Rain events during September and October have created challenging conditions not only for timely harvest of corn and soybean crops but also for the impact harvest will have on the soil. These wet conditions coupled with a drop in air temperature will slow harvest operations. Soils are too wet for traffic from heavy equipment, making them susceptible to compaction during harvest operations. When soils are near saturated conditions, heavy equipment loads weaken soil structure where water works as lubricant, leading to the collapse of soil aggregates. This will cause significant surface compaction, rutting, and deep subsoil compaction.
Damage from soil compaction can have significant impact on water infiltration, root development, and ultimately grain yield the following season. It is generally estimated that yield loss due to soil compaction caused by wheel traffic ranges between 10-20% depending on the extent of soil compaction experienced. Over the past decade the size of Iowa farms has increased, leading to larger and heavier equipment. An axle load from a 12-row combine with full grain tank is estimated at 26 tons/axle and a single-axle fully loaded grain cart is estimated at 22 tons/axle.
Avoiding field conditions prone to soil compaction will likely be unavoidable this fall as farmers make an effort to get grain harvested without further field pre-harvest yield losses and worsening grain quality. Soil compaction will be worse the wetter the soil conditions are. Care should be taken to avoid wet and near saturated soil conditions; ideally, field operations would be delayed until soil water is below field capacity. Tillage energy, time, and costs could increase a hundredfold to remediate excessively compacted soils during harvest operations.
Tips to minimize soil compaction during and after harvest
Ruts made when harvesting soybean under saturated soil conditions.
- Dedicated travel lanes. Many combine operators use “on-the-go” unloading into a grain cart to speed harvest. In areas that have received excessive rainfall since Sept. 1, farmers may want to consider having dedicated travel lanes for the grain cart. It has been documented that 60-80% of soil compaction occurs from the first wheel passes, subsequent field operations account for a much smaller amount of compaction.
- Don’t run at full capacity. Reduce the axle loads of both the combine and grain carts by not loading them to full capacity. This may not be an attractive option in high-yielding cornfields and where harvest has already been delayed. This is much easier to implement in soybean where the grain volume is much less than corn. A compromise may be to try and keep axle loads lower in the far reaches of fields and achieve the highest axle loads (full capacity) nearest the end rows where grain will be transported out of the field.
- Tire size and inflation pressure. Use appropriate tire sizes for the conditions and adjust tire air pressure to match the axle load being carried. Larger tires with lower air pressure provide more surface area, allow for better flotation, and reduced pressure on the soil surface.
- Concentrate non-harvest field activates near the point of exit from the field. While it is tempting to move semi-trailers and tractors with wagons along the field edge as harvest continues, this practice increases compaction along the end rows.
- Harvest around the wettest areas. The wettest areas are the most at risk for soil compaction but are also an accident risk that ultimately could lead to longer harvest delays. Additionally, buried equipment may come with large financial penalties. Come back to these areas later in the fall once the soil conditions are drier or have frozen.
- Avoid or minimize tillage. Remember to hold off tillage operations until soil conditions are drier than field capacity. It is important to consider the soil moisture at the depth of tillage. Tillage in wet conditions results in further compaction and smearing of soil instead of the intended fracturing of the soil. If it is absolutely necessary to cosmetically fill in ruts, use a disk unless soil conditions are dry enough for fracturing of the soil through use of more aggressive implements.
In wet conditions, the best choice farmers can make is to stay away from the field and avoid traffic on wet soil to reduce soil compaction. How you approach fieldwork after a heavy rain event can impact your soil for future growing seasons. More consequences of soil compaction can be found in Top 10 Reasons to Avoid Soil Compaction.Easy test to check soil conditions for field operations
Most of Iowa's soils have medium textures. For these soils, a simple method of checking soil moisture is the "feel" method. Probing the top 12-18 inches with a hand soil probe to assess the field's soil moisture conditions is time well spent.
Check the soil moisture status by pushing a ribbon of soil from between the thumb and index finger. If it breaks off within one or two inches, the potential for creating compaction is less. However, if the ribbon stretches out to four or five inches, it is still too wet. The chances are good that being in the field under these conditions may cause more problems than it will solve.
Another soil consistency test is to roll soil against the palm of your hand to determine if it will clump and roll or if it is fragile and falls apart. Soil that clumps together is more prone to compaction.Additional Resources:
Crops: CornSoybeanCategory: Crop ProductionSoilsTags: soil compactionyield losstillage
This year continues the chain of years with unusual harvest conditions driven by rapid weather changes in the latter part of the growing season. In mid August, crops were significantly ahead of schedule in terms of maturity. Heat and moisture in May and June accelerated the pace of development, to the point that signs of maturity were evident by the 15th of August. Rains followed by above average temperatures began over Labor Day weekend, and have been repeated nearly every weekend to date. The forecast for the weekend of October 7 is more of the same – very heavy rains with intermittent warm, sunny and high humidity periods. The 2018 crop is now at a point where the wet conditions are affecting quality.
In some areas, flooded streams inundated mature crops. Please see this ICM News article written on September 27, for guidance in handling these crops.
Corn moisture contents vary widely but field mold is showing up. Most field molds grow on corn after blacklayer; rapid drydown normally prevents significant further problems. Fields should be scouted for molds because some species have the potential to produce mycotoxins. Those fields should be harvested as quickly as possible, and dried rapidly without a long period of wet holding. Field molds normally do not grow in storage after drying but wet holding and slow drying can cause growth and toxin increases before the corn is dry. The lower temperature and air drying systems will have difficulty in the weather conditions predicted to continue for the next 10 days. Run bin dryers as warm as manufacturer recommendations allow, and try to fill bins in stages to reduce depth and increase drying rates in individual bins. Rotating fills may require documentation for crop insurance purposes, to identify grain traceable to specific fields.
Good descriptions and photographs of field molds, mycotoxins and their impacts are available at https://cropprotectionnetwork.org/library, in the training modules section.
- CPN-2001 Corn Ear rots
- CPN 2002 Corn Mycotoxin FAQs
- CPN 2003 Corn Grain sampling and mycotoxin testing
- CPN 2004 Storing mycotoxin-affected grain
A two-part narrated presentation on mycotoxin effects on animals and on handling and management for toxin-affected grain is available on the Iowa Grain Quality Initiative website.
A recommended scouting/sampling procedure for mold identification is given below:
- Check at least 100 ears selected from throughout the field.
- If more than 10 percent of plants have an ear rot, harvest the field early.
- Dry and cool harvested corn quickly.
- Test moldy grain for mycotoxins before feeding to livestock
This year, the highest risks for mycotoxins are for vomitoxin (DON) in northern and SW Iowa, and for some aflatoxin in SE Iowa (where drought conditions prevailed until after Labor Day).
