Phytophthora root and stem rot, caused by Phytophthora sojae, thrives in cool, wet conditions, as they allow spores to colonize young soybean roots. Though it’s primarily associated with cold springs, Phytophthora sojae is responsible for 8% to 10% of soybean crop losses nationwide — even in normal crop years.
Battling phytophthora got more complicated several years ago when it became clear that one of the industry’s top resistance genes, Rps1a, is no longer effective in many areas. Other resistance genes, Rps1c and Rps1k, may provide protection from the disease for now. A pair of newly discovered resistance genes from the soybean genome may add to the arsenal in years to come.
Meanwhile, strobilurin fungicides in seed treatments can add some suppression, and high rates of metalaxyl or mefenoxam can control the pathogen in highly susceptible and moderately susceptible varieties.