Source
Dean Malvick
The Bulletin - Pest management and crop development information for Illinois

Seed and seedling diseases often dramatically influence establishment of soybean stands. Although many soybeans remain to be planted in Illinois, many have been planted, and the wet weather in some areas and fluctuating temperatures may favor development of seed and seedling diseases. This article provides a general overview of seed and seedling diseases of soybean and describes a project designed to enhance our knowledge of these diseases in Illinois.

Soybean seed and seedling diseases can develop soon after the seed is planted and continue beyond the V2 stage. Although other factors can also cause these effects, reduced germination and/or emergence that results in thin stands is an indication of seed and seedling diseases. Another indication is damping-off, which kills seedlings after emergence. These disease problems are usually associated with wet soil conditions, although major problems may not be noticed until a week or more after the wet conditions occurred.

The major pathogens that cause seed and seedling diseases of soybean are the fungal or fungal-like pathogens Pythium, Phytophthora, Rhizoctonia, and Fusarium. However, we don't have a good understanding of which of these pathogens are most common in Illinois. Please read the last paragraph in this article to get information about a project to help answer this question.

These pathogens all survive and persist in soil. The tan-brown, soft-rot symptoms on roots and stems caused by Pythium and Phytophthora are similar and are difficult to differentiate without laboratory testing. Reddish to dark brown lesions, often sunken, are typically caused by Rhizoctonia. Fusarium damping-off and root rot typically causes light to dark brown lesions. Proper diagnosis of the major problem can help with determining best management practices.

Infection of soybean hypocotyl by Phytophthora or Pythium.

Once a problem with seed and seedling diseases of soybean is noted, several management options can be considered this year and in the future. Whether or not to replant is based on several factors, including the magnitude of the stand reduction and the planting date. Additional information on replanting decisions can be found in the Illinois Agronomy Handbook, which is available from University of Illinois Extension. If the decision is made to replant due to loss from disease, fungicidal seed treatments and perhaps a cultivar with good resistance to Phytophthora should be considered.

Seed treatments will sometimes provide a significant benefit, while at other times they don't provide a clear benefit. Influential factors include planting date, history of seed and seedling diseases, types of seed and seedling diseases present, risk tolerance, soil compaction, seeding rates, field drainage, and tillage.

Seed treatments differ in activity against various seed and seed rotting pathogens. The systemic compounds Allegiance-FL and ApronXL are most effective for control of the "water-mold" pathogens Phytophthora and Pythium. The other group of fungicides, including Rival, Maxim-4FS, captan, and several other products, protect against Fusarium, Rhizoctonia, and other true fungal pathogens. Azoxystrobin is a new systemic fungicidal seed treatment that recently became available for soybean in the product SoyGard, primarily for control of Rhizoctonia and perhaps Fusarium. For full-spectrum control of different pathogens, combinations of these products are often used. In addition, a biological fungicide (Yield Shield) is available to protect soybean seed against Fusarium and Rhizoctonia.

We have limited information on which seed and seedling pathogens of soybean are commonly causing problems in Illinois. We have initiated a project to get more information about these pathogens in Illinois and are requesting your help. We want samples of soybean with seedling diseases from anywhere in Illinois to determine which pathogens are causing problems. If you visit fields or get calls regarding soybean disease at the seedling or stand establishment stage, please collect plants or have plants collected (10 plants per field, along with exact location, soybean variety, and seed treatment if any). Send them to Dean Malvick, University of Illinois, Department of Crop Sciences, N533 Turner Hall, 1102 South Goodwin Avenue, Urbana, IL 61801. Results will be reported next fall or winter.