In the spring, spores are produced on infected debris and wind blown to healthy plants, where they cause new infections. Symptoms first appear on the undersides of leaves as bright orange, powdery pustules. As these pustules develop, yellow to orange colored spots become visible on the upper leaf surface. Young stems and sepals may also become infected, causing curling and distortion of plant tissue. Eventually, the fungus produces reproductive structures called uredia, which appear on the leaves and canes as reddish-orange pustules.
Rose cultivars vary widely in their susceptibility to rust and also in their reaction to infection. Some varieties may support many pustules per leaf without defoliation, while a single pustule may cause defoliation of others. Still other susceptible cultivars may wilt in mid-summer. Since moist, cool weather favors disease development, there is little spread of the disease during the summer months. In late summer and early fall, the fungus produces other structures called telia (black in color), which overwinter in the fallen leaves and infected canes.
In Minnesota, it may be possible to achieve control of rose rust with the following practices. Remove infected leaves from the plants as soon as they appear. Remove all fallen leaves and any left on the plant before winter. If pustules appear on the canes, prune all infected canes. If disease is severe, fungicide applications of triforine (Funginex), sulfur, or lime sulfur may be applied. These chemicals provide good control of black spot and powdery mildew as well as rust, and thus would be good choices for an all-purpose spray.
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Chad Behrendt, Crystal Floyd