Photo on left: Aspergillus flavus (aflatoxin) on an ear, Photo on right: Giberella (vomitoxin) on an ear
The longer the wet weather persists the more risk of mold and toxins that there will be. The moderately warm temperatures forecast for the next 10 days will accentuate mold growth. Moldy grain does not automatically contain mycotoxins; some species are not toxigenic, and not all toxigenic species always produce toxins. This is why end users (ethanol plants, feed mills, wet mills) will be screening composite samples of early harvest deliveries to determine if there are concerns. The often-present “polka dots” from Cladosporium are an example of field mold, that will grade damaged but that does not produce a toxin.
Stalk strength is low; there will be increasing amounts of downed or broken stalks. Harvest downed corn first regardless of moisture content because mold growth is accelerated, and drydown rates are reduced. Consider cleaning this corn if possible because the larger mass of material through the combine will create more airflow clogging fines and foreign material. It is always recommended to remove the center of bins before long term storage. That need will be greater this year.
General corn quality is average at best, as indicated by test weights. The kernel fill was not as complete as last year; dry corn test weights probably will be around 54-56 lb/bu. This still meets grade standards but expect a shorter storage life than last year. Because of reduced fill and kernel size, protein contents will probably be below the 7.5% long term average. The critical management actions this year will be rapid drying without long holding periods and cooling as quickly as possible to preserve future storage life. Always, actions right at harvest are the most crucial in determining future quality the following spring and summer.
The table below shows the time-moisture and temperature relationship for safe storage.
Both temperature and moisture content are important in grain preservation, but often temperature control is the most immediately important if drying capacity is limited.
Soybeans experience fewer in field mold problems than corn, because field moistures are normally low (<13%). However, if the soak-dry cycles continue with more heavy rain, expect pod splitting and eventually grey colored beans. Freeze-thaw cycles would further accentuate the splitting but currently there are no forecasts for frost in the near future. Do not try to store field molded soybeans; the oil will become rancid and continued deterioration is likely. The grey beans will grade damage in the market, but likely will get worse in storage. Aerate them for cooling, then market them as soon as possible.
The same needs for removing the center core of bins and dropping the temperature as rapidly as possible apply to soybeans as corn. Notice in the storage time table above that soybeans spoil at a rate equal to 2% points wetter corn. Soybeans are normally 13-14% or less in the field.
Soybeans are often stored in unaerated bins or buildings; this removes both the cooling and drying capability of aerated bins. Wet fields and warm temperatures may require special handling to be sure that beans in unaerated bins are cool and dry.
None of these wet-conditions problems affect grain yield; that was established earlier in the growing season. The predicted high yields and corresponding long term storage will make at-harvest management very important this year. Future ICM articles will update these issues, and identify long term storage management needs as weather conditions develop.Crops: CornSoybeanCategory: Grain Handling and StorageTags: grain storagemoldwet conditions
There have been flooded fields with water over the grain in Southwest, Northwest and East Central Iowa this year. This was caused by intense rains over Labor Day and the following weekends. Poor stalk strength causing downed corn has increased the amount of grain covered by flood waters. Grain submerged by uncontrolled flood waters is considered adulterated under the Food Drug and Cosmetic Act. This policy dates to 2008 when grain storage in Cedar Rapids were inundated and has been applied to several situations since then. Adulterated material cannot be put in commercial facilities of any type, where there would be a chance of entering human or animal food. The Food and Drug Administration (FDA) updated its flooded food guidance on September 17, 2018. Grains are considered food crops.
All grains for which the edible portion was submerged are considered adulterated. This would include grain from fields that had shorter plants flooded but not taller ones, if both were harvested together. By rule, adulterated materials cannot be used for human or animal food in any circumstance. For any adulterant, however, there is provision for the holder to ask the FDA to approve a reconditioning plan. In Iowa, the contact point for the FDA is the Iowa Department of Agriculture and Land Stewardship, Feed and Fertilizer Bureau. This process may take considerable time and effort.
Grain not flooded but was near to the flood water on partially submerged plants is subject to testing for contaminants based on a risk-based evaluation of the specific situation (case by case basis). This places the responsibility for the risk assessment on the producer. Discuss the situation with crop insurance and liability insurance carriers. Document any decisions made, as clearly as possible – how much area, what did you do, why. Food safety regulations are now very reliant on documentation of actions and reasoning.
The FDA provided a list of potential hazards in flooded grain in the guidance referenced above. Mycotoxins, heavy metals, pathogenic organisms, pesticide residues, and polychlorinated biphenyls are the main concerns. The Iowa State University Veterinary Diagnostic Laboratory, accessible through any veterinarian in Iowa, is one source of testing and support.
The FDA makes a specific distinction between uncontrolled waters (streams, rivers, etc.) and isolated low places in fields. The guidance states “Flooding is the flowing or overflowing of a field with water outside a grower’s control. Pooled water (e.g., after rainfall) that is not reasonably likely to cause contamination of the edible portions of fresh produce is not considered flooding”. Uncontrolled flowing water may have picked up contaminants, while ponds in low places are less likely to do so.
Grain elevators and grain processors should be aware of any flooded grain situations in their trade areas. Because flooded grain is adulterated, this grain should not be offered for sale nor knowingly taken into a commercial facility. Processors have flooded grain as one of the reasonably foreseeable hazards being controlled in their Food Safety Plan under the Food Safety Modernization Act. Without an approved reconditioning plan, this grain is effectively a total loss with no portion recoverable from the market.
The continued rainy weather is increasing the chances for mold growth on all corn. Grain that has not been flooded but is showing mold growth should be harvested and dried as quickly as possible.
Crop: CornCategory: Grain Handling and StorageTags: CornSoybeangrain qualityflood damage
Corn in previously flooded field. Photo credit: Brian Lang
Cases of tar spot in corn have been reported over recent weeks in 12 Iowa counties. Iowa State University Extension and Outreach plant pathologists have been able to confirm the presence of tar spot in four counties via the Iowa State University Plant and Insect Diagnostic Clinic. Agronomists and collaborators have confirmed the remaining cases throughout the state. The current counties with confirmed tar spot presence include: Jones, Jackson, Johnson, Muscatine, Fayette, Clayton, Black Hawk, Buchanan, Delaware, Dubuque, Clinton and Scott counties.
Knowledge of yield impacts of tar spot in corn is limited. In Mexico and Latin America, tar spot, caused by the fungus Phyllachora maydis, does not affect yield. However, if P. maydis co-infects with another fungus, Monographella maydis, causing tar spot complex, then substantial yield loss may occur. To date, M. maydis has not been reported in the continental U.S. University of Wisconsin plant pathologists have suggested however that P. maydis is capable of forming a complex with other organisms present in the Midwest, and the impacts of these complexes are currently unknown.
