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Strategic Farming: Let's talk crops focused on emerging corn and soybean diseases

Phyllis Bongard, Extension content development and communications specialist, and Dean Malvick, Extension plant pathologist

Northern corn leaf blight
In recent years, several corn and soybean diseases typically observed further south have made their way into Minnesota. Dr. Dean Malvick, Extension plant pathologist, joined moderator Claire LaCanne, Extension educator-crops, to discuss emerging and familiar crop diseases, and how to identify and manage them in the final 2024 session of Strategic Farming: Let’s talk crops.

Corn diseases

Gray leaf spot

Gray leaf spot (GLS) is not a new disease in Minnesota, but it is less of an issue for us than it is for our neighbors to the south. The fungus that causes GLS is favored by prolonged high humidity and warm temperatures, so southern Illinois, for example, is an area that has seen high very levels of GLS. While we certainly experience high humidity during the summer, it doesn’t tend to be on a broad enough scale to favor significant levels of GLS disease.

GLS is common in the fall in southern Minnesota, but typically at low levels in scattered fields. The classic symptoms include gray, rectangular lesions that are bordered by veins. However, the disease can display other lesions, too, making diagnosis a challenge.

If Minnesota were to have wet, humid summers, GLS issues could increase. Management for this disease includes planting resistant hybrids and using fungicides when needed.

For more information, see Gray Leaf Spot of Corn and Fungicide efficacy for control of corn foliar diseases from the Crop Protection Network (CPN).

Northern corn leaf blight

Northern corn leaf blight (NCLB) is widespread and more common than gray leaf spot in Minnesota. The long, canoe-shaped lesions are typically easy to find in Minnesota fields, but like gray leaf spot, the disease rarely causes significant yield losses in the state.

NCLB is favored by cool, wet conditions. Dew also plays a major role in spore germination. One hour of dew is needed for spores on the leaf surface to germinate and another four hours are required for infection to occur.

Resistant hybrids are available to help manage NCLB. Tillage and crop rotation play a role by reducing the amount of inoculum available in infected residue. Fungicides are also an effective tool in managing NCLB.

For more information, visit CPN’s Northern corn leaf blight of corn.

Stalk rots

Stalk rots have been problematic in Minnesota for a long time. The primary types occurring in Minnesota include Anthracnose, Gibberella and Fusarium stalk rots. Anthracnose is usually the easiest to diagnose with its dark discoloration on the outer rind of the drying stalk.

Gibberella sometimes has a pinkish discoloration associated with it. Since the same pathogen also causes Fusarium head blight of small grains, a rotation of wheat or small grains with corn can increase inoculum and cause problems with both crops. Fusarium stalk rot is quite common in Minnesota, although it’s not the easiest to diagnose.

Finally, Physoderma stalk rot has been seen more frequently in the last 6-10 years. It can cause issues on the leaves as well as at stalk nodes.

While the incidence of stalk rot varies by year, it is widespread and common in most places that corn is grown. If it occurs early enough, it can affect yield. However, yield loss is less common than early lodging and harvest challenges due to weakened stalks.

There is no simple formula for managing stalk rots because there are so many pathogens and factors involved. The key is to reduce stress on corn plants and minimize leaf disease and damage. Anything that reduces photosynthetic area also reduces the amount of energy that can go into strengthening the stalk. In addition, a corn on corn rotation increases inoculum and the potential for reinfection in following years.

For more information, see Anthracnose stalk rot, Gibberella stalk rot, Fusarium stalk rot, and Physoderma stalk rot.

Tar spot

Tar spot is an emerging corn disease in Minnesota. It was first reported in north central Illinois and Indiana in 2015 and confirmed in southeastern Minnesota in 2019. While it has been detected at low levels in additional Minnesota counties since then, the highest disease levels have remained concentrated in the southeastern corner of the state.

Until recently, researchers thought the disease required a lot of moisture and cool conditions to develop. While temperatures below 73F do favor tar spot, they’ve learned that it can develop at higher temperatures, such as in very warm central Illinois. There’s much yet to learn, but researchers speculate that cooler nighttime temperatures may play a role in keeping the disease going.

What about moisture? Tar spot has been able to develop under drier conditions than originally thought. In U of M inoculation trials, tar spot developed and spread in the field despite the dry 2023 conditions. Tar spots were visible 19 days after inoculation and secondary spread of the disease was detected 20 to 22 days after that.

