The fungus Pythium, which is commonly found in the soil, can infect geranium root and stem tissue and cause plant death. Wet, poorly drained soil favors disease development.
Infected plants are usually stunted. Leaves turn a yellow or brown color, wilt, shrivel and eventually die. Lower leaves often show symptoms first. 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.
Diseased plants cannot be "cured" and need to be removed and destroyed. If the geraniums were growing in containers, discard the soil. Wash the pots and any tools that may have been contaminated. Rinse tools and pots in a 10% solution of household bleach. To prevent the disease in future plantings, plant only healthy geraniums (inspect before purchasing) in a well- drained soil. Avoid plant stress by providing adequate moisture and fertilizer (but not excess!).
This article originally appeared in the August 25, 1995 issue, p. 131.