In early stages of the disease, plants may show recovery at night, but wilt again the following day. To diagnose Pythium root rot, dig an unhealthy plant and wash the soil from the roots. Diseased plants will show rotted, brown roots. Often the outer layer of root tissue is missing and few roots are present. In addition, the fungus can progress up the stem and cause rot above the soil level.
Infected stems turn black starting at the soil line.
This black appearance spreads up the stem. Because of this dark discoloration, the disease is referred to as blackleg or Pythium blackleg of geranium. If the geraniums were growing in containers, discard the soil. Wash the pots and any tools that may have been contaminated. Prepared by Paula Flynn , Department of Plant Pathology Iowa State University , Ames, Iowa.