Field Notes talked corn and soybean diseases

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

While some crop disease has been reported, most corn and soybean diseases are still in the early stages of development. Most of the diseases that cause significant issues may start appearing in the next week or two. To help know what to watch for, Dr. Dean Malvick discussed several corn and soybean diseases, conditions that favor them, and their management options in the July 23rd Strategic Farming: Field Notes episode.

Soybean diseases

White mold

White mold, a potentially damaging disease, is favored by rain during the flowering stage. However, it prefers cool temperatures for development and while there have been a few cool nights, the warm temperatures we’ve been experiencing may slow its progression. Watch for this disease, especially if cool weather and more rain is in the forecast.

Cultural practices that speed canopy closure, such as planting in narrow rows at high plant populations, can enhance disease risk in susceptible varieties. Plant varieties that have some resistance to white mold. Since completely resistant varieties aren’t available, planting partially resistant varieties will be helpful but not completely effective against this disease. Reducing stand density, increasing row spacing, and reducing soil fertility (especially manure) can create a less favorable environment for the pathogen.

Fungicides can also be an effective tool against this pathogen if applied at R1 to R2 growth stages (early bloom) before infection occurs. Fungicides can reduce damage and help maintain yield if conditions favor this disease.

Figure 1. White mold in soybean field

Figure 2. White mold on soybean stems

Figure 3. Frogeye leaf spot.

Frogeye leaf spot

Frogeye leaf spot is a serious disease in the southern half of the U.S. and was common across Minnesota a few years ago. Since then, however, there have been few reports of this disease. Look for small, brown, elliptical to round spots with a darker margin. With soybeans rows filling and significant amounts of moisture, conditions are favorable for this disease to develop.

Sudden death syndrome (SDS)

Sudden death syndrome has been an issue in most parts of the state for a long time. Dr. Malvick suspects that we’ve had favorable conditions for this disease to develop again this season. Initial infections occur early in the plant’s growth when the radicle, or new root, is just developing. However, it takes a long time for symptoms to show up, as we don’t often see them until the first week or two of August. Scattered yellow spots between leaf veins become large necrotic blotches, while veins remain green.

Figure 4. Sudden death syndrome.

More seed treatments are available for managing SDS. Take note of seed treatments you use and their effectiveness.

SDS has been found across most of southern Minnesota and as far northwest as Clay County. While it hasn’t been confirmed further north in the Red River Valley in Minnesota, it has been found in North Dakota and as far north as the Canadian border. There’s no reason why it couldn’t be developing further north in Minnesota. If you see this disease in this region, please consider sending samples to Dr. Malvick (dmalvick@umn.edu).

Brown stem rot

Brown stem rot is a soybean disease that causes leaf symptoms that are similar to SDS, although they normally don’t appear until later August. The major difference is in how the pith of split stems appear. The pith of a BSR-infected plant with be discolored and brown, while the SDS pith will not. 

Figure 5. Diseased soybean stems comparing white pith of sudden death syndrome (left) to brown, diseased pith of brown stem rot (right).

Phytopthora rot

Figure 6. Phytopthora rot.

With all the rainfall in many areas, conditions have been very favorable for Phytopthora rot. This pathogen causes dark brown discoloration of the lower stem and can kill plants at any time during the growing season. Plants are often killed in patches in the field.

More cases are emerging where resistance genes bred into soybean varieties are no longer effective against Phytopthora. If you notice plants dying in the field, this may be a potential cause.

Corn diseases

Corn diseases are also starting to develop in Minnesota.

Corn rusts

Corn rusts, like all rusts, cannot overwinter in Minnesota, so must arrive from southern states. Common rust is frequently seen in Minnesota, especially in cooler summers. While it is here nearly every year, it rarely causes significant damage or is of great concern on most grain hybrids. By the time it gets here, the temperatures are usually too warm for common rust to develop to high levels.

Southern rust, on the other hand, is more concerning. After a major outbreak in 2024, are we in for a repeat in 2025? Unlike common rust, Southern rust is favored by warmer temperatures and recent 80-degree weather has been ideal. This disease has been reported across Iowa with a few reports from Minnesota. This is another disease to watch for.

How can you tell the two rusts apart? Common rust pustules tend to be round, darker and brown. They’re common on both upper and lower surfaces. Southern rust, on the other hand, has orange to light brown pustules that are more elliptical and found mostly on the upper leaf surface.

Figure 7. Common rust.

Figure 8. Southern rust.

Bacterial leaf streak (BLS)

Bacterial leaf streak of corn is a relatively new disease in Minnesota. While it has been here about 11 years, it has not been a major problem. It’s been about 6 years since it was more common, but it has popped back up this year in southern Minnesota. The disease levels were not concerning, but it is something to pay attention to, especially since it is favored by frequent rainfall or irrigation and warm temperatures.