Tar spot in corn was first confirmed in the U.S. in 2015 in Illinois and Indiana. The disease was first reported in Eastern Iowa in 2016, the same year that it was detected in Michigan and Florida. Since then, the disease has been reported each growing season, which suggests that the fungus is overwintering in the Midwest. Reports of tar spot in eastern Iowa in 2018 have been received late in grain fill, and severity in most fields is low.Symptoms
Tar spot in corn is recognized as small, raised, black-irregular-shaped spots scattered across the leaf surface. These spots are fruiting structures of the fungus, known as ascomata P. maydis.
Figure 1. Photo by Ed Zaworski
Figure 2. Photo by Adam Sisson
If a small section of the ascomatum is viewed under a microscope, hundreds of sausage-shaped asci filled with ascospores are visible (Figure 3).
Figure 3. Photo by Ed Zaworski
As with most diseases, tar spot does have “look-a-likes” – namely, common and southern rust. At the end of the growing season, both rust fungi switch from producing orange-red uredinospores, to black teliospores. Rust pustules filled with teliospores can be mistaken for tar spot ascomata. Remember that rust spores burst through the epidermis and the spores can be scraped away from the pustules with a fingernail. Tar spots cannot be scraped off the leaf tissue.
Researchers postulate that the pathogen is spread via wind and rain water. It has been proposed that spores of the pathogen arrived in the U.S. in a storm that originated in Mexico and Latin America.
If you have observed corn with tar spot symptoms, please notify an Iowa State University Extension and Outreach plant pathologist (Alison Robertson and/or Ed Zaworski) or tweet at @isu_ipm, with a photo (if possible) and the name of the county in which the disease was found, so that we may continue to add to our database and keep track of the disease in Iowa.Crop: CornCategory: Plant DiseasesTags: Corndiseasefungustar spotIPM
Harvest is just around the corner for many Iowa farmers and now is a good time to consider options to reduce movement of weed seed between fields with harvest equipment. While we may not think of it during harvest time, combines are extremely effective at transporting seed from field to field. A few precautions leading up to harvest and during harvest can help manage any escaped problem weeds.
Prior to harvest, scout fields for escaped weeds since weeds are easier to see after crops have matured. This is important to identify problem fields or areas for next year. Your notes about weed problems are critical to choosing effective management tactics for next year, so make this a priority prior to harvest. In some situations scattered weeds could be removed from the fields prior to harvest. It is much easier to manage weed issues before they drop mature seed or before that mature seed goes through a combine.
Waterhemp is an example of a potentially herbicide-resistant species that may need to be contained, especially when it is out of control in only a few fields on the farm. Palmer amaranth and burcucumber are examples of two species that may be either new or in few enough fields that it is valuable to prevent them from spreading further. These species are especially difficult to manage and preplanning harvest can help reduce problems in future years.
If weeds cannot be rogued prior to harvest, decide whether planned harvest order needs to change to avoid spread of certain species to other, uninfested fields. Another precaution when harvesting fields, especially given all the drowned out spots from this spring’s rains, would be to harvest around those areas in the field. These steps are especially important if fields are suspected to have herbicide resistant weeds that are not present elsewhere on the farm or if the fields have a new or unusual species that should be kept from spreading to other fields.
Harvest equipment can carry significant material, including weed seed, between fields.
Finally, steps should be taken to minimize movement of weed seed between fields on harvest equipment. In the future, new technology like the Harrington Seed Destructor (HSD) will make weed management at harvest simpler, but until then, relying on good clean-out practices is necessary in some situations. Combines can retain more than 150 pounds of biomaterial including crop seed, plant material, and weed seed after it has been run empty. A few short steps to perform a self-cleaning of the combine and about 20-30 minutes of time before moving on to another field can help further reduce movement of new weed problems to another field. Read more about those steps in our between-field combine clean-out document.Crops: CornSoybeanCategory: WeedsHerbicide ResistanceTags: weed seed movementweed managementharvest weed management
I recently moved offices for the first time since arriving at Iowa State University in February 1990. While moving, I uncovered several things that I had not seen in many years. One of those items was a faded news article that a colleague sent me written by Doug Lindner in the Algona Upper Des Moines newspaper originally published on September 2, 1988 (see figure below, article is available online here).
The article described a visit to north central Iowa in 1988 by newly appointed Dean of the ISU College of Agriculture David Topel. He traveled to speak with farmers, legislators, and agribusiness and extension personnel about an emerging threat to soybean production in Iowa - the soybean cyst nematode (SCN).SCN in Iowa - a retrospective
Reading the news article from 1988 again made me reflect on the SCN situation in Iowa over the past several decades. Looking back, some things that stand out to me include:
- SCN spread from a single county in Iowa (Winnebago County) in 1978, to 34 counties in 1988, to all 99 Iowa counties in 2016. Also, SCN currently is found in every soybean-producing state in the United States except West Virginia.
- SCN consistently has been identified as the most damaging pathogen of soybeans in Iowa and in the United States since the mid 1990s.
- The number of SCN-resistant soybean varieties for Iowa has increased from about 20 in the early 1990s to more than 1,000 in 2017.
- Nematode-protectant seed treatments were introduced as a much-needed new tool for managing SCN in the last decade.
Perceptions about SCN in Iowa have changed over the decades - from grave concern over severe damage in the 1980s (see figure above), to a unified call for testing of fields for SCN in the 1990s (“Take the Test, Beat the Pest” - The SCN Coalition), to effective management of SCN in the 2000s. What about now?SCN in Iowa - now and in the future
Managing SCN used to be pretty routine. Once a field was found to be infested with SCN, a farmer simply had to grow resistant soybean varieties in rotation with corn in that field. And there were hundreds of SCN-resistant soybean varieties for Iowa available by the end of the 1990s. Currently, few soybean varieties are described as being not resistant (i.e. susceptible) to SCN.
Almost all SCN-resistant soybean varieties since the early 1990s were developed from the same soybean breeding line, or “source of resistance”, named PI 88788. These resistant varieties controlled most (> 90%) SCN reproduction for many years. But researchers from several states who were monitoring the reproduction of SCN populations on resistant soybean varieties detected a troubling trend over the years – SCN populations were becoming resistant to the resistance.
Currently, SCN populations that reproduce well on resistant soybean varieties with the PI 88788 source of resistance are common and widespread throughout Iowa and the Midwest.
Put simply: soybean varieties with PI 88788 SCN resistance no longer control SCN well in many (most?) fields in Iowa and in several other states. Also, farmers are losing yield as a result of increased SCN reproduction on resistant soybean varieties.So, what’s a farmer (or agronomist) to do?
Managing SCN for the long term requires active effort like never before. The SCN Coalition from the 1990s has re-formed now to emphasize this situation. Recommendations from the SCN Coalition are:
- Collect soil samples from fields before every second or third soybean crop to monitor changes in SCN population densities (numbers) over time.
