Cedar-apple rust diseases can be very harmful to the thriftiness of apple trees. If severe infections of cedar-apple rust continue for several seasons, apple tree death can be the result. Damage to apple is brought about chiefly by premature defoliation. Affected fruit are smaller, deformed, and undesirable for marketing.
On red cedar and ornamental cedar, Juniper species cedar-apple rust can be harmful. Under some backyard conditions where both hosts (cedar and apple) are located close together, both trees have been killed by this disease. There is no repeating spore stage for either host; therefore, the cedar-apple rust fungus can persist only where both hosts are present.
On apple trees, the disease first appears on the leaves as small greenish yellow spots which gradually enlarge, changing to orange-yellow and becoming surrounded at the border by concentric red bands. On the upper leaf surface, the spots become stippled with black specialized fruiting structures. On the underside of the leaf, lesions are formed and hair like projections can be observed. The leaf thickens around these projections, causing the cuplike appearance. These cuplike lesions can also appear on immature fruit of apples, causing dwarfing and malformation.
Cedar leaves are infected during summer months, and by June the following summer, small greenish brown swellings appear on the upper or inner foliage surface. These swellings enlarge and by autumn appear as chocolate-brown, somewhat kidney-shaped galls. Each gall is covered with small circular depressions. The somewhat kidney-shaped galls vary from one-sixteenth of an inch to over two inches across. The next spring, in moist weather, the pocket like depressions in the galls put forth orange horns. These horns are a gelatinous material that swells immensely and may reach the size of a small orange
Control is most readily accomplished by removing either host from the vicinity of the other. The prevailing winds will affect the distance the hosts need to be kept apart to prevent disease development. From one-quarter of a mile to more than a mile distance is required.
In a backyard situation, it is possible in late winter to remove all cedar-apple rust galls by pruning them out of the cedar trees. To break the disease cycle, galls have to be removed before horns are formed. No benefit will be received from this method if your neighborís tree has cedar apple rust galls.
Apple trees can be protected from cedar-apple rust by following a fungicide spray schedule starting at blossom time and continuing at seven-day intervals until the cedar galls have stopped spreading spores. Control on cedars can be obtained with a fungicide spray schedule from June through September at two-week intervals. Many flowering crabapple varieties are very susceptible to cedar-apple rust
Some apple trees are more resistant to cedar apple rust than others, however if there is wet weather in the spring the junipers and cedars can be affected with the orange gelatinous horns that seem to appear overnight. Now is the time to be spraying these evergreens to prevent the cedar apple rust next spring. Next spring is the time to start spraying to prevent cedar apple rust on apples or