Anthracnose of caneberries—Elsinoe veneta =Sphaceloma necator
Anthracnose causes discolored spots on caneberry shoots and sometimes leaves. Infections reduce fruit quality and can cause the dieback of infected canes and premature drop of leaves.
Identification
In spring, young canes that become infected develop small purplish spots that are slightly raised or sunken. Spots later enlarge and their centers become gray with slightly raised purple margins. As the center of spots changes from purple to gray, spots can turn intermediate colors such as pinkish. Late in the growing season the spots are entirely gray giving canes an unhealthy appearance sometimes called gray bark. Dark specks (fungal fruiting bodies) may develop on the gray bark.
Anthracnose spots on stems can merge into cankers. Bark that is heavily cankered may split. If cankers entirely circle stems, canes become girdled and die outward and upward from the cankers.
Infected leaves may develop spots about 1/16 inch in diameter. Spots are purple or gray with purple margins. The center of leaf spots may drop out leaving small holes in leaves.
Life cycle
The fungus overwinters in lesions on canes. Spores are produced in the lesions when conditions are foggy or rainy or when canes are wetted by sprinklers during late winter or spring when new shots are formed. The spores spread in splashing irrigation water and rain and with the wind. The spores cause infection of young shoots and sometimes leaves. The fungus can develop and cause new infections throughout the growing season if young plant tissue and wet conditions occur together.
Damage
Anthracnose can canker and girdle stems, causing cane dieback and the loss of fruit production. Fruit on infected canes may fail to develop to its mature size and ripening can be delayed, reducing fruit quality. If leaves are heavily infected, they may drop prematurely, which weakens plants and can reduce fruit production.
Solutions
Purchase caneberries from a reputable suppler that provides disease-free plants. If anthracnose is a problem in that growing area seek anthracnose-resistant cultivars. Resistant red raspberries include Autumn Bliss, Chilcotin, Heritage, Meeker, Nootka, and Willamette.
Provide plants with a good growing environment and appropriate cultural care so they grow well and are more tolerant of disease damage. Avoid the application of excess fertilizer because this stimulates excessive growth of young plant tissue that is susceptible to anthracnose infection. Growing caneberries inside macrotunnels (hoop houses covered with clear plastic) can greatly reduce the abundance of anthracnose and certain other diseases because the surface of plants is kept relatively dry.
Switch from sprinkler irrigation to drip or flood to keep foliage drier and less susceptible to anthracnose. If watering by hand, direct the spray to the base of older canes and away from leaves and young shoots. Irrigate early in the day so surfaces dry more quickly.
After harvest and during dry weather, prune off infected canes. Dispose of cuttings away from valued caneberries. Consider removing any wild caneberries growing nearby. Unmanaged caneberries can be a source of anthracnose spores that spread to caneberries grown for their fruit.
Do not crowd caneberry plants. Prune to thin canes and improve air circulation. Increased air circulation hastens the drying of wet plants and can reduce anthracnose infections and disease development. Control weeds near valued caneberries because weeds also impede air flow and can slow the drying of nearby caneberries.
If the above cultural practices have been followed and anthracnose continues to be a problem, the caneberries can be sprayed with a fungicide. Make the disease-prevention application during the late dormant or delayed dormant season, when buds swell but before they open. Thoroughly spray the entire plant with Bordeaux mixture, another copper fungicide, or sulfur. If fog or rain are frequent after the application, make a second spray when new canes are 6 to 12 inches long.
Adapted from Anthracnose of Raspberry and Blackberry, Ohio State University and Raspberry (Rubus spp.)-Anthracnose, Pacific Northwest Plant Disease Management Handbook. For more information see Pest Notes: Anthracnose.
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