Damon Smith, Oklahoma State University
Anthracnose is most common on young shoots and fruit but can be found on any succulent plant part. Lesions on shoots and leaves are often sunken and can take on a reddish appearance, especially near the margins. On leaves, the centers of the lesions can fall out, producing a "shot-hole" appearance. Leaves also may curl and distort. On fruit, lesions may be sunken and appear more reddish-black in color. As the lesions enlarge (up to 1/4 inch), the center will become increasingly sunken and turn gray. Fruit also may crack as the lesions expand.
Sanitation is extremely important. Remove old, infected plant material, which is the primary source of spores for new infections. Proper dormant pruning and destruction of old canes, clusters, and other plant parts can significantly reduce the number of spores. Also, canopy management during the season can help to increase airflow, which reduces leaf wetness duration (primary component for infection). Practices such as shoot positioning and strategic leaf pruning can reduce drying time.
In areas with a history of the disease, apply lime sulfur sprays during the dormant season. This fungicide helps to reduce the amount of primary inoculum. Subsequent fungicide sprays every 10 to 14 days from bud break until veraison may be necessary where anthracnose is severe. Check with your county Extension office for a list of fungicides effective for controlling anthracnose.
Anthracnose is caused by the fungus Elsinoe ampelina. The disease is of European origin, therefore anthracnose is generally worse on American grapes. Anthracnose can be severe in years with heavy rainfall, when conditions are warm. The disease is sporadic in its occurrence, but once established in a vineyard it can be a persistent problem in subsequent years.
The fungus overwinters as survival structures (sclerotia) found on old, infected plant material. In the spring when conditions are predominately wet (24 hours or more of wetness), sclerotia will germinate to form mycelium that produces spores (conidia). A fruiting structure (ascocarp) can also be produced from the sclerotia, which also will produce another type of spore (ascospore). Regardless of the type of spore, once transported to susceptible tissue (via wind), temperatures between 35ºF and 90ºF are suitable for infection. However, optimal conditions for disease development are 75ºF to 79ºF. Once the fungus has parasitized the host, it also can produce fruiting bodies (acervuli) that produce pinkish, slimy masses of spores when conditions are wet. The spores can be splashed to adjacent plant tissue and cause new infections.
Anthracnose of Grape, Ohio State University
Anthracnose, University of Minnesota
Anthracnose, Michigan State University
Field Guide for Integrated Pest Management in Pacific Northwest Vineyards, Washington State University
Reviewed by Bruce Bordelon, Purdue University and Stephen Jordan, University of Wisconsin-Madison