Old plants are not easily killed. Infected plants are destroyed after flowering. No chemical control is listed. Black rot attacks the plant crown within a few inches of the ground. The roots are destroyed and the ends are stubby and black.
Infected plants are dwarfed, pale yellow and sickly.
The stems are easily removed from crowns. The disease is worse in heavy cold soils. Avoid replanting where the disease is a problem. No chemical control is listed. Downy mildew may cover the leaves with a gray mold during moist weather.