Anthracnose is a common disease problem on raspberries. The fungus that causes the disease survives the winter in infected canes, so a primary means of control is sanitation.
Thus, properly pruning raspberries in the spring is important to maximize fruit production, but it also plays an important role in disease control.
Anthracnose, caused by the fungus Elsinoe veneta, is easy to recognize. In early stages, the disease symptoms appear as small purple spots on canes. As the spots enlarge, they become oval shaped, with sunken tan centers and dark purplish margins. Numerous spots can weaken and may even kill the canes. The fungus may also infect leaves, petioles, fruit buds, and fruit. Fruits of infected plants may be small, dry, and seedy.
Since the raspberry anthracnose fungus needs water to infect the plant, cultural control practices that promote rapid drying of plant tissue are helpful. Keep the widths of rows between 1 and 2 feet, and select a site with good air movement. Avoid overhead irrigation; water the soil instead.
Prune out diseased canes of summer-bearing raspberries in the fall or early spring. Anthracnose is not usually a problem on fall-bearing raspberries because the entire cane is removed at ground level in the fall or spring.
Fungicides can be used to control anthracnose of raspberry. Liquid lime sulfur is commonly used and is applied at the end of the dormant period. Timing of the application is important. Apply the product when the plants are breaking dormancy; after the leaf buds open, but before the leaves are fully expanded. Using liquid lime sulfur later in the season can cause damage to the leaves.
If anthracnose lesions are observed on leaves or canes during the growing season, additional fungicide sprays with labeled products can be used, typically at 14-day intervals. If the growing season is dry, fewer sprays will be necessary because the fungus requires leaf wetness for infection.
This article originally appeared in the March 21, 2003 issue, p. 25.