Management of Soybean Diseases in the Buckeye State
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Updated from “Profitable Soybean Disease Management in Ohio” 2002. Ohio State University Extension, Bulletin 895.
Soybean farmers have several tools in their toolbox to manage diseases in their fields. Determining the most effective approach will depend on the field’s disease and crop history, environmental conditions, and accurate diagnosis.
Several factors have contributed to an increase in disease incidence and severity of soybean diseases over the past ten years. Soybeans have been increasingly used as a continuous crop in production fields due to the government support prices for soybeans as opposed to wheat and corn. Systems implementing no-till or low tillage practices may leave plant debris with overwintering pathogens near the soil surface where they may germinate and infect plants in the next growing season.
Pathogen populations have been adapting to resistance genes used in several available commercial varieties, rendering these resistance genes ineffective. In order to properly manage diseases, these factors need to be taken into consideration and a variety of methods should be integrated into the management program.
Basic Soybean Management Strategies
- Identify soybean disease problems. Scouting fields at specific times of the year will help determine what diseases are present in your field. There is not a one-size-fits-all management program for all fields or all problems. It’s important to know your field’s crop and disease history as well as receive an accurate diagnosis for the symptoms you observe in your own field in order to properly design management strategies and ultimately reduce yield losses.
- Plant resistant varieties. After receiving an accurate diagnosis, choose soybean varieties that have resistance genes that will work against the pathogens in your field. If the right resistance genes are deployed, little to no disease will develop even when environmental conditions are favorable. This is the most effective tool in the soybean grower’s toolbox.
- There are two types of resistance, race specific resistance and partial resistance, incorporated into commercial varieties. These two types will appear different in the field, but both can be effective in limiting losses due to soybean pathogens.
- Race specific resistance. This type of resistance is typically controlled by a single dominant gene. When the soybean has this single gene, the pathogen can no longer successfully infect the plant. However, pathogens are biological organisms with the capacity to adapt to previously resistant varieties. Over time, the number of individuals in the pathogen population that recognize the gene in the soybean will begin to build. When this happens, the soybean variety is no longer resistant, but is susceptible. Once this happens, the genes will no longer be effective. This shift in population is apparent in Phytophthora sojae and soybean cyst nematode (SCN) populations. To maintain resistant variety effectiveness, it is important to rotate sources (genes) of host resistance.
- Partial resistance. This type of resistance is effective against all races of a pathogen that may exist in any given field. However, it is not complete resistance. Some disease does develop, but it is limited when compared to a susceptible variety. This type of resistance is controlled by several genes and is subsequently more difficult to incorporate into commercial varieties. Both Phytophthora and Sclerotinia can be managed with this type of resistance. Partial resistance is also called field resistance or tolerance.
- There are two types of resistance, race specific resistance and partial resistance, incorporated into commercial varieties. These two types will appear different in the field, but both can be effective in limiting losses due to soybean pathogens.
- Rotate crops. A good crop rotation program is essential for long-term productivity of fields. Wheat and corn in a crop rotation scheme are essential in reducing soybean pathogen populations. Without soybeans, soybean pathogens begin to decline. For example, soybean cyst nematode populations are reduced by half in the years that corn or wheat is grown.
- Consider tillage. Tillage is not essential every year, but should be considered as a management tool when one of the following situations applies: high incidence of disease, especially residue-borne pathogens; wet years after disease losses due to Phytophthora; or when crop debris is very thick on soil surface, causing soil to retain moisture.
- Tile fields. Phytophthora root and stem rot, brown stem rot, and sudden death syndrome are all diseases that require excess moisture in order to infect soybean plants. Tiling fields will reduce the amount of time fields stay wet, thereby limiting the time these fungi are active.
- Use seed treatments. Ohio weather is quite variable from location to location and year to year. Seed treatments can protect seed and young seedlings from numerous pathogens that infect soybeans at early growth stages.
- Maintain fertility. There are many pathogens that attack weakened plants as well as those that are favored by over-applications of fertilizers. A balanced soil fertility program enables soybeans to limit disease development caused by some pathogens. Thus, lush growth due to excessive nutrients promotes an environment favorable for fungal pathogens by maintaining humidity and leaf wetness.
- Follow good agronomic practices. Purchasing high-quality conditioned seed, planting seed at the proper planting density and depth, and having good soil tilth will help maintain crops through many seasonal stresses.