Of the two factors, temperature was the most important factor for tar spot development according to a new study reported in Scientific Reports. The optimum range was between 64 and 73F, with monthly average temperatures above 73 hindering disease progression. They also found that tar spot developed when relative humidity was below 90% over a 2-to-3-week span. In fact, extended periods of high humidity, especially with high temperatures, seemed to hinder disease development. While there are still questions about the conditions required for tar spot development, it shows that cool temperatures can favor tar spot and this disease may not need as much moisture as originally thought.

To manage tar spot, avoid the most susceptible hybrids. Start scouting fields in mid-July to assess risk and if severity exceeds 5%, fungicides can be effective and profitable if conditions are favorable for continued disease development. However, application is needed when the epidemic starts with optimal timing between VT and R3. Managing residue is less important for tar spot, since spores can be dispersed by wind.

For more information, see Tar spot of corn.

Southern rust

Southern rust is another emerging corn disease in Minnesota, although it has not been a concern during the recent drought. Keep an eye out for it in August because it has the potential to become a bigger problem.

For more information, see Southern rust of corn.

Soybean diseases

Frogeye leaf spot

Frogeye leaf spot (FLS) is an emerging soybean disease in Minnesota. Prior to 2017, this fungal disease was uncommon in the state. Since then, it has spread and reached levels of concern in some fields in non-drought areas. In the southern half of the US, frogeye leaf spot is a significant problem with yield losses of up to 30% reported.

FLS can be identified by brown/tan leaf spots that are surrounded by a purplish ring. It’s favored by warm and humid weather and yield losses can be substantial.

FLS can be managed with crop rotation, tillage, and fungicides. However, managing FLS with a fungicide is complicated due to the pathogen’s resistance to QoI (strobilurin) fungicides. A non-QoI fungicide, such as a triazole or SDHI type, needs to be selected instead. If a fungicide mixture contains the QoI fungicide, non-QoI fungicides need to be in high enough concentrations for the mixture to be effective. See the Crop Protection Network’s publications , Fungicide efficacy for control of soybean foliar diseases and Frogeye leaf spot of soybean for more information.

White mold

White mold has been a problem for decades and while there’s nothing new and emerging with this disease, there are a few management tips to consider. This disease thrives in cool temperatures and wet conditions, particularly under the canopy. If it’s feasible, reduce planting density and increase row width to manage known risk and minimize disease. Plant the most resistant soybean varieties and know your fields and WM risk in each one. Finally, consider fungicide applications at late R1 or even early R2 growth stages when rows are filling in and risk is high, depending on weather and crop conditions.

For more information, see White mold on soybean.

Soybean cyst nematode

Soybean cyst nematode (SCN) has been in Minnesota since the 1970s, but its distribution is still expanding and it’s now found in most counties where soybeans are grown.

While not developed as nematicides, there is evidence that the fungicides Illevo and Saltro can affect nematodes. Do they have any effect on SCN? A newly published paper helps answer this question.

The objectives of this 13-state plus Canada study were to evaluate 6 seed treatments on early and full season SCN reproduction, sudden death syndrome (SDS) symptoms, and soybean yield over three years. The overall conclusion was that no seed treatment offered significant SCN control compared to the non-treated check when pooled across all the locations and years. While Illevo and Saltro did provide more consistent yield increases, that was likely due primarily to their strong effect on SDS.

Charcoal rot

If we continue to have periods of drought and heat in the summers, charcoal rot of soybean is a disease we may see more of. It’s common across the US and while we know it’s in Minnesota, we don’t know how widespread it is.

Charcoal rot is favored by hot (>85F), dry soils. It has a very wide host range that includes soybean, corn, alfalfa, sunflower, and dry edible beans.

Symptoms usually begin in the driest parts of the field with premature leaf yellowing. To diagnose charcoal rot, you can cut into the lower part of a stem or root and use a hand lens to look for small black specks. Crop rotation with small grains may be helpful and some soybeans have partial resistance.

If you see plants that you suspect of having charcoal rot, Dr. Malvick would like to hear from you ( He welcomes any plants or soil from fields that are infested with charcoal rot or Phytopthora and there’s no charge for this analysis.

Digital Crop Doc

Every year is different and we never quite know what we’re going to find or what’s going to develop in crop fields. That’s where Digital Crop Doc comes in. When disease is developing in the field and you’re wondering what it might be or you’re looking for confirmation, submit photos to the online form and one of our experts will contact you with a diagnosis. If we can’t give a definitive diagnosis, we might request more information or suggest that samples be submitted to the plant disease clinic.

This service is free and open for corn, soybean, small grains, sugarbeet and forage submissions.

Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.

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