Plant Pathology
Factsheet

Warm, humid weather can favor development of foliar diseases of alfalfa during summer.

Leaf spotting diseases are common in Kentucky alfalfa fields, although they are often present at minimal levels. However, during extended periods of humid weather, leaf spot diseases can cause yield loss from leaf drop or crop stunting. They can also cause losses in hay quality from leaf drop. Spots range in color from white to tan to brown; they sometimes have a dark brown border and may also have a yellow halo. Stemphylium leaf spot, Summer Leaf Spot and Black Stem, and occasionally Common Leaf Spot can be observed. Agents can find descriptions of these diseases in their copies of the Compendium of Alfalfa Diseases, and the Diagnostic labs can positively identify problem cases. However, the control recommendation is the same for all leaf spot diseases: cut early to minimize leaf drop and allow a new cutting to begin growth. Varieties may differ in their susceptibility to leaf spot diseases, but very little public information is available on varietal reactions.

Web Blight is another common foliar disease of alfalfa. Affected leaves and shoots turn grayish green and collapse, becoming tan as they dry. Characteristically, dead leaves stick to neighboring leaves and stems because hyphal strands hold them in place. These fungal strands are visible to the naked eye if you look carefully before the dew has fully dried. My research indicates that this strain of Rhizoctonia only infects leaves and does not infect crowns, so stand loss is not expected from this disease. Again, the best recommendation is to cut early and allow a new cutting to begin growth. Varietal resistance is not available.