Cane blight usually affects only canes that have been wounded in their vegetative year. It sporadically attacks canes of all Rubus species. Black raspberry is more susceptible to this disease than the other brambles.
All symptoms of cane blight occur in close association with wounds. Infection occurs in late spring or early summer through pruning and insect wounds. In the spring, buds fail to break dormancy, lateral shoots wilt, or fruiting canes die when the fruit begins to ripen. Canes are usually brittle at the point of infection, and may break if bent. Symptoms appear late in the season on new shoots where plants have been pruned. Infected areas are brownish purple and develop from the cut ends. Branches originating in the infected areas wilt and die. Fruiting canes show a sudden wilting of branches when the fruit begins to ripen. Weakened canes are more susceptible to winter injury.
Cane blight is caused by the fungus Leptosphaeria coniothyruim, which also causes a canker on roses and a fruit rot of apple and pear. The fungus requires a wound or damaged tissue to infect a plant. The fungus overwinters on dead canes, which is where spores form for spring infection. These spores are spread by splashing rain, wind, and insects from early spring to late fall. Old stubs can continue to produce inoculum for several years. Black raspberries are more susceptible to cane blight than other brambles.
Any practice that improves drying of foliage, such as keeping fruiting rows narrow and weeded, will help in the control of cane blight. A major consideration in the control of this disease is the prevention of damage to or wounding of the canes. Eliminate weeds and thin out weak canes to speed up the drying of plants. Prune out and dispose of old, diseased canes promptly after harvest. Choose a planting site with good air movement, and time pruning so that cuts have 3 days to dry before a rain. Fertilize to promote plant vigor, remove old canes after harvest, and control insect pests to reduce plant injuries. If fungicides are necessary, they should be applied during bloom with additional applications made during harvest if needed. Refer to Table 7.5 for pesticide recommendations.