BLACK LEG OF GERANIUM By Dr. Steve nameth Dept. of Plant Pathology The Ohio State University, Columbus Black Leg of Geranium is caused by the fungal pathogen Pythium sp. Pythium is a commonly found in greenhouses were bedding plants are grown. Pythium is usually associated with root rot of various floriculture crops including poinsettia, petunia, Impatiens and geranium. In geranium Pythium can infect the roots under conditions of saturated medium. The fungus initial infects the root tip and works it way up the root to the geranium crown. If the fungus is left unchecked and no chemical controls are put into place, the fungus will continue to advance from the crown area and up the stem. Because of the semi-woody nature of the geranium stem the entire stem can be affected by Pythium and the plant will not collapse, however, it will turn dark brown or black in color. Hence the term Black Leg. Black Leg can attack geraniums at any stage of production (Photo 1) (Photo 2) (Photo 3). Since Black Leg is an indication of an advanced case of Pythium Root Rot there are usually no healthy roots left on the affected plant. Once symptoms of Black Leg are observed it is very likely that the plant is destined to death and nothing can be done to salvage the infected host. If at the fist sign of the disease the plant is not removed from the greenhouse the fungus will continue to colonize the entire stem and may eventually move from the stem into the leaf petioles and the leaves. Under these circumstances the affected petioles will turn black, wilt and collapse to the media (Photo 4). The best way to control Black Leg is to avoid getting it in the first place. Since Black Leg is an advanced Pythium Root Rot all of the controls usually initiated to control root rot will also help to control Black Leg. These would include not allowing the growing media to become saturated with water for extended periods of time, always practicing good greenhouse sanitation, and avoiding any damage to the root or root tips. Damage to the roots helps to accelerate the Pythium infection process. If necessary, applications of chemical fungicides may be necessary. The same fungicides labeled for Pythium control on geranium are also the fungicides used for Black Leg. These would include, but are not limited to: Subdue MAXX and Subdue 2E, Banrot and Banol, and Heritage. It is best to apply these products prior to the onset of Black Leg since Black Leg is a sure sign of advanced Pythium.