Red and purple raspberries are more affected by spur blight than black raspberries. Blackberries appear to be immune. In extremely overgrown and weedy plantings, the disease can cause a loss in yields, especially if excessive nitrogen is applied.
Symptoms and Disease Cycle
Spur blight is caused by the fungus Didymella applanata, which overwinters in infected canes. Symptoms appear in late spring or early summer as chocolate-brown, dark-blue, or purplish spots or bands on new canes and petioles. These lesions enlarge until the cane is girdled. By late summer, canes may crack and split lengthwise, at which time the reproductive, pimple-like, black fruiting structures of the fungus can be seen. These structures overwinter and the following spring discharge spores into the air or may ooze to the surface of the stem during wet periods. Symptoms on leaves appear as chocolate-brown, angular, or wedge-shaped areas. The effects of spur blight are increased plant susceptibility to winter injury and reduced yield as a result of the withering and eventual death of infected laterals. Leaflets also wither and drop prematurely.
Suggested controls are the same as those for cane blight, botrytis blight, and anthracnose. Keeping the plant canopy open to ensure that leaves and canes dry rapidly after rain will help in controlling this disease. Thorough pruning, including the removal of pruned canes from the planting, is helpful. Lime sulfur and sanitation are useful, and keeping row width narrow to increase air movement is particularly effective. Producing only the fall crop on primocane bearers has greatly reduced the incidence of the disease. Several fungicides can be used during the season. Refer to Table 7.5 for pesticide recommendations.