google Maps™ that shows pesticide applicators the locations of registered sensitive crops and beehives so they can make informed decisions regarding potential pesticide applications. FieldWatch® replaced the Iowa Department of Agriculture and Land Stewardship Sensitive Crop Directory in 2017.
Recently FieldWatch® launched two new mobile apps that make it easier to access and input data. The FieldCheck app is designed to give applicators easier access to information from their mobile device while in the field. BeeCheck is designed to help beekeepers make changing the location of beehives easier and faster. Both apps are available free of charge for Android and iOS devices.
Winter seems to be never ending, and spring not arriving. This could lead to a compressed period for field work before corn planting begins. There are conversations underway about switching planned spring preplant anhydrous ammonia to another nitrogen (N) product like urea-ammonium nitrate solution (28 or 32% UAN) or granulated urea. And likely discussions about changing from preplant to sidedress applications. What should be considered? Perhaps the most important item is to have a conversation between dealer and farmer to ensure product availability when desired, equipment needed for application, and any associated change in costs.Preplant Applications
Urea, urea-ammonium nitrate solution, and other products
If planned N fertilizer applications can be made without an undue delay in planting, then go ahead and make the applications. For materials such as urea or UAN solution, or polymer coated urea, those can be broadcast and incorporated with normal tillage before planting. Incorporate or inject rather than leaving the fertilizer on the soil surface to avoid volatile N loss from granulated urea or urea in UAN as it converts to ammonium, or runoff if a rapid rainfall (or snowmelt) event occurs. If time is critical and UAN application is to be made with preemerge herbicides, then surface application is an option, although more risky due to potential volatile N loss from being sprayed/broadcast and the applied N remaining on the soil surface (especially in no-till) if there is not sufficient rain to move the urea into the soil. A rainfall of at least 0.25 to 0.50 inch within approximately two days after application will eliminate volatile loss concern. UAN is half ammonium nitrate and half urea, therefore volatile loss potential from UAN is half of that with urea. Banding UAN on the soil surface will also reduce volatile loss to about half that with broadcast application. Predicting the amount of volatile N loss is difficult, but increases with high surface crop residue (especially no-till), moist-to-drying soils, warm soil temperatures, many days without rainfall, high soil pH, low soil cation exchange capacity, and higher N application rates. Although an added cost to decrease risk of volatile loss, a urease inhibitor can be added to slow urea conversion which provides time for rainfall to move urea into the soil. Preplant or preemerge applications can be part of a weed-and-feed or split-N system, with a full N rate or rate to supply part of the total N application need and the remainder applied sidedress.
Another fertilizer option is polymer coated urea, designed to delay urea release until soils warm. To avoid product runoff, incorporate into the soil. Surface broadcast options, especially adapted to no-tillage that generally do not have volatile loss concern, are ammonium nitrate and ammonium sulfate. These products are not used extensively in Iowa as a primary N material, so would likely have limited availability.
If disturbing soil is a concern in no-tillage from injecting N, then broadcast application is an advantage but also has the large disadvantage of potential volatile losses, surface runoff, or immobilization of N with surface residue, and is not a highly recommended application.
Anhydrous ammonia before planting
Anhydrous ammonia has certain considerations. It must be injected, and the ammonia band will initially have high pH and considerable free ammonia which can damage (burn) corn seedlings and roots. There is no exact “safe” waiting period before planting, and injury can happen even if planting is delayed for a considerable time period. The risk of ammonia injury depends on many factors, with several that are not controllable. Risk increases if application is made when soils are wet and then dry (ammonia moving up the injection track), with higher application rates, when soils with high clay content are wet (sidewall smearing of the injection track and ammonia moving toward the soil surface during application), and when soils are very dry and coarse textured (larger ammonia band). A few management practices can reduce the risk of ammonia damage. Wait and apply when soil conditions are good, have a deep injection depth (six to seven inches or more), or wait several days until planting. If the injection placement relative to future corn rows can’t be controlled, apply at an angle to reduce entire sections of corn rows from being damaged. If the injection track can be controlled with GPS guidance positioning technology, then offset a few inches from the future corn rows – with this guided system no waiting period is needed. There would be a similar free ammonia and/or salt issue with shallow banded urea or UAN solution. Anhydrous ammonia nitrifies more slowly than products like urea or UAN solution, so is a preferable fertilizer for soils with greater potential for losses in wet conditions.Sidedress Applications
Best options for sidedressing, in approximate order from most to least preferable (and depending on crop emergence and size):
- injected anhydrous ammonia, UAN, or urea.
- broadcast granulated ammonium nitrate or ammonium sulfate.
- surface applied urease inhibitor treated urea or UAN.
- surface dribble UAN solution.
- broadcast urea.
- broadcast UAN.
