Orange rust is a serious disease of brambles. Black and purple raspberries are susceptible, as are most varieties of blackberries. Orange rust does not occur on red raspberries.
The new shoots of infected plants appear weak and spindly as they emerge in the spring. Leaves are small, yellowish, and sometimes misshapen. Within 2-3 weeks the undersides of leaves show small, orange spots (pustules). Infected leaves eventually wither and fall.
Canes may appear to partially recover during the growing season, but the plant is still diseased. The orange rust fungus is systemic, meaning that it grows throughout the roots, crown, and shoots of an infected plant. Infected canes often appear bushy, bearing little or no fruit.
Infected plants cannot be cured. They must be removed (including roots) to prevent further spread of the disease. To prevent the occurrence of this disease start with disease-free, certified nursery stock. Inspect any nearby wild brambles for orange rust. Maintain good air circulation by proper pruning of canes after harvest and by controlling weeds. Fungicide sprays are not effective for control of orange rust.
This article originally appeared in the May 23, 1997 issue, p. 75.