- Use a broad-based, multi-faceted approach to manage SCN. Available management strategies include SCN-resistant varieties, nonhost crops (like corn), and nematode-protectant seed treatments.
- Seek out and grow SCN-resistant soybean varieties with sources of resistance other than PI 88788. Currently, “Peking” is the most common non-PI 88788 source of SCN resistance available.
Go to www.TheSCNCoalition.com for more information about the SCN Coalition, including a timeline of SCN history, data illustrating the gradual buildup of resistance-breaking populations of SCN and associated yield losses, lists of SCN experts and soil-processing laboratories in every state in the United States and in Canadian provinces, and state-specific SCN management recommendations. The SCN Coalition is funded by the soybean checkoff through the North Central Soybean Research Program and the United Soybean Board and by industry partners.Crop: SoybeanCategory: Plant DiseasesTags: SCN
Since early August, farmers and consultants have been reporting what they believed were potassium (K) deficiency symptoms in soybean leaves located in the middle or upper canopy. This is not surprising in fields or portions of fields with soil-test values in the very low or low K interpretation categories. Moreover, K deficiency symptoms could develop at these growth stages with drought conditions, even in fields with adequate soil-test K levels. Sometimes symptoms occur in late summer with rainfall events after a dry period. Potassium deficiency symptoms are very common and well known at early growth stages, but due to poorly understood reasons, in the last couple of decades deficiency symptoms in upper leaves at middle to late reproductive stages also have become common.
In low-testing soils or droughty soils, K deficiency symptoms may develop from the V3 stage up to more advanced vegetative stages mainly in the older leaves, but with severe deficiency symptoms may progress to the upper leaves. The symptom is yellowing of the leaflet margins with mild deficiency that may become brown and necrotic with extreme deficiency. The K deficiency symptoms on older leaves sometime remain until the reproductive stages, but often may not be seen because the leaves have been shed or partially decomposed. The reason symptoms are observed mainly in the older leaves at early vegetative growth stages is because K is very mobile within the plant and with a deficiency it is translocated from older leaves to new leaves. Figure 1 shows examples of typical soybean K deficiency symptoms at early growth stages.
Figure 1. Soybean potassium deficiency symptoms at early vegetative growth stages.
The K deficiency symptoms at early vegetative stages should not be confounded with soybean iron deficiency chlorosis (IDC), which often occurs in high-pH (calcareous) soils. In contrast to K deficiency, the IDC symptoms are yellowing of the interveinal area of mainly entire young leaflets. With extreme iron deficiency, however, brown and necrosis may occur in leaf margins. The ICM News article “Is It Iron or Potassium Deficiency?” refers to IDC symptoms in soybean.
The K deficiency symptoms in soybean middle or upper leaves at intermediate to late reproductive stages are similar to the ones observed early in older leaves. Figure 2 shows typical examples of soybean K deficiency symptoms at reproductive stages. The physiological reasons for late-season development of deficiency symptoms during the last couple of decades are not entirely clear. Reasons might be that with increasing soybean yield potential there is more translocation from leaves near developing pods and grain, resulting in deficiency symptoms.
Figure 2. Soybean potassium deficiency symptoms during the reproductive growth stages.
Observations during many years have shown that severe K deficiency can advance soybean maturity. Therefore, it is not surprising to see senescing soybean, with most leaves yellow or brown, in low-testing field areas a few days before plants in other parts of a field. Figure 3 shows and example observed in research plots. It should be remembered, however, that deficiency of other nutrients or conditions such as excessively wet or dry soil also can advance soybean senescence.
Figure 3. Potassium deficiency advances soybean senescence.
Several soybean diseases caused by fungi and viruses can also produce yellowing of soybean upper leaves, which also may advance plant and leaf senesce. Sometimes, the disease symptoms and K deficiency symptoms occur at the same time. This should not be surprising because Iowa research has demonstrated that K deficiency aggravates the incidence or severity of several soybean leaf diseases. Part of this research was summarized in the proceedings article for the 2016 ISU Extension and Outreach ICM conference “Watch potassium management - It also affects corn response to nitrogen and soybean diseases”. Additional field observations suggest possible interactions with soybean cyst nematode (SCN) and aphid infestation levels. That is, upper canopy K deficiency symptoms can develop in field areas associated with SCN or aphids.
Sometimes it is very difficult to distinguish between K or disease symptoms unless the plants or leaves are submitted to a plant pathology lab for study. Soil and leaf K testing of apparently normal and affected field areas also may help identify the cause for the symptoms. Recently published interpretations for K tissue testing can be useful for soybean plants at the V5 to V6 vegetative growth stages or for upper leaves at the R2 to R3 reproductive growth stages, but not for later growth stages. This is because leaf K concentrations decline during later growth stages. You can see the tissue test interpretations in ISU Extension and Outreach publication CROP 3153 “Phosphorus and Potassium Tissue Testing in Corn and Soybean”.Crop: SoybeanCategory: Crop ProductionSoil FertilityTags: potassium deficiencySoybeansoil fertilitydeficiency symptoms
I have been reluctant to provide estimates of soybean acres damaged from dicamba applied to Xtend soybean due to the difficulty in developing a realistic number of affected acres. While there has been a significant number of acres damaged by dicamba, I am sure it is less than five percent of Iowa’s nearly 10 million soybean acres. Due to this relatively small number of acres affected (in relation to total soybean acres), dicamba injury will not significantly impact Iowa’s productivity in 2018. However, if you are a farmer whose crop has been damaged by dicamba, the fact that the majority of soybean in the state were not affected is of little consolation.
To get a better handle on the extent of dicamba injury across the state, I asked ISU Extension and Outreach field agronomists to complete a brief on-line survey. Half of the agronomists stated the number of soybean acres damaged by dicamba was similar to 2017, whereas the remainder were split between fewer acres and more acres damaged in 2018 than 2017. When I’ve asked commercial agronomists the same question, the range of responses was similar to those of my extension colleagues.
More than 75% of ISU Extension and Outreach agronomists felt volatility was involved in at least 25% of the drift cases they investigated, while 25% thought movement following application played a role in over 50% of the incidences they investigated.
Complaints to state regulatory agencies is one measure that the Environmental Protection Agency (EPA) will consider in their upcoming decision regarding future use of dicamba on Xtend soybean. We know the reported incidences represent a very small fraction of total drift cases as farmers are reluctant to involve regulatory agencies. The majority of ISU Extension and Outreach agronomists reported that Iowa Department of Agriculture and Land Stewardship (IDALS) was contacted in less than 25% of the dicamba cases, and nobody reported IDALS was contacted in the majority of cases.
The majority of growers using the Xtend system are happy with the increased performance in weed control obtained with dicamba compared to alternatives. However, one ISU Extension and Outreach agronomist stated that farmers planting non-dicamba resistant soybean “are really upset with the continued off-target movement of dicamba”. It is my opinion that the new label restrictions placed following the 2017 growing season, and the training required for applicators of the new dicamba products, has failed to reduce off-target problems to an acceptable level.
