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Update on Soybean Diseases and Diagnosis in Illinois

June 8, 2001
After a period of dry and warm weather in many parts of Illinois, various diseases of soybean in the V1 to V2 stages are being reported as the weather has become wet and cool. We have received reports of damping-off as well as root and foliar disease symptoms on soybean. Robert Bellm, Extension educator, Edwardsville Extension Center, reported two farms in Shelby County with soybean leaves developing brown spots, turning yellow, and falling off. To date, most reports have come from the central and western parts of the state.

A common symptom being reported is brown lesions and discoloration of soybean roots. Depending on the type of lesion and discoloration, this may be associated with several fungal pathogens, including Rhizoctonia, Fusarium, Pythium, and Phytophthora. Wilting, damping-off, and rot of the roots and hypocotyls also have been reported and may be caused by these pathogens. These pathogens can also cause sub-lethal infections that result in stunted plants with inferior root systems. For more information on these pathogens and their management, see issue no. 2 (April 6, 2001) and issue no. 6 (May 4, 2001) of the Bulletin. The cool weather may make plants more vulnerable to infection and may be preventing plants from outgrowing some infections. A return to normal warmer temperatures may significantly reduce the development and impact of lesions that are developing on roots. Continued wet or cool conditions will likely result in damping-off and stand reduction.

Various foliar lesions and symptoms have also been reported on soybean. Several pathogens can cause soybean foliar diseases at all stages of plant development. Examples include bacterial blight, which prefers cool and wet weather, and bacterial pustule, which prefers warm and moist weather. Brown spot, anthracnose, and frogeye leaf spot are fungal diseases that prefer warm and moist conditions. These fungal diseases are initiated by primary inoculum (spores and mycelium) that overwinters on stem and leaf tissues or seeds. Infections at the early stages of plant development may provide secondary inoculum to spread the disease later in the season if weather conditions are favorable. Spread and development of these and most other foliar diseases should be slowed or halted when dry and warm weather returns.

In some locations, root and foliar disease of soybean has raised the issue of the value and need for replanting and seed treatments. Remember the three conditions required to have a disease problem: presence of a pathogen, environmental conditions favorable for disease, and susceptible plants. You know that all of these conditions are being met in a field if you are seeing disease now. If you are thinking about replanting, you may want to consider seed treatments for diseases affecting roots and hypocotyls or a different cultivar for foliar diseases. Resistance is not available for some foliar diseases, however, so be sure to check with your seed dealer for information on available cultivars.

There is no definite answer for whether you should replant using seed treatments or a different cultivar. These decisions depend in part on the pathogens causing the problem and the expected risk of favorable weather for disease. Most seed treatments are generally only effective for 10 to 14 days after planting. Seed treatments or different cultivars may provide minimal benefit if you have a tilled field and the weather turns warm and dry. On the other hand, if the weather stays wet or cool and you have a no-till field, seed treatments will reduce the risk of reoccurrence of a root rot problem, and there is a greater chance that seed treatments will provide a yield advantage.

We have been receiving calls about diseases along with requests for information about how to manage them. Although we often can narrow the possibilities about what may be occurring based on the verbal descriptions we receive, more information is usually needed to make definite recommendations. The best recommendations are based on a thorough disease diagnosis that can be conducted for a small fee at the University of Illinois Plant Disease Clinic (see issue no. 5, April 27, 2001, of the Bulletin). If you have a serious concern regarding a potential disease problem, call the clinic at (217)333-0519 and arrange to send a sample for diagnosis as soon as possible after the sample is collected. Another option for disease diagnosis is the University of Illinois Extension Digital Distance Diagnostics Imaging (DDDI) system. With the DDDI system, you can take samples to regional Extension offices equipped with microscopes and digital cameras and have digital images of plant and pest samples produced that are sent to plant pathologists, entomologists, or agronomists for diagnosis (see article on the DDDI system in this week's issue of the Bulletin).--Dean Malvick

Author: Dean Malvick

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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