Phytophthora root and stem rot (PRSR) is a common disease of soybeans that can ultimately cause death of soybeans at any stage of development. The disease can cause stand losses and severe yield reductions in susceptible soybean varieties.
PRSR is caused by the water mold Phytophthora sojae, a soilborne organism that survives in soil via specialized survival spores called oospores. Oospores are produced in infected soybean plants, and can survive for many years in the soil after soybean residues decompose. Oospores germinate when soil moisture is high, producing reproductive structures called sporangia. These structures can germinate directly and infect soybean roots, or can produce numerous microscopic spores called zoospores that are released when soils are saturated or flooded. Zoospores are attracted to, and can swim towards, germinating seeds or roots. P. sojae tends to be most active when temperatures are between 58°F and 77°F, as opposed to Pythium species (the causes of Pythium root rot) which tend to be active over a wider temperature range (50°F to 95°F).
Watch for symptoms of PRSR in fields or areas of fields with poor drainage. Once plants emerge, PRSR can lead to yellowing, wilting, and death (called post-emergence damping-off). Infected seedlings can be easily pulled from the ground because of damage to developing roots. Symptoms of PRSR in older plants (particularly those infected before flowering) include root decay, browning and water-soaking of stems extending 6 to 12 inches above the soil line, yellowing of leaves, wilting, and eventual death, with leaves on dead plants remaining attached.
Once soybean plants become infected by P. sojae, there is no cure. Therefore, management of PRSR relies on preventing infections from occurring. To avoid problems with PRSR, use PRSR-resistant varieties as a primary means of disease management. In addition to using resistant varieties, consider using seed treatments containing metalaxyl or mefenoxam. These active ingredients have been shown to be effective in providing early protection of soybean seeds and seedlings against P. sojae. Also improve soil drainage to promote drier soils that are less favorable for P. sojae growth and reproduction.
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