Anthracnose

Symptoms

Light to dark brown diamond-shaped lesions with dark borders form on lower parts of stem. Small, black bodies of the fungus may sometimes be observed in the center of lesions.

A "shepherd's crook" may develop on young dead shoots. As disease development progresses, the fungus may girdle and kill stems, crown buds and the crown. Dead straw-colored stems become scattered throughout the field and a thinning of the stand becomes evident. Look for a single stem killed as well as entire crown.

Crown infections are characterized by a dry rot with blue-black coloration of the internal tissues and lower stems.

Cause

Anthracnose is caused by Colletotrichum trifolii. Hot, moist weather conditions in summer and early fall are favorable for anthracnose development. Dense, rank stands reduce air circulation and provide conditions favorable for infection. The crown rot phase of this disease is often serious in hot, dry weather.

IPM Techniques

• Examine fields every four weeks in mid-summer and fall (July 15 to October 1). Examine plants within a 20 foot radius. Report severity of the disease according to the following rating scale:

0 = no symptoms on stems or crowns of any plants

1 = a few lesions found on some stems of a few plants, but no crown infections, little or no death of plants

2 = lesions found on a number of plants, some dead, straw-colored stems scattered throughout the area

3 = lesions observed on most stems of most plants, numerous dead stems found, thinning of stand apparent with anthracnose symptoms associated with crowns of dead of dying plants. (If entire crowns are dead, indicate as such.)

• Plant resistant cultivars with an R (resistant) or HR (highly resistant) rating to anthracnose.

References and Additional Information

• IPM-1 Kentucky Alfalfa IPM Manual.
• PPA-10d Kentucky Plant Disease Management Guide for Forage Legumes, P. Vincelli, Extension Plant Pathology, University of Kentucky
• ID-104 Managing Diseases of Alfalfa, P. Vincelli, Extension Plant Pathology, and G.D. Lacefield and J.C. Henning, Agronomy, University of Kentucky
• Compendium of Alfalfa Diseases. 2nd ed. Stuteville, D.L. & D.C. Erwin, (eds.). 1990.

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