The EPA recently held two teleconferences with academic weed scientists from states where the new dicamba products are registered. There was near unanimous agreement that the level of off-target injury observed in 2018 is unacceptable. The EPA asked for suggestions on label modifications that could reduce problems in the future. Following are ideas that were put forward:
- All products containing dicamba should be Restricted Use Products
- Volatility is viewed as a contributing factor to off-target damage, thus some sort of temperature restriction should be implemented
- Date restrictions are viewed as more ‘workable’ than the current growth stage restriction, but they would need to be state specific
- There needs to be better clarification of sensitive/susceptible crops
- Buffers need to be 360 degrees rather than downwind
The EPA stated they plan to announce their decision in the near future so that people will know the status of the technology before making 2019 seed purchases. Off-target movement of dicamba is complex, there is no simple solution, and whatever action the EPA takes will not make everyone happy.Crop: SoybeanCategory: WeedsTags: dicambadriftvolatility
Iowa State University Extension and Outreach field agronomists have reported the appearance of frogeye leaf spot in soybeans as much of the crop across Iowa enters the R3-R5 growth stage.
Frog eye leaf spot — caused by the fungus Cercospora sojina — can occur at any growth stage in soybean, but most often occurs after flowering. Typically, the symptoms can be observed from beginning flowering (R1) through beginning maturity (R7). Young, newly unfolding foliage is the most susceptible to fungal infection, which is why symptoms are mostly observed in the upper plant canopy.Symptoms
Disease symptoms typically start out as small, water-soaked spots (or lesions) in the upper plant canopy. As the disease progresses, lesions enlarge and become round to angular. Eventually, the lesion center changes color to gray or brown and is surrounded by a narrow reddish-purple margin. In some soybean varieties, a light green halo around the lesion border can be observed. If environmental conditions are favorable, fungal sporulation can occur which gives the underside of lesions a fuzzy gray appearance. The lesions can then begin to coalesce to create blighted areas on leaves. When the disease is severe, plants could experience premature defoliation.
Lesions on a soybean leaf turning from gray to brown.
In addition to foliar symptoms, the pathogen can infect stems and pods late in the growing season, though these symptoms can be challenging to identify. When lesions appear on the stem, they are elongated. When lesions appear on pods, they tend to appear oblong, and resemble the foliar symptoms. If pods are severely diseased, seeds can become infected and can experience discoloration, turning them a light purple-to-gray color. Infected seeds may also be symptomless.
Frogeye leaf spot can be difficult to diagnose correctly in the field, as it is easily mistaken for other diseases and disorders, including herbicide injury. It is recommended that symptomatic plant samples be sent to a diagnostic laboratory to confirm the disease before implementing a management strategy if diagnosis is unclear. Frogeye leaf spot can be confused with the following diseases and disorders:
- Phyllosticta leaf spot
- PPO herbicide injury
- Paraquat herbicide injury
Conditions that favor frogeye leaf spot include warm, humid weather, with frequent rains that persist over an extended period of time. Several days of overcast weather can also increase the spreading of the fungus. Field conditions can also increase the susceptibility of plants to the disease. These conditions include continuous soybean production (the fungus that causes frogeye leaf spot can survive in infested soybean residue for at least two years), short rotations between soybean crops, practicing conservation tillage as well as planting a susceptible soybean variety in a field with a history of frogeye leaf spot.Management
There are soybean varieties that are resistant to frogeye leaf spot, though be aware that some varieties marketed as resistant to the disease might not entirely be so. The resistance gene, known as Rcs3, has been effective against all races of this fungus known to occur in North America. Crop rotation and tillage can also be effective in reducing the amount of fungal inoculum available to infect the next soybean crop. Long rotations may be necessary if the disease has been severe in a particular field. Well-timed foliar fungicide applications can effectively control frogeye leaf spot. Research shows that applying a fungicide during pod development (R3-R4) is most effective for managing the disease. There is not a set threshold for foliar disease management of soybean, but growth stage, disease risk and varietal susceptibility should be considered when making treatment decisions.Resistance Management
Resistance to quinone-outside inhibiting (QoI/strobilurin) fungicides has been reported in the frogeye leaf spot pathogen in North America. It is important to use fungicide products that contain active ingredients from different fungicide classes for resistance management purposes. Never rely on only one class of fungicide to manage frogeye leaf spot, and always consider the risk factors of variety susceptibility, cropping history and and environmental conditions listed before you apply a fungicide, in order to minimize the risk of further fungicide resistance developing.
If you decide to apply a foliar fungicide, scout fields two weeks after the application to determine if the fungicide is adequately managing disease. Although many factors influence fungicide efficacy (such as low-volume spraying, nozzle choice, carrier-water quality, etc.), inadequate control may indicate that the fungus is resistant to the fungicide you used. Also remember that no fungicide will ever provide 100 percent control on a susceptible variety. If you believe fungicide resistance may be an issue in your field, contact your local extension specialist.Crop: SoybeanCategory: Plant DiseasesTags: frogeye leaf spotsoybeanscrop scoutingfoliar fungicide
Soybean aphid is the most important insect pest of soybean in Iowa. Foliar insecticides, mostly pyrethroids and organophosphates, have been the primary management tactic for soybean aphid in Iowa since 2001. Regular scouting and timely treatments will protect yield. Our research and extension program at Iowa State University (ISU) is focused on evaluating insecticide efficacy for soybean aphid on a wide range of products. We are also screening soybean aphid populations for pyrethroid resistance in northern Iowa.
Photo 1. Soybean aphid. Photo by Matt Kaiser.
Evaluations are established at two locations every summer (see 2005-2017 reports). Here, we show a two-year summary from the ISU Northwest Research Farm, near Sutherland, IA. There were 12 treatments in 2016 and 17 treatments in 2017. Soybean aphids were counted on whole plants from June through September. Soybean aphids colonized plots in late June and July in both years. Foliar insecticide applications were made with a back-pack sprayer when populations reached the economic threshold. The seasonal exposure of aphids on plants was converted into cumulative aphid days (CAD) and yield was estimated to compare treatments. We expect to see treatment differences when CAD exceeds the economic injury level of 5,500 (red lines in Figures. 1A and 2A).
Figure 1. 2016 summary of A) cumulative aphid days (CAD) and B) yield. Means with a unique letter are significantly different (alpha = 0.10). Asterisks indicate pesticidal seed treatment.
Figure 2. 2017 summary of A) cumulative aphid days (CAD) and B) yield. Means with a unique letter are significantly different (alpha = 0.10). Asterisks indicate pesticidal seed treatment.
- Aphids reached the economic threshold in both years; however, aphids were sprayed earlier in 2016 (9 August) than in 2017 (18 August).
