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Orange rust

Orange rust is a serious disease of black raspberries and blackberries that is caused by two closely related fungi. Arthuriomyces peckianus is the rust fungus that infects black raspberries while Gymnoconia nitens infects blackberries and dewberries. Orange rust does not infect red raspberries. Although uncommon in Minnesota, orange rust can be devastating when it does occur. Black raspberry plants with orange rust are unproductive. The plants survive, and produce new canes each year, but they no longer produce flowers or fruit.

Identification

orange-rust

Orange Rust on a young black raspberry cane.

Michelle Grabowski, UMN Extension

Orange rust is unmistakable in the spring on young black raspberry shoots. When black raspberry canes sprout in the spring, infected shoots are weak and spindly, with small pale green or yellowish leaves. After 2 or 3 weeks, the fungus makes distinct cushion-like spore producing structures on the lower surface of the leaf. Initially, the spore producing structures are pale orange and waxy, but they rupture to produce visible reddish orange spores. After the fungal spores are released, the infected leaves fall off, and the primocane will often grow normally.

Important biology

Orange rust is one of the few fungal diseases that spreads systemically through the plant. The fungus grows throughout the plant, from the roots to the canes. Once a plant is infected, all the canes produced by that plant will have the disease for the life of the planting. The fungus survives the winter within the infected crown. The disease spreads by spores produced from weak primocanes in early spring or by spores produced in late summer. The spores infect new leaves, shoots or buds, and the disease spreads towards the roots.

Management

The best time to scout for orange rust is in early spring, when the new primocanes are less than a foot high. Also scout nearby wild black raspberries for the distinctive orange young canes each spring, and kill any wild plants with the disease. Always remove or kill infected plants in early spring before the new canes release powdery orange spores which will spread the disease.


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