There is a wide time period for sidedress applications. Sidedress injection can begin immediately after planting if corn rows are visible or GPS guidance positioning equipment is used. Be careful so that soil moved during injection does not cover seeded rows or small corn plants. It is easiest to inject in the row middle and there is no advantage in attempting to place the injected band close to the corn row. Corn roots will reach into the row middle at a small growth stage. Injected N can also be applied between every-other-row, and this will provide equivalent response as when placed between every row. For many soils, when planting corn after soybean there can be adequate N in the root zone to meet the needs of small corn plants. For corn after corn, there is a greater likelihood that additional N is needed for early growth. Preplant or starter N can help meet early plant needs, and is especially important if sidedressing is delayed significantly or there will be a planned mid-to-late vegetative growth stage application in either rotation.
With sidedressing, a urease inhibitor with surface applied and non-incorporated urea and UAN could help reduce volatile loss, similar to that described above with preplant applications. A dry soil surface may be more common within the growing season, which will reduce volatile loss potential. The rate of N applied, and hence the amount of potential N loss, has to be large enough to offset the inhibitor cost. Rainfall will eliminate volatile loss and is needed to move surface applied N into the root zone.
Broadcasting granulated urea, ammonium sulfate, or ammonium nitrate across growing corn can cause leaf spotting or edge browning where fertilizer granules fall into the corn whorl. Damage will be greatest with ammonium nitrate, but that product is not readily available or used in Iowa, with damage from ammonium sulfate more than urea. The chance of damage increases with larger corn and higher application rate. As long as the fertilizer distribution is good, not concentrated over plants, and the rate reasonable, the leaf damage should only be cosmetic.
Broadcast application of UAN solution across growing corn has the potential to cause leaf burn and reduced early growth. Depending upon the severity of damage, reduced plant growth may be visible for several weeks after application. Research conducted in Minnesota indicated that when corn plants were at the V3 growth stage (vegetative leaf stage defined according to the uppermost leaf with a leaf collar visible – in this case three leaf collars visible), phytotoxic effects were worse at rates greater than 60 lb N/acre (rates applied were 0, 60, 90, and 120 lb N/acre), but damage was not permanent and did not adversely affect stand or yield. When plants were larger than the V3 stage, plant damage was worse and some yield depression occurred with the 120 lb N/acre rate. Many herbicides are applied using UAN as the carrier to minimize trips across fields. However, this strategy is only recommended prior to crop emergence. Almost all herbicides prohibit application in N solutions after corn has emerged. Check herbicide labels closely.
If N is going to be sidedress applied, then rates can be adjusted from results of the late spring soil nitrate test (LSNT). Soil samples, 0-12 inch depth, are collected when corn is 6-12 inches tall with rate adjustment based on the measured nitrate-N concentration.
If corn becomes too tall for normal sidedress equipment, it is possible to use high clearance equipment to apply N. The N source often will be UAN solution, with equipment available to either dribble the solution onto the soil surface with drop tubes or shallow inject with coulter-shank bars (coulter-disk injected), and urea which can be broadcast spread across the top of corn.
Research in Iowa has shown corn can respond to N application at mid-to-late vegetative corn growth stages when there is deficient N supply, but there can be loss in yield potential. Reduced yield occurs more frequently when soils are dry at and after application (applied N not getting into the root zone) and with severe N stress. Best responses occur with sufficient rainfall shortly after application to move N into the active root zone.
If attempts to get N applied preplant or early sidedress have failed, or there are concerns about N supply from early fertilizer or manure applications, then mid-to-late vegetative stage application can be a helpful rescue. Having some non-N limiting (approximately 50% more than normal rate) reference strips or areas in fields are helpful for comparisons. These areas can be used to visually determine if corn would respond to additional N, as a check to see if earlier N applications are not sufficient, and determine if plants are showing growth or coloration symptoms that are not due to N deficiency. These reference areas are also needed for N stress sensing tools (such as chlorophyll meters, active canopy sensors, or satellite images) to help guide application rates and understand N stress across landscapes. These reference areas should be planned and N applied early in the season, or be field areas that are known to be non-N deficient. Plant and canopy sensing can begin when corn is at approximately the V8-V10 growth stage. If late N application is needed, it should be applied as quickly as possible and not later than the tassel/silking stage.In Summary
- Fertilize before planting if it does not greatly delay corn planting; otherwise consider split or sidedress N.
- If you decide to change planned N applications, make certain needed N fertilizer products and sidedress or high-clearance equipment will be available; or if hiring applications the dealer/custom applicator can accomplish the applications.
- Consider the N volatilization potential of different N materials when applying without incorporation or injection into the soil.
- Try not to make poor N management decisions just to get applications completed.
- Communication between farmer and dealer is key.