Symptoms: There are more than 5,000 known species of rust on plants. Common rust (Phragmidium spp.) is a fungal disease that affects roses, snapdragons, bluegrass and woody perennials. It is most often found on mature plants where symptoms appear primarily on the surfaces of lower leaves.
Early on, look for white, slightly raised spots on the undersides of leaves and on the stems. After a short period of time, these spots become covered with rust-colored spore masses. Later, leaf postules may turn yellow-green and eventually black. Severe infestations will deform and yellow leaves and cause leaf drop.
Rust diseases are favored by 4 to 8 hours of low light intensity, warm temperatures and moisture — high humidity, heavy dew or light rain — followed by 8 to 16 hours of high light intensity, high temperatures and slow drying of leaf surfaces.
Black spores are visible on leaves and stems in fall and overwinter inside infected stems, which are distinguished by dark, corky blotches at points of infection. The spores are spread by wind or by water splashing back up onto the foliage.
Control: Choose resistant varieties when possible. Pick off and destroy the infected leaves, and rake under the plants frequently to remove all fallen debris. Water in the early morning hours (avoiding overhead watering if possible) to give plants time to dry out during the day. Drip irrigation and soaker hoses can be used to help keep the foliage dry. Do not compost infected leaves or stems and thoroughly clean up garden areas in the fall to reduce over wintering sites for the fungal spores. Prune plants and remove weeds to improve air circulation.
Sulfur or copper powders/ sprays should be applied weekly, starting when foliage begins to develop in the early spring and continuing throughout the growing season. Spray early in the day, and avoid applications during hot weather. These organic fungicides will not kill the rust, but prevent the spores from germinating.
Photo Credit: University of Illinois, College of Agriculture