- When aphids exceeded the threshold before seed set in 2016, foliar insecticides provided significant yield protection (Figure 1B). When aphids exceeded the threshold after seed set in 2017, there was no yield benefit to the foliar insecticides (Figure 2B).
- Plots with insecticidal seed treatments had significantly higher CAD than foliar insecticide treatments in both years (Figures. 1A and 2A).
- Foliar insecticides provided good efficacy in both years. We have not observed pyrethroid resistance at either location to date.
- Insecticidal seed treatments are not recommended to manage soybean aphid in Iowa, as they do not provide a consistent reduction of CAD (Figures. 1A and 2A).
- Scouting and using foliar insecticides when aphids exceed the economic threshold (250 aphids on 80% of plants) before seed set will provide a 5-10 bushel yield increase (Figure 1B).
- Late-season aphid infestations may not impact yield; applying insecticides after seed set may not result in a return on investment (Figure 2B).
- Pyrethroid resistance is an emerging issue in the Midwest. It is important to check for efficacy after applications to look for resistant aphids.
Acknowledgments: ISU Research Farms; Iowa Soybean Association and the soybean checkoff; BASF, Dow AgroSciences, FMC, and Syngenta Crop Protection.Crop: SoybeanCategory: Crop ProductionInsects and MitesTags: aphidpest
According to the U.S. Drought Monitor, southeastern and south central Iowa are experiencing prolonged heat and moisture stress. In early August, there were twospotted spider mites detected in corn and beans. I recommend scouting corn and soybean fields for mite infestations this month, especially in these areas.
Spider mites generally reach economically damaging levels in late July or early August when conditions are favorable for its growth. However, twospotted spider mite can start building populations in June during years with early-season temperatures greater than 85°F, humidity is less than 90 percent, and moisture levels are low. These are ideal conditions for the twospotted spider mite and populations are capable of increasing very rapidly.Scouting
A hand magnifying lens is recommended to scout for twospotted spider mites (< 1/60 inches long). They can be mistaken for specks of dirt to the naked eye (Photo 1). Twospotted spider mite larvae have six legs, and nymphs and adults have eight legs. Mites can be removed by shaking leaves onto a white piece of paper and then look for moving mites. Twospotted spider mites are typically a cream or green color when feeding on corn or soybean. They can also be an orange to red color when conditions are unfavorable for their growth.
Photo 1. Twospotted spider mite. Photo by Frank Peairs, www.ipmimages.org.
Twospotted spider mites can aggregate at the field edges, especially if there are weeds surrounding the borders. Eventually they can disperse with the wind to develop a field-wide infestation. I encourage people to look at the edge rows first to see if mites can be found. If their presence is confirmed, then estimate populations throughout the field by walking a “Z” or “W” pattern.
Twospotted spider mites begin feeding on the bottom of the plant, and move to the top as the plant’s health deteriorates. Although they lack wings, twospotted spider mites disperse with the wind to move from dying plants to areas with healthy plants. Therefore, it is important to scout healthy areas of an infested field that are downwind from damaged areas. Early symptoms of twospotted spider mite injury will appear as small yellow dots or stipples on the lower leaves of the plants. Prolonged feeding will cause the infested leaves to turn completely yellow, then brown, and eventually the leaf will die and fall from the plant. The webbing is visible on the edges and underside of leaves, and is an indication of prolonged colony feeding (Photo 2). Twospotted spider mite is capable of reducing soybean yield by 40-60 percent when left untreated; drought-stressed plants could experience even more yield loss.
Photo 2. Heavy twospotted spider mite infestation on corn. Photo by Adam Sisson, Iowa State University.
Exact treatment thresholds for spider mites in corn and soybean do not exist. Instead, the decision to treat should take into consideration how long the field has been infested, mite density including eggs, mite location on the plant, moisture conditions, and plant appearance. A general guideline for soybean is to treat between R1-R5 (i.e., beginning bloom through beginning seed set) when most plants have mites, and heavy stippling and leaf discoloration is apparent on lower leaves (Photo 3). Foliar insecticides are recommended in corn from R1-R4 (i.e., silking through dough stage) when most plants have mites at or around the ear leaf and 15-20 percent leaf discoloration.
Treatment of twospotted spider mites may not be required when temperatures drop below 85°F and humidity levels are greater than 90 percent for an extended time because a naturally-occurring fungus can control populations. Mites that are infected by the fungus will appear brown, and will not move on the piece of paper used for scouting.
Photo 3. Twospotted spider mite injury on soybean. Photo by Whitney Cranshaw, www.ipmimages.org.
Bruce Potter and Ken Ostlie (University of Minnesota) developed a twospotted spider mite rating scale to help make treatment decisions:
0 – no spider mites or injury observed
1 – minor stippling on lower leaves and no premature yellowing observed
2 – stippling common on lower leaves and small areas on scattered plants with yellowing observed
3 – heavy stippling on lower leaves with some stippling progressing into the middle canopy and leaf yellowing and some leaf loss observed; mites scattered in the middle and top canopy [Economic threshold]
4 – lower leaf yellowing readily apparent and leaf drop common; stippling, webbing, and mites common in the middle canopy; mites and minor stippling present in upper canopy [Economic injury]
5 – lower leaf loss common and yellowing moving to the middle canopy; stippling and distortion of upper leaves common; mites in upper canopy observed.
Organophosphates are the recommended insecticidal chemistry for twospotted spider mite control (e.g., dimethoate and chlorpyrifos). Bifenthrin is the only pyrethroid to show efficacy against twospotted mite. Insecticides may not kill the eggs, thus a treated field should be scouted 7-10 days after application to determine if a second application is necessary. As always, refer to the label for the appropriate rates and re-entry intervals. To improve application coverage, consider increasing the water volume to make contact with spider mites. Border treatments may also be a cost-effective option if heavy spider mite populations are restricted to edge rows.
Robert Wright and Julie Peterson from the University of Nebraska-Lincoln summarized a list of pesticide groups labeled for twospotted spider mites in corn and soybean:
Mode of action class 1B; organophosphates
- Dimethoate in soybean and corn. E.g., Dimethoate 4E, 4EC, 400, Dimate 4E, 4EC.
- Chlorpyrifos in soybean. E.g., Lorsban 4E, Lorsban Advanced, Chlorpyrifos 4E, Govern 4E, Hatchet 4E, NuFos 4E, Warhawk 4E, Yuma 4E.
Mode of action class 3A; pyrethroids
- Bifenthrin in soybean and corn. E.g., Bifenture 2E, Brigade 2E, Discipline 2E, Fanfare 2E, Sniper 2E, Tundra 2E.
Mode of action class 6; chloride channel activators
- Abamectin in soybean, including spider mite eggs. E.g., Agri-Mek SC.
