There are more than 5,000 known species of rust on plants. The majority require two unrelated host plants to complete their life cycle (heteroecious) and others need only one (autoecious). Some rust fungi can produce up to five (5) different kinds of spores to complete its life cycle.
Rust can be easily identified. Rusty-yellow to bright orange spots form on leaves. These spots are actually filled with a powdery mass of spores and can be easily removed by scraping off the surface. On twigs of cedar, you will find galls with orange-gelatinous spore horns.
In the Midwest, the more common rust diseases include cedar-apple rust, cedar-hawthorne rust, and cedar-quince rust. Rust does not directly kill a plant but may contribute to its decline. It can cause defoliation, stunting, and branch dieback.
This spring, the Plant Disease Clinic has received rust samples on rose, buckhorn, and cedar. With a prolonged wet weather, we expect to receive more plant samples with rust problems. There is no single control measure for rust. If feasible clip infected leaves, twigs, and branches. Removal of any known alternate host plants within the vicinity may not be practical, since rust spores can be blown in great distances. Use rust-resistant varieties when available. Finally, some fungicides are labeled for controlling rust on certain plant species.
This article originally appeared in the May 21, 1999 issue, p. 61.