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

By: Liz Stahl, UMN Extension Educator – Crops and Dean Malvick, Extension Plant Pathologist

Tar spot on corn. Photo: D. Malvick

Key corn and soybean diseases to watch out for in MN were the topic of the March 5th Strategic Farming: Let’s Talk Crops program hosted by University of Minnesota Extension. Dr. Dean Malvick, Extension Plant Pathologist with UMN Extension, joined moderator Claire LaCanne, UMN Extension Educator - Crops, for a discussion on “Corn and Soybean Diseases – Looking back as we look forward”. This was the ninth weekly episode of the 2025 Strategic Farming: Let’s talk crops! webinars. The series runs through the end of March.

The following highlights some of the diseases discussed. 

To watch this and other episodes: http://z.umn.edu/StrategicFarmingRecordings

Corn diseases

Tar Spot

Tar spot was first discovered in the U.S. in 2015 and in MN in 2019. Since its initial discovery in MN, it has been spreading, causing yield losses in some fields, particularly in SE MN, that have exceeded 50 bu/ac. Tar spot overwinters in MN on corn residue, and the inoculum is currently present in many fields across the state, although the disease has not been a yield limiting factor in most areas. Wind borne spores and infected corn residue are sources of the disease inoculum.

The most severe and the only yield-limiting levels of tar spot to date have occurred in SE MN. Thus this area is at higher risk from this disease than others so far, although there has been a pattern of inconsistency where tar spot severity can vary greatly in adjacent counties. Having a significant level of disease the previous year also does not necessarily mean there will be an issue the following year. Much research has been going into developing risk models for the disease. Although it appears the risk is low for the disease to develop across a wide area of the state in a given year, moderate temperatures (60 to 74°F), and an average relative humidity (more than 70% but less than 90%) seem to favor the disease.

The initial signs of the disease are black spots on corn leaves that will not rub off. As the disease progresses, infected leaves will eventually turn brown and die. When the disease develops early in the season, if conditions are conducive, the disease can complete several cycles in the field. About 18 days to three weeks are needed from the time plants are infected before the disease signs and symptoms appear. If the first signs of tar spot don’t develop until early August, there may not be enough time left in the growing season for the disease to be a significant yield limiting factor.

A fungicide application applied around VT (tasseling) and R1 (silking) up to the R3 (blister) stage of corn has been shown to decrease disease incidence when the disease is present in the field. If the disease develops early in the season, particularly when conditions are conducive for disease development, the likelihood of seeing a return to application increases. The Crop Protection Network, using research results from Universities throughout the Upper Midwest, has a corn fungicide efficacy table for tar spot and other corn diseases. Planting a tolerant hybrid is also a useful management strategy to reduce tar spot. Extending the crop rotation or tillage appear to provide little benefit

Although the tar spot pathogen overwinters in corn residue, the disease often does not seem to be worse in no-till fields vs fields with more aggressive tillage. Tar spot progression throughout the US can be tracked through the IPMPIPE (Integrated Pest Management Pest Information Platform for Extension and Education) tar spot website.

Goss’s Leaf Blight and Wilt

Goss’s Leaf Blight and Wilt, first confirmed in MN in 2009, has been shown to decrease yields up to 40%. Since this disease is caused by a bacterial pathogen, fungicides are ineffective against it. The disease starts out as water-soaked lesions, which can resemble northern corn leaf blight. As the disease progresses, the lesions will expand and leaf tissue will die. Although Goss’s wilt is still present in fields in Minnesota, it has not been a significant issue for most areas of the state over the past 10 years. Widespread use of resistant hybrids has dramatically reduced the potential for significant disease incidence and ultimately yield losses.

Physoderma Brown Spot & Node infection

This disease is favored by warm (75 to 85F) and wet weather when corn is in the vegetative stages. Small, yellowish oval or round spots on the leave, often in bands, are a characteristic of this disease. Black to dark purple spots may develop on the midrib of leaves, and infection can lead to node or stalk rot.

Southern Rust

This disease was widespread across Minnesota in 2024, and it remains to be seen if it will return in 2025. Most corn hybrids have a low level of resistance to southern rust. Spores must be blown into MN from the south as this disease does not overwinter in the state. Yield losses due to southern rust can be significant, but a fungicide applied from the R1 to R3 stage can be an effective management strategy.

Soybean diseases

White mold

White mold has been a problem in soybean in MN for many years. Cool, wet weather in mid to late July during flowering is needed for this disease to develop. High plant populations, high fertility, narrow rows, and a thick lush canopy that limits air circulation all favor the disease.

Planting a resistant variety is a key management strategy. Reducing seeding rates and increasing row width when feasible, can also help. A timely fungicide application, applied around growth stage R2 can also help manage the disease. The Crop Protection Network provides an efficacy table for fungicides to manage white mold in soybean.

Soybean cyst nematode (SCN)

SCN is a problem across most of the state. General recommendations for managing this pest have been pretty stable over the years. This includes taking soil samples to check SCN numbers in the field, rotating with a non-host, rotating soybean varieties, and rotating sources of resistance to SCN. Nematicide seed treatments may help as well.

Sudden Death Syndrome (SDS)

SDS is primarily a root disease, but it can affect soybean leaves and cause defoliation. SDS is often found in fields with SCN. SDS foliar symptoms look familiar to Brown Stem Rot. To manage SDS, use soybean varieties with resistance to SDS. Several seed treatment fungicides are labelled for managing SDS and are effective for managing this disease.

For further details on these diseases and more, check out the U of MN Extension websites on soybean and corn diseases.

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