M. Grabowski, UMN Extension Educator
Black leg is a stem infection of Geraniums (Pelargonium spp.) that results in a distinctive black discoloration of stems. As infected stems rot, they become soft and often bend over. This disease is caused by several species of the water mold Pythium. Pythium spp. are soil dwelling organisms that thrive in wet conditions and can survive in infected plant debris and soil. Black leg symptoms often start at the soil line and move up the plant.
To avoid black leg, inspect all geraniums prior to purchase and select only healthy symptom free plants. If repotting geraniums, use new clean potting mix and new pots or pots that have been cleaned with a solution of 10% household bleach. Take care to keep tools, watering cans, and hose heads off the floor and away from dirt and plant debris. Pythium spp. can be introduced into clean potted plants by tools that have contacted contaminated dirt or plant debris. Do not over water plants.
If plants do become infected, they will not recover. Discard the plant and the potting mix. Clean the pot with a solution of 10% household bleach before reusing it.