Anthracnose

Anthracnose, commonly called "cane spot" or "gray bark," occurs in several species of Rubus. It is considered an extremely serious disease of black, purple, and susceptible varieties of red raspberry. Severe yield loss can result due to defoliation, wilting of lateral shoots, death of fruiting canes, and reduction in fruit size and quality.

Symptoms
Anthracnose symptoms are most conspicuous on canes but can also occur on leaves, petioles, flower buds, and fruit. In the spring, reddish-purple spots appear on young canes. As the disease progresses, the spots enlarge and the centers become sunken. These early lesions on the cane are called pit lesions. By late summer or early fall, the typical "gray bark" symptom can be observed, especially on the red raspberry. Within these lesions, spores are produced and then are spread by running water, splashing rain, and wind. Canes weakened by anthracnose are more susceptible to winter injury and eventually may die. Cankered canes also might produce abnormal fruiting branches with malformed fruit, especially in seasons of drought. Fruit infections are not common unless there is a high level of anthracnose in the plantings. Infected fruit is typically dry and seedy. Most economic loss results from defoliation, reduction in fruit size and quality, and death of canes, either directly from the disease or from winter injury.

Disease Cycle
Anthracnose is caused by the fungus Elsinoe veneta, which overwinters on canes infected the previous season. In the spring, fungal spores are produced on these diseased canes. These spores are spread to very young green tissue, and infection takes place. The primary damage to plants is caused by these early infections. Black and purple raspberries are more susceptible than red raspberries.

Disease Management
Infections that take place early in the growing season cause the most damage, so controls should be instituted early in the season. Anthracnose can be managed by sanitation and spraying. Although sanitation is labor intensive, it is an effective management practice for the control of anthracnose. Planting clean, disease-free nursery stock is important. Cut out all diseased canes, cane "handles," and any infections observed on new plants. Good air movement through the planting should be provided by the removal of weeds and spindly canes. If possible, all noncultivated brambles within the vicinity should be rouged because these wild plants will also harbor the pathogen. If fungicides are necessary, a dormant to delayed-dormant application of lime sulfur is the most effective method of reducing the incidence of this disease. Refer to Table 7.5 for pesticide recommendations.