How to Manage Pests
UC Pest Management Guidelines
Anthracnose can affect leaves and stems, but crown rot is the most important phase of the disease. The most obvious symptom is the bluish-black, V-shaped rot in the crown. Dead stems associated with such crowns are sometimes bleached white. Because stems die suddenly, the dead leaves do not drop from the stem.
Anthracnose also causes small, irregularly shaped blackened areas on stems that become large, oval, or diamond-shaped straw-colored lesions with black borders. Black fruiting bodies (acervuli), which under a hand lens look like small dots, develop in the lesion. As lesions enlarge, they may coalesce, girdle, and kill affected stems. In summer and fall, dead shoots (white in color) are scattered throughout the field.
Anthracnose is a common problem in older alfalfa stands. The fungus persists in alfalfa debris and crowns. The disease reaches maximum severity during late summer and early fall coincident with warm and humid weather. During the growing season, spores on stem lesions are a source of inoculum. Splashing rain and irrigation water disperse spores onto growing stems and petioles. Spores may also be spread with seed contaminated during the threshing process.
Control of anthracnose involves use of resistant cultivars or cultural practices. Start looking for signs of anthracnose in early summer. If an infestation is found, harvest alfalfa before losses become too severe.
Rotation with crops other than clover and alfalfa for at least two years will eliminate sources of inoculum in the field. For more information, see CROP ROTATION.
UC IPM Pest Management Guidelines: Alfalfa