BLS symptoms can often be confused with those of gray leaf spot (GLS). GLS is a fungal disease with lesions that tend to be long and linear and more squared than BLS. BLS lesions are wavier on the edges.

Remember that fungicides may be effective against GLS, but they will not work on BLS.

Figure 9. Bacterial leaf streak (left) and gray leaf spot (right).

Goss's leaf blight and wilt

Goss’s wilt was a significant issue in Minnesota 10 to 15 years ago. Since then, its prevalence has declined so much that it’s rarely seen outside of small outbreaks. This is largely due to good levels of resistance in most hybrids. It’s favored by continuous corn, infected residue, susceptible hybrids, and hail damage.

Symptoms are long, linear to oval lesions that tend to be bordered by freckles. It can be easily diagnosed with lab tests, including cutting the lesion open and watching bacterial ooze flow right out of the leaf tissue.

Figure 10. Goss's leaf blight and wilt.

Tar spot

Tar spot, caused by the fungus Phyllachora maydis, was first confirmed in Minnesota in 2019 in southeastern Minnesota. Since then, it has spread across southern Minnesota, into South Dakota, and all the way up to the southern Red River Valley and North Dakota. Fortunately, most of that spread was at low levels of disease severity. So far it has been confirmed in three counties this season: Fillmore, Faribault, and Watonwan Counties. To see tar spot’s full US distribution, visit the Corn ipmPIPE website.

After an incubation period (time from infection to development of spots) of about three weeks, tar spot spores produce small, raised black spots on leaves and husks. These spots are usually elongated, go all the way through the leaf, and cannot be rubbed off. In early stages, it can be challenging to identify this disease. As it develops, the spots become more prevalent and, in some cases, the spots are surrounded by a lighter colored halo.

Figure 11. Tar spot of corn.

Risk factors for disease development

Tar spot infections can come from two spore sources: 1) Corn residue where the pathogen has overwintered, and 2) Windborne spores. While it might make sense that the amount of infected corn residue would directly impact disease risk, that does not often seem to be the case. Farmers in the southeastern part of the state have reported higher levels of disease in fields that didn’t follow corn than those that did. Dr. Malvick has also seen severe infection levels in fields where corn was not the previous crop, suggesting that rotational history cannot predict tar spot incidence or severity. 

Moderate temperatures favor tar spot and is the most important environmental variable influencing its development. Temperatures between 64 and 73F and lasting for extended periods (30 days) are optimum for development. Temperatures above 73F seem to slow its progression.

Moisture plays less of a role in tar spot development. Moisture is favorable to a point, but too much can result in less tar spot. Additionally, high relative humidity levels above 90% seem to decrease its development. There are still many unknowns around risk factors for tar spot severity.

Scouting for tar spot

Scouting for tar spot can be challenging. Fortunately, it’s often found near the field edges. In many cases, if tar spot is not found anywhere near the field edge, there can be some level of confidence that there’s not a lot of disease in that area. In addition, if tar spot isn’t developing by early August and corn is at its typical stage of development, the chances of it developing to a severe level and causing yield loss diminish quickly. So far, field incidence and severity levels have been very low, but it’s early so be vigilant. Not finding tar spot now is not a predicter of how much risk there is!
Management

So you’ve found a few spots in the field. When does it pay to spray? While there is some debate on the economic threshold, a fungicide application could potentially be justified if 1) there is a consistent level of 5 to 10 lesions on two leaves across a large number of plants in the field, 2) the corn is at tasseling to milk stage, and 3) the forecast is favorable for disease development. The optimal time for most fungicide applications is between VT and R2, with a preference toward later applications, if possible, to avoid two applications. There are multiple fungicide options for treating tar spot. For product comparisons, visit Fungicide efficacy for control of corn foliar diseases.

Forecast

Timing is everything! If we were into the 2nd week of August with current observations, the likelihood of high levels of disease across large parts of the state would be low. However, it’s early enough so that tar spot could develop. The key is to monitor fields, forecasts, and scout.

For more information

• Crop Protection Network – crop disease information from a multi-state collaboration of Extension specialists and public/private professionals.
• Corn ipmPIPE – Corn disease and insect pest tracking from a multi-university collaboration.

Join us again on August 6 for corn and soybean insect pests!

The Wednesday, July 30 Field Notes session has been cancelled due to conflicts with the Field School for Ag Professionals. Join us again the following week on August 6 when we welcome Drs. Bob Koch and Fei Yang, Extension entomologists, for updates on corn and soybean pest activity.

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