Mode of action class 10B
- Etoxazole in soybean and corn, including spider mite eggs and immatures. E.g., Zeal.
Mode of action class 12C
- Propargite in corn. E.g., Comite.
Mode of action class 23; tetronic and tetramic acid derivatives
- Spiromesifen in corn, most effective on spider mite eggs and immatures. E.g., Oberon.
- Hexythiazox in corn; does not control adult spider mites. E.g., Onager.
- Hero (zeta-cypermethrin and bifenthrin) labeled for soybean and corn.
- Cobalt (chlorpyrifos and gamma-cyhalothrin) labeled for soybean.
- Swagger (bifenthrin and imidacloprid) labeled for soybean.
- Tundra Supreme (chlorpyrifos and bifenthrin) labeled for soybean and corn.
The soybean cyst nematode (SCN) reproduction can be affected greatly by soil conditions. SCN numbers are positively correlated with soil temperature and negatively correlated with soil moisture. In short - greatest SCN reproduction occurs in hot, dry growing seasons.
Temperatures in Iowa were unusually warm in May 2018. In one field in central Iowa, adult SCN females were seen on roots on June 5, just 26 days after planting (read more here). Typically, SCN females do not appear on roots in the spring until 35 or more days after planting.
June and July have continued to be warm and parts of the state remain dry. The USDA National Agricultural Statistics Service reported at the end of July that corn and soybean crop development in Iowa was 7 to 14 days ahead of schedule. It is likely that SCN development has been accelerated this season, too. And hastened SCN development could result in greater-than-normal increases in egg numbers.Patches of soybeans maturing early? A tell-tale symptom of SCN
Soon, patches of early-maturing plants will be appearing in Iowa soybean fields. There can be many different reasons for early senescence of soybeans. A common cause of early-maturing soybeans that is not often discussed is SCN feeding. The results of a study conducted at Iowa State University’s Northern Research and Demonstration Farm south of Kanawha, Iowa, illustrate this phenomenon visually.
A sampling grid pattern was established in the study area shown in the figure below, and three soil cores were collected from each intersection of the gridlines in the spring. The three cores were combined and mixed into one composite soil sample, and the SCN egg population density present in each soil sample was determined. In mid-September, aerial images of the study area were collected.
A distinct pattern of early-maturing soybean plants is seen near the top edge of the field in the aerial image. And grid cells with high SCN egg population densities are in a similarly-shaped pattern in the map of SCN numbers. The soybean plants matured early where SCN population densities were the greatest.
Actively manage SCN for the future
Map of initial soybean cyst nematode (SCN) egg population densities (top) and an aerial image (bottom) taken in September of the sampled area in an SCN-infested field at the Iowa State University Northern Research and Demonstration Farm near Kanawha, Iowa.
It is not possible to eliminate SCN from a field once the field has been infested. Instead, an active, integrated management approach is needed to keep SCN population densities in check to preserve the productivity of infested fields for growing soybeans in the future.
Integrated management of SCN includes growing nonhost crops such as corn in rotation with SCN-resistant soybean varieties. Also, farmers should seek out soybean varieties with different sources of resistance to grow in different years. And nematode-protectant seed treatments now are available to bolster the performance of SCN-resistant soybean varieties. Fields should be sampled in the fall prior to every second or third soybean crop to monitor SCN population densities.SoybeanCategory: Plant DiseasesTags: SCN
Since 2010, aphids have been colonizing corn later in the summer and are building up to striking levels in Iowa. They can be found at the base of the stalk, around the ear and sometimes above the ear leaf. It seems these aphids have been sighted in corn again this summer (Photo 1).
Photo 1. Aphids have been detected in northern Iowa again. Photo by Brian Lang, ISU.
Aphids have been confirmed in cornfields for about two weeks, particularly in northeast Iowa. This is about 10-14 days earlier than previous growing seasons. Some of these infested fields will likely be sprayed in early August. From my observations this week, I noticed aggregated colonies at the end rows, but some areas have aphids throughout the field.
One important observation I've noticed is that cornfields can have two aphid species - corn leaf aphid and bird cherry-oat aphid. They are closely related and look very similar in size and color. You can see more than one species in a field and even on a single plant. The bird cherry-oat aphid have an orange-red saddle between the cornicles (Photo 2). Other aphid species can also be found, including greenbug and English grain aphid, but are not as common in corn this year. Species identification is not critical for management at this point (i.e., an aphid is an aphid).
Photo 2. Bird cherry-oat aphid. Photo by David Cappaert, www.ipmimages.org.
All aphids have piercing-sucking mouthparts and feed on the sap from the plant phloem. They excrete sugar-rich honeydew that can cover the aboveground portion of plants. The honeydew can promote a sooty mold that interferes with plant photosynthesis. We know soybean plants covered with mold and aphids can have serious yield loss, but we don't know the extent of yield reduction caused by aphids in corn.
Currently, there are no treatment thresholds for aphids in corn past tasseling. But regular sampling will help you make educated decisions about a foliar application at this time. Sample field-wide (30 plants for every 50 acres) to determine the average density. Here are some considerations to make before applying an insecticide for aphids in corn:
- Are 80% of the plants infested with aphids or are they aggregated around the field perimeter?
- Are aphids colonizing the ears, or the ear leaf and above (Photo 3)?
- How long has the field been infested and is the density increasing?
- Do you see honeydew and/or sooty mold on the stalk, leaves or ear? Mold can interfere with photosynthesis and interfere with the grain-filling process. Moldy ears could also reduce grain quality and make harvest difficult.
- Are you seeing winged aphids or nymphs with wing pads? This may be a sign of migration out of the field.
- Is the field under drought stress? Dry weather will make amplify potential feeding damage to corn.
- Do you see any bloated, off-color aphids under humid conditions? Natural fungi can quickly wipe out aphids in field crops. Also, parasitized aphids are common to see in corn and are a result of wasp biocontrol (Photo 4).
- What is the corn growth stage? Fields reaching hard dent may be past the point of a justified insecticide.
- What is the expected harvest date? Some insecticides have a 60-day pre-harvest interval. Check the label and calendar.
- Are you able to use high volume and pressure of an insecticide application to reach the aphids? Ideally, small droplets should make contact with the aphids for a quick knockdown. Don't expect residual to protect the corn from fluid feeders.
Photo 3. Scout for corn aphids after tasseling and look for colonies around and above the ear.
Photo 4. Heavy infestations of corn aphids usually have parasitized mummies. Photo by Adam Sission, ISU.
Typically, fields with more than 500 aphids per plant field wide will benefit from a foliar insecticide. I strongly encourage you to leave an untreated check strip or two in fields that you spray. Try to leave a strip that is a fair comparison to the majority of the field - not just along the field edge. If you decide to treat for aphids in corn, I would like to hear about the yield comparisons. Your pooled data will help me formulate treatment guidelines for the future.Crop: CornCategory: Crop ProductionInsects and MitesTags: aphidscoutingpestIPM
In 2016 and 2017, there were isolated reports of soybean injury by soybean gall midge in northwest Iowa. Confirmations were reported in 2011 from Nebraska and in 2015 from South Dakota. In 2018, the distribution in Iowa has spread to eight Iowa counties (Figure 1). This article hopes to raise awareness about a new soybean pest and confirm any additional infested Iowa counties.
Figure 1. Confirmations of soybean gall midge in 2018 indicated by Iowa counties in orange.
Much is unknown about the soybean gall midge. Entomologists cannot even confirm the species at this point (we are working on it!). Here are some details about the biology and plant injury we have observed so far:
- Injury is most severe at field edges; this possibly indicates adults fly to new soybean fields the following growing season.
- Larval feeding and plant injury is usually restricted to the base of the plant (Photo 1).
Photo 1. Spilling open soybean stems will reveal midge larvae including decaying plant tissue. Photo by Ryan Rusk.
- Many midges can infest a single plant (Photo 2).
Photo 2. Soybean gall midge larvae are clear and eventually turn bright orange as they mature.
- Vegetative and reproductive plants can be infested.
- Initially, infested stems look swollen (Photo 3), eventually turn brown (Photo 4) and break off resulting in plant death.
Photo 3. Look for discolored, swollen soybean stems near the soil line.
Photo 4. After prolonged midge feeding, the plants break off and die.
- In some cases, plants were infected with a fungal disease. But this wasn’t always true as we were able to collect infested plants that did not have any fungal infection present.
- Cultural control practices did not seem to make a difference, including variety selection, time of planting, row spacing, tillage or manure application.
- Insecticidal seed treatments did not appear to effectively suppress the midges.
We are assuming the soybean gall midge can complete at least two generations in Iowa, but we do not know how long a generation takes to develop. We also assume it can overwinter in Iowa and does so as a pupa in the soil or leaf litter similar to other midges.
Midges are a fly in the Cecidomyiidae family. There are about 6,000 midges worldwide and at least 1,100 species in North America. Midges are small (2-3 mm), have long antennae and unusually hairy wings. Most midges are fragile and weak fliers. Many midges are considered economically important plant pests (e.g., Hessian fly, sunflower midge, wheat blossom midge); however, some are predatory on aphids and mites. The maggots are not mobile and must be located on or near the host plant to survive. Many midges larvae feed within the host plant tissue, creating abnormal growths called galls.
If you see these midges infesting a soybean field in Iowa, please let me know via email (email@example.com) or Twitter (@erinwhodgson)!Crop: SoybeanCategory: Crop ProductionInsects and MitesTags: midgepestflyIPMscouting
Watch a video on testing and sampling for bacterial leaf streak!
Iowa State University Extension and Outreach plant pathologists have identified and confirmed several different cases of bacterial leaf streak in corn this growing season. Bacterial leaf streak is relatively new to the United States, with the first case identified in 2016, though symptoms of the disease have been present in Nebraska since 2014. To date, the disease has been reported in nine states including: Colorado, Illinois, Iowa, Kansas, Minnesota, Nebraska, Oklahoma, South Dakota and Texas.Scouting and identifying
Because the disease isn’t widely recognized and can often be misdiagnosed in fields, it’s important to understand how to properly scout for and identify this unique disease.
Bacterial leaf streak is caused by the bacterium Xanthomonas vasicola, and has been observed on field corn, seed corn, popcorn and sweet corn. Symptoms begin as narrow leaf lesions with wavy edges that occur between the veins of corn leaves and can range between one-to-several inches long. Lesions may be yellow, tan, brown or orange and look greasy or water-soaked. These lesions can occur anywhere on the leaf blade, sometimes close to the midrib, and can appear translucent with bright yellow halos, which when backlit, are easy to see extending from the lesions. Over time, these lesions can expand to cover larger areas of the leaf. In extreme cases, these lesions may extend along the entire length of the leaf, and grow together to form large, necrotic areas.
Yellow, tan and brown lesions form in bands across the leaf blade. Holding the sample up to the light will sometimes illuminate these bands to aid in diagnosis.
Not much is known about the disease cycle of bacterial leaf streak, due to the recency of the identification in the U.S. Researchers presume that the bacterium overwinters in infected crop residue because the disease has been observed on volunteer corn present in a soybean field, where the disease had been identified the previous year in corn. It is assumed that irrigation, splashing rain and wind-driven rain spread the bacterium, but it is unknown how far the bacterium can travel. It is also assumed that a plant needs to be wounded for infection to occur.
Disease symptoms have been observed in corn as early as the V7 growth stage, in which lesions first appear on lower leaves, though because of the disease's recent arrival to the Midwest, not much data has been collected on the earliest appearances of the pathogen for scouting windows. Under favorable conditions — continuous corn production, overhead irrigation or rainfall during hot weather — the disease spreads up the plant canopy, though symptoms could also appear only in the upper canopy. Symptoms in the upper canopy are more common when the disease occurs after tasselling. Some reports indicate that bacterial leaf streak appears after high wind and rain events. Under these favorable conditions, disease severity can approach 30 percent.Similar diseases
Scouting for bacterial leaf streak can be difficult, as its symptoms can be mistaken for other common diseases; grey leaf spot and common rust. A laboratory can easily distinguish bacterial leaf streak from fungal diseases through bacterial streaming. Bacterial streaming is an identification process that entails placing a small section cut from the edge of a leaf lesion in a droplet of water on a microscope slide. Under a microscope, you can observe the bacterial cells streaming out from the cut edge of the lesion. However, under a microscope, other bacterial diseases also produce this streaming, so you should confirm the presence of the bacterium by sending a sample to the Iowa State University Plant and Insect Diagnostic Clinic.
An example of bacterial streaming. When water is applied to the infected tissue sample and viewed under a microscope, the bacteria will
stream" out of the sample and into the water. Edward Zaworski
Currently, there is little information available when it comes to managing bacterial leaf streak. Field observations suggest that corn hybrids differ in susceptibility. Once hybrids can be screened for resistance, using resistant hybrids will be the best way to manage bacterial leaf streak. Like other bacterial diseases (such as Goss’s wilt) there are no effective chemical controls. Researchers do not recommend tillage to reduce the risk of bacterial streak, due to the need to manage soil erosion. While bacterial leaf streak has been most commonly observed in overhead-irrigated fields, it is also known to occur under both flood irrigation and dryland conditions.Yield impacts
Likewise, researchers only have limited information about bacterial leaf streak’s impacts on yield. Researchers anticipate losses to be minimal if symptoms develop late in the season, or if extensive leaf blight does not occur before or during grain fill.Crop: CornCategory: Plant DiseasesTags: bacterial leaf streakCorn diseaseCorndisease
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