Issue 6, May 28, 2010
Raspberry Orange Rust
Customers growing raspberries other than red raspberry may be seeing an orange growth on the leaves now. This is a fungal disease called orange rust that infects blackberry, black raspberry and purple raspberry. Red raspberry is resistant. Raspberry orange rust should not be confused with the late leaf rust disease of red raspberries. The major symptom of raspberry orange rust is the presence of masses of yellow-orange spores covering the lower leaf surface (see image) by late May, at least in Illinois. These spores serve to spread the disease to other plants. In addition, heavily infected leaves may die, and infected shoots will be weak, spindly and have very few, if any, thorns.
This disease is systemic, meaning the orange rust fungus invades the entire plant, reducing blossoming and fruit set. Although the pathogen does not kill plants, heavily infected plants are of no value. They will not recover; and unless destroyed, infected plants will persist as a source of inoculums, spreading the disease to other plants.
Management of this disease includes planting only healthy black raspberry and blackberry stock. Eradicate infected wild blackberries and black raspberries (including roots) near your raspberry patch. Remove and destroy infected plants as soon as they appear in the spring. Promote good air circulation by thinning healthy canes and controlling weeds. There are no effective fungicides at this time.--Jim Schuster