Phytophthora root rot of soy was first identified in Indiana in 1948 and has spread throughout soy-growing regions of the US and Canada. The disease is a widespread problem in Iowa, occurring in most areas where soy are grown.
The fungus Phytophthora sojae survives from season to season as a resting structure called an oospore or as threadlike structures called mycelia. The fungus survives in the soil or on plant debris associated with the previous crop. The oospores can survive in soil almost indefinitely.
Phytophthora root rot is prevalent in heavy clay soils or in soils with poor drainage during wet weather. The disease can affect soy from the seedling stage to near maturity. Stand reduction occurs when the disease infects the plants at the seedling stage and causes seed rot and damping off. Infection of older plants causes interveinal yellowing of leaves and eventual wilting and death. The leaves turn gray or brown and remain attached to the plant. Infected roots become discolored, rotten, and eventually die.
Resistance is the most effective tool for disease control. P sojae has different races that can be controlled by soybean with specific resistance genes. Over 70 races of Phytophthora have been identified in Ohio soils. Studies have shown that races of P. sojae vary geographically. It is very important to select cultivars with resistance genes that are effective for the races present in your fields.
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Article made possible through the contribution of Iowa State University Extension.