Phytophthora root and crown rot (Phytophthora root and stem rot)
Root- and crown-infecting Phytophthora cause leaves to wilt, discolor, remain undersized, and drop prematurely. Twigs and branches die back and the entire plant, especially when young, can be killed as roots and vascular tissue die. Plants infected when they are mature grow slowly and may gradually decline.
Small brownish lesions encircling feeder roots are an early symptom of infection. As disease progresses, the small, fibrous feeder roots become scarce. Where present, the small roots are black, brittle, and dead from infection. Woody roots decaying fromPhytophthora alone are firm and brittle but eventually soften as secondary decay organisms develop.
Depending on environmental conditions and the species of pathogen and host plant, sap that is black, brown, or reddish may ooze from darkened areas of trunk bark. Cutting away bark from the basal trunk and roots often reveals a brown or reddish streak, stain, or canker in infected wood with a water-soaked margin separating it from the healthy whitish or yellowish wood.
Several items should be considered for management of Phytophtora root and stem rot. Try to create a field environment that reduces the likelihood of saturated soil by taking steps to increase drainage where possible. Treatment of seed with the highest labeled rates of fungicidal compounds such as mefenoxam (Apron XL) or metalaxyl (Apron) can be beneficial. In all areas where Phytophthora root and stem rot has been a problem, resistant cultivars should be planted. The most common resistance genes that are widely effective are Rps 1c and 1k. Rps1a can be effective, but is not as effective in many areas as the other genes. Rps 3 and other genes can also be effective. For areas where the Rps genes are not working well due to the presence of pathogen races that overcome the resistance, cultivars with high levels of field tolerance (or partial resistance) should be planted. Crop rotation and tillage may be of some benefit.