Distinctively "rusty" spots on geranium leaves are likely caused by the fungus Puccinia Pelargonii-zonalis. This disease, also known as geranium rust, is only problematic on Pelargonium x hortaruma, the florist’s geranium; scented leaf, Martha Washington, ivy and wild geraniums are resistant to geranium rust. Correctly identifying rust as early as possible and a vigilant treatment program will help to control this pathogen.
Geranium leaf rust first appears as small, yellow spots on the lower leaf surfaces that rapidly increase in size and become rusty brown spore pustules. Irregular, concentric rings of rusty pustules form around the original spot. Small yellow spots appear opposite the pustules on the upper leaf surface but generally do not develop rings. Spots on the upper leaf surface turn brown as the disease progresses, and heavily infected foliage turns yellow, dries and drops, often leading to complete leaf loss on the geranium.
The Puccinia Pelargonii-zonalis fungus persists as yellow-brown spores produced on infected plants. These spores, known as urediospores, are dispersed by wind, water, infected debris, hands or tools. Spore germination is best in moist, cool conditions; germination and spot formation are inhibited at temperatures above 81 degrees Fahrenheit. Spores germinate in free moisture and penetrate the geranium leaf through its stomata. Seven to 10 days pass between this penetration and the appearance of spots. After another seven to nine days, the pustules release spores. Spores remain viable for up to 12 weeks after release.
Prevent and control geranium rust by carefully inspecting plants before buying them. Use amendments, if necessary, to ensure a well-drained soil. Avoid overhead irrigation and the moist, cool conditions that this fungus prefers. Remove any rust-affected leaves every day and destroy debris. Remove and destroy any severely infected plants. Ensure that plants are well spaced to allow ample air circulation. If necessary, several fungicides may be applied to control or prevent this disease.
A number of other diseases caused by bacteria, fungi, viruses or environmental conditions may lead to the development of leaf spots on geraniums, although not all of these spots can be characterized as "rusty." These diseases include botrytis blight, bacterial leaf spot caused by Xanthomonas campestris, impatiens necrotic spot virus, tomato spotted wilt virus, Alternaria leaf spot, Cercospora leaf spot and oedema.