Rust Diseases of Brambles
Orange rust is a fungal disease that occurs only on brambles, particularly blackberries, dewberries, and black raspberries. This disease is not known to affect red or purple raspberries. This is a systemic disease. Once the plant is infected, the entire plant is infected for life.
The diagnostic symptoms of orange rust occur early in the spring when the new shoots begin their growth. The new leaves are stunted, deformed, and pale green or yellowish. Waxy blisters cover the undersides of the leaves. These blisters later become bright orange and powdery, the characteristic that gives the disease its name "orange rust." Canes produced on the diseased plants may appear healthy. However, these infected canes are usually spineless and do not produce blossoms. The diagnostic orange pustules will be produced on the leaves of these canes the following spring. Infected plants generally take on a bushy appearance since many short, upright shoots arise from one bud.
Orange rust is caused by two fungi: Arthuriomyces peckianus and Gymnoconia nitens. The disease occurs only on black raspberries, blackberries, dewberries, and possibly purple raspberries. The two fungi that cause the disease are very similar. The disease is not known to affect red raspberries. The fungus is systemic and overwinters in diseased roots and canes. Orange rust generally is favored by cool wet conditions. When the orange spore pustules mature and break open in June or July, the spores are spread to other plants by the wind. The fungus enters the plant through the leaves and grows internally through the canes, crowns, and roots. Newly infected plants seldom show symptoms until the following spring.
Many initial problems in the bramble planting can be prevented by starting with certified, disease-free nursery stock. Inspect all plants in the spring for symptoms of infection. As soon as symptoms of orange rust are detected, remove the entire plant. Remove and destroy all wild blackberries and raspberries in the area that might serve as a source of disease. Any practice that speeds the drying of foliage, such as keeping plantings weeded and rows narrowed back, will assist in control since spores need a relatively long period of leaf wetness in order to be able to germinate and penetrate the leaves in the spring. Avoid tipping canes in the fall because transporting inoculum on hands is easy during this operation. No chemical control is known for this disease. Some blackberries, specifically Ebony King, Eldorado, and Raven, are reported to exhibit resistance. If fungicides are used, they should be applied from the time orange pustules are first seen until the leaves on which they were produced die and dry up, and then again during late summer or fall when temperatures cool. Refer to Table 7.5 for pesticide recommendations.