In Ohio, diseases of soybeans have increased in incidence and severity over the past few years as a result of intensified soybean production. Soybeans are now continuously cropped in many areas mainly due to economics of wheat and corn production and government support prices for soybeans. In addition, the incidence and severity of soybean diseases have also been affected by the increased use of reduced tillage and a greater number of fields that are infested with pathogens. Adaptations of some soybean pathogens to current soybean varieties have also added to the increased losses in yield from diseases.
No-till and reduced tillage crop production systems reduce the amount of soil loss from erosion. However, these production practices also allow for the buildup of pathogens that survive on crop residues. Many soybean, corn, and wheat pathogens depend on residue for survival from one crop to the next. Production systems that leave residue on the soil surface favor pathogen survival, resulting in increased potential for diseases.
Pathogen populations change and adapt to resistance genes that have been deployed in commercial varieties. This is especially true for Phytophthora sojae, which causes seed, seedling, and stem rot diseases of soybean. In fields where the same specific resistance gene has been used for six to eight cropping seasons, some resistance genes have become ineffective. Soybean producers need to learn to manage resistance genes so that the available resistance genes may be useful over a longer period of time.
Two soybean pathogens, Sclerotinia sclerotiorum (white mold) and soybean cyst nematode, have been introduced into an increasing number of acres and fields through a variety of ways — on equipment, by flooding, and on seed. Although these two soybean pathogens are difficult to control, an integrated disease management program can limit losses.
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| Figure 1. Soybean field in 2000, with large areas wiped out by Phytophthora sojae and Pythium spp. pre-emergence damping-off. |
Environmental conditions play an important role in the development of soybean diseases. In addition to susceptible varieties and the presence of high populations of soybean pathogens, the appropriate temperature and moisture conditions must be present in order for diseases to develop. In 2000, heavy rains in much of the state followed planting, which resulted in very high losses to Phytophthora damping-off. As much as 20% of the soybean acreage had to be replanted that year. This was the result of a very favorable environment for Phytophthora in addition to the widespread planting of susceptible soybean varieties. In 1997, cool, wet conditions during flowering provided the perfect conditions for infection and subsequent development of Sclerotinia stem rot (white mold).
As Ohio soybean producers intensify soybean production, it is imperative that good disease management programs are in place to limit losses when favorable environmental conditions occur. This bulletin addresses the essential components in assembling an effective disease management program.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-1868