What does all this rain mean for corn and soybean diseases in Minnesota in 2024?

 Dean Malvick, Extension plant pathologist

Soybean in flooded field.

The frequent and excessive water in many crop production fields increases problems that affect plant growth and health. This includes nitrogen loss, immersion of plants, poor root development, etc. – all which can result in stunting and poor growth without the help of any disease. However, the wet soil and frequent rains also can set up conditions for infection and development of various diseases, some of which may be damaging roots or stems of plants now. Scouting is recommended to see when and where disease may be developing. Other diseases may also be getting established only to do most of their damage later if weather conditions are favorable later in the summer. This article highlights a few observations and diseases.

Root diseases

Soybean plants killed by Rhizoctonia root rot.

Generally, root rots are favored by wet or moist soil. They are more common and problematic in soybean than corn, but root rots and root disease can also damage corn. Prediction of when and where they will occur and how serious they will be is challenging. Warm and moist to wet soils are generally favorable for Phytophthora root and stem rot of soybean and Rhizoctonia root rot of soybean and corn. These diseases are occurring now.

Specific seed treatments are effective for these two diseases. Resistant soybean varieties can also be effective for Phytophthora root and stem rot -although not all resistant varieties are effective in all fields. Scouting and accurate ID of problems now is important to understand which problems are occurring and where. Field visits can also reveal how well seed treatments and resistance genes have protected plants from disease and can help in planning for management tactics to be used in the future.

Soybean SDS and white mold

Wet soil and frequent rains in the early season can increase the risk of sudden death syndrome (SDS) and white mold of soybean. However, the weather in July and August will determine whether these diseases develop to high levels. SDS is favored by periodic rains and moist soil in the mid to late summer. White mold prefers cool and wet weather in early July to mid August when soybeans are flowering. If we return to very dry weather in early July – these two diseases are much less likely to result in yield loss. There is nothing to be done to manage SDS this growing season. White mold can be managed with application of selected foliar fungicides at the R2 or late R1 growth stages.

Leaf diseases

Most of the significant leaf disease pathogens of corn and soybean in Minnesota overwinter on crop residue and can be favored by wet or humid weather. This includes northern leaf blight, Physoderma brown spot, and tar spot of corn; in soybean, this includes bacterial blight, frogeye leaf spot, Septoria brown spot, and Cercospora leaf blight. In fields with residue infested with the pathogens that cause these diseases, the continued wet soil may be enhancing pathogen growth as well as production and spread of spores that can start new infections. The extended periods of wetness on leaves of growing plants may promote infection of the leaves by the spores. This does not mean early season foliar fungicides are warranted, as applications at the R3 stage for soybean and VT- R1 stage for corn have generally been most cost effective for most leaf diseases. The corn disease crazy top is especially favored by flooding, but fortunately this rarely occurs in Minnesota.

The wet weather may be setting up significant disease problems in some fields – especially if the weather favors those diseases in July and August. Most fields will likely not have major problems- but most is not all and that is concerning.

Disease diagnostic resources, photos, and additional information

Digital Crop Doc https://z.umn.edu/DigitalCropDoc

Plant Disease Clinic https://pdc.umn.edu/

Soybean diseases and insect pests https://extension.umn.edu/soybean/soybean-pest-management

Soybean diseases (Soybean Research and Information Network) https://soybeanresearchinfo.com/soybean-diseases/

Crop Protection Network https://cropprotectionnetwork.org/