California alfalfa growers will want to watch for “flags” of wilted or dead stems in fields this time of year. If they’re there, that’s likely a sign that the crop is suffering from the fungal disease anthracnose, says Carol Frate, University of California (UC) Extension farm advisor.

Lower portions of infected plant stems often have oval- or diamond-shaped lesions or spots, Frate notes in a recent post on UC’s Alfalfa & Forage News blog. The spots will have a straw-colored center with dark brown or black borders. Within the lesions, the fungus produces dark fruiting bodies that look like small black dots when seen with a hand lens.

The disease can also attack alfalfa crowns, eventually killing plants. Infected crown portions are usually v-shaped, dry and bluish to black in color. As the infection moves deeper into the root, the discoloration turns to tan, then to brick red.

Anthracnose thrives under warm, humid conditions typical of the canopy of irrigated alfalfa during summer months. The fungus survives winter in infected debris and the crowns of infected plants.

As conditions turn warm and humid in summer, spores are produced that can be moved with dew or irrigation water. The disease usually becomes more widespread as summer continues. Infected plant parts carried on harvesting equipment can spread the disease to other fields.



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“Ideally, younger alfalfa fields should be cut before older ones to reduce the spread of anthracnose and other diseases that tend to be more widespread in older fields,” Frate says.

Rotating to other crops for two years or more should greatly reduce the disease pressure when alfalfa is planted again, she says. Using varieties with resistance to anthracnose should, too. (See the National Alfalfa Alliance publication, “Winter Survival, Fall Dormancy and Pest Resistance Ratings for Alfalfa Varieties.”)

“If this disease has been a problem in previous years or is noticeable in current stands, select highly resistant varieties for future plantings.”

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