Crabapple and apple leaves showing symptoms of cedar-apple rust have been arriving in the Plant Disease Clinic. This disease is easy to recognize. Initially the rust fungus causes small yellow spots on the upper surfaces of leaves. These spots enlarge and turn bright yellow-orange. They are commonly 1/4 inch in diameter. As the disease progresses, raised tubular structures are formed on the lower surface of the leaves, directly under the spots. Spores are released from these structures and travel to a nearby susceptible red cedar or juniper host. Symptoms on the fruit of apples occur near the blossom end and are similar to leaf lesions.
The disease is called cedar-apple rust because the rust fungus spends part of its life cycle on apple or crabapple and part on red cedar or juniper. Spores released from the tubular structures on the under sides of the crabapple or apple host travel to the twigs and leaves of junipers or red cedars and cause round, brown galls (1/4 to 2 inches in diameter) to develop. During warm spring rains these galls swell and exude bright orange gelatinous tendrils. Fungal spores are produced in these tendrils and are blown by wind to nearby apples or crabapples. The spores can be blown for several miles, although most infections occur within a 1/2 mile. Galls do not produce spores until the second spring, but there are usually many mature galls available each year. Once the spores reach a susceptible apple or crabapple host the disease cycle is complete.
Control of cedar-apple rust involves interrupting the disease cycle. Plant apple varieties that are resistant to cedar-apple rust. Red Delicious, Grimes Golden, Redfree, Jonafree, and Prima are examples of resistant varieties. Avoid highly susceptible varieties such as Jonathan, Rome, Wealthy, and York Imperial. Examples of disease resistant crabapples include Prairifire, Professor Sprenger, Profusion, Adams, Indian Magic, and Sargent crab. Check catalogs and nurseries for other resistant varieties.
Removing the cedar host within the vicinity of a crabapple or an apple is usually not a feasible option for homeowners because the trees are often planted in neighborhoods for ornamental purposes. Rust galls on small cedars and junipers can be pruned out in late winter or very early spring. Susceptible cedars and junipers are eastern red cedar and its varieties, and Rocky Mountain juniper and its varieties.
Fungicide sprays can provide satisfactory control when applied properly. Commercial growers should refer to Pm-1282 "Fruit Tree Spray Guide" for control recommendations. Homeowners can refer to Pm-175 "Home Fruit Insect and Disease Management". Daconil 2787 is labeled for the control of cedar-apple rust on crabapple.
This article originally appeared in the July 29, 1994 issue, p. 125.