Cane blight on raspberry (Rubus)
Cane blight of raspberries is a late-season disease caused by the fungus, Leptosphaeria coniothyrium. Infected canes may blossom normally, but they usually wilt and die before they can set fruit or while they are heavy with fruit. Black raspberries are more susceptible than red or purple raspberries. Cankers on apples and roses are also caused by this fungus.
Symptoms and Diagnosis
Brown-purple areas develop around cuts or wounds in infected canes. The discoloration will slowly spread down the cane, encircling the stem. Small, smutty patches of olive-green fungal conidia (spores or conidiospores) develop on the bark. The spread of the disease through the canes blocks water movement through the plant, causing wilting and, eventually, death. Cane blight reduces yields because symptoms appear later in the growing season, when branches may be laden with fruit.
Spores of the fungus overwinter on dead infected canes. In the late spring, fungal spores are spread from plant to plant by the wind and splashing water. Infection takes place when there is sufficient moisture allowing the spores to enter the plant through wounds. Spores may continue to live on dead, infected canes for 2 or more years.
Integrated Pest Management Strategies
1. Prune and destroy all infected stalks. Blighted canes cannot be cured.
2. Do any pruning work during dry weather to allow wounds to dry out and prevent infection by the fungal spores.
3. Prevent damage and wounds to canes by controlling cane borers. Vigorous, adequately fertilized, and watered plants are more resistant to injury and less likely to attract harmful insects. If borers have infected canes, the tips will begin to wilt; prune these tips well below any insect punctures.
4. Plant resistant varieties. Red-fruited or purple varieties of raspberry are less susceptible to cane blight.
5. Use fungicides. Pesticides registered for use include copper and mancozeb.
Strategies 1, 2, and 4 are strictly organic approaches. The use of organic fertilizer would be a viable organic approach to Strategy 3. Of the fungicides listed in Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper products.