Answer: Cedar-apple rust, caused by the fungus Gymnosporangium juniperi-virginianae, is a problem when crabapple and certain species of juniper and red cedar grow in close proximity. This rust requires two different hosts to complete its life cycle. The fungus causes brilliant yellow-orange spots or lesions on crabapple leaves and occasional lesions on the calyx end of the fruit. On cedar, the fungus produces brown to reddish-brown galls, ?-2" in diameter. During rainy periods in the spring, distinctive bright orange, gelatinous spore-horns protrude from the surface of these galls. The spores are blown by the wind to crabapple trees where they infect and produce their characteristic lesions.
Removal of unwanted junipers or cedars located in close proximity to the crabapple tree usually reduces the number of spores capable of infecting, although this is often not practical. Galls can also be removed from juniper branches during dormancy. However, selection and planting of resistant varieties are the most important and effective methods of control. Examples of rust-resistant varieties are: Ellwangerina, Henry Kohankie, Ormiston Roy, and Red Baron. Where the disease seldom occurs or few leaves are infected, no control is necessary. Where disease is frequent and severe, fungicide sprays can be applied to the rosaceous hosts. Among the fungicides registered for use in your area are chlorothalonil, ferbam, mancozeb, triadimefon, triforine, and myclobutanil. Applications can be made at budbreak and repeated at label intervals as necessary. More frequent sprays may be necessary in wet weather. Fungicide control for Juniperus hosts is usually not practical since it often requires a season-long spray program.
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