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.

Symptoms

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. 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 as many short upright shoots arise from one bud.

Disease Cycle

Orange rust is caused by the fungus Arthuriomyces peckianus, which overwinters in the diseased roots and canes. Orange rust is generally favored by low temperatures and high humidity. 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.

Orange rust is a systemic disease. Once the plant is infected, the entire plant is infected for life

Disease Management

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 may serve as a source of disease. No chemical control is known for this disease. Some blackberries, specifically Eldorado, Raven, and Ebony King, are reported to exhibit resistance.