DISEASES OF ALFALFA - VIOLET ROOT ROT

Violet root rot is a relatively uncommon alfalfa disease, but can occur in alfalfa stands grown on the sandy soils of central Wisconsin. The fungus, Rhizoctonia crocorum, causes this disease. Infected plants are characterized by having roots where the exterior layer very easily sloughs away. The interior of the root has a very corky or spongy appearance. Often fungal threads are present on and in the root to such an extent that the root has a purple or violet color, thus the name of the disease. Violet root rot is commonly found in sandy soils. These soils tend to have a low pH, which may influence violet root rot development. An absence of micronutrients, particularly aluminum, in low pH soils may make alfalfa plants more susceptible to the disease.

Crop rotation is the most effective control for violet root rot. Other leguminous crops such as clover and bird’s-foot trefoil can serve as alternate hosts for the pathogen and should not be used in rotation with alfalfa when violet root rot has been a problem. Crops such as corn or small grains seem to be good rotation options in such situations.