Geranium Rust
by Dr. Steve NamethDepartment of Plant Pathology
The Ohio State University
Geranium rust is an economically important disease associated with florist's geranium production all over the world. The disease is caused by the fungus Puccinia pelargonii-zonalis. The fungus is spread from plant to plant via spores in the air or in splashing water. The spores land on the leaf surface, and in the presence of free water they germinate and enter the geranium through the stomate. About seven days after the fungus enters the host the first visible symptoms of the disease can be seen. Small yellow spots appear on the lower and upper surface of the leaf (Figure 1). About one week later, brownish spore masses (pustules) appear in the center of the yellow spots, mostly on the lower leaf surface (Figure 2). The "rust-like" spot continues to expand in a concentric ring type growth and the leaf spot takes on a bulls eye configuration (Figure 3 and Figure 4). If left unchecked the leaf spots will continue to expand until the spots coalesce. Severely infected leaves will turn yellow and drop. Spore pustules may appear on the upper surface of the leaf, however, they tend not to form the concentric type of growth. Eventually all of the leaves of the infected plant become cover with rust and the overall appearance of the plant declines. At this point the plants are not salable and only serve as a source of infection for other geraniums.
There are a variety of cultural and chemical ways of controlling geranium rust. First off make sure that the geraniums that you bring into your greenhouse are free of the disease. Most of the big four geranium producers are well aware of this disease and they go to great measures to produce stock and cuttings that are disease - free. Once you have secured disease-free material you want to do your best to keep the conditions in the production facility unfavorable for disease development. The optimum temperature for disease is 60 to 70 degrees F and remember that the fungal spores must have free moisture on the leaf surface in order for the spores to germinate. Keep overhead watering to a minimum and if you must overhead water, try to water in the morning as to allow the leaves more time to dry. If you find geranium rust in your greenhouse, remove the infected plants immediately and destroy them. Fungicide applications must be started in order to protect uninfected plants. Products that contain chlorothalonil, mancozeb, or triadimefon work well to control rust. These products must be applied in a timely manner as the disease can spread rapidly once identified.
