Phytophthora, a genus of plant-damaging oomycetes, is a soilborne water mold that can spread from an infested field to a new field through the movement of water in the soil or on the surface. Phytophthora is relatively difficult to identify, due to most indications being underground. While leaves turning reddish-purple in the autumn are a symptom, this is gradual and may lead to unsalvageable crops. Several inches of soil must be removed around the base in order to analyze the discolored inner bark layer and crown.

Several Phytophthora species cause root and crown rotting. While not all soilborne, all are fungi that are commonly found in orchard soils. If not a common inhabitant, the fungi is usually present due to the introduction of contaminated planting stock or contaminated soil. The species vary in how destructive they may be depending on what is infected, however all species are common in needing extremely wet soils to properly infect. Rootstocks are at peak susceptibility during spring and autumn when soil temperatures are more prone to fungus growth and zoospore production.

Source: Ohio State University Extension

Top photo by Joseph O’Brien, USDA Forest Service, Bugwood.org; Bottom photo by William M. Brown Jr., Bugwood.org