Underside of a leaf severely infected with geranium rust. The pustules are rust colored and surrounded by rings of smaller pustules. Leaf tissue in and around pustules turns yellow.
Photo: Chad Behrendt
Symptoms are visible as small yellow leaf spots, within one week after the initial infection. Within two weeks, a rust-colored pustule containing spores develops in the center of each spot on the lower leaf surface. A short time later, rust-colored concentric rings may begin forming around the initial pustule (illustration). These rings are made up of pinhead sized pustules. Infected leaves eventually yellow and drop.
Prevention and cultural practices play a large part in the control of geranium rust. Before purchasing geraniums, inspect the leaves carefully for rust pustules and yellow leaf spots to avoid purchasing infected plants. Continue to inspect plants frequently after purchase. Rust spores can blow through window screens, so indoor geraniums may become infected. Destroy severely infected plants so they cannot produce spores, which infect healthy plants. Lightly infected plants may be saved by diligently removing the infected leaves, as soon as symptoms are observed. Water plants carefully to avoid wetting leaves. Space outdoor plants to ensure the leaves dry quickly. Always wash hands and tools after touching infected plants. Be careful when taking cuttings from plants grown outdoors, as they may be infected but not yet showing symptoms. It is unlikely for geranium rust to survive winter outside in Minnesota, but removing infected debris may help prevent infections the following spring.
If rust has been a problem in previous years, use resistant cultivars when planting. If a geranium planting is of particularly high value and rust has been a problem in previous years, fungicides may be applied to prevent infection. Read and follow all label instructions whenever a pesticide is used.
Rytter, JL. 1993. Rust, in Geraniums IV, JW White, ed. Ball publishing, Geneva, IL, pp. 230-233.
Harwood, CA and RD Raabe. 1979. The disease cycle and control of geranium rust,Phytopathology 69:923-927.
Chad Behrendt, Crystal Floyd