Plants may be infected but not show symptoms until the plant undergoes some form of stress. Symptoms Black root rot is commonly confused with Pythium root rot. Above-ground symptoms of both diseases include yellowing, stunting and under certain conditions wilting or death of the plant. Sometimes, plants affected with black root rot may have black stem lesions at or near the soil line. A close examination of the roots will reveal different symptoms for each disease.
Pythium usually attacks roots from the ends or tips, causing a soft, brown rotting as it progresses.
Oospores of Pythium can be seen in the infected roots with a microscope. Black root rot begins by attacking the middle of the root and forms cankers. The black root cankers can be seen relatively easily by washing roots free of growing media and then viewing them carefully with a hand lens. A plant diagnostic lab can be consulted if you suspect black root rot. Be sure and tell them you think black root rot may be involved so that the lab will examine the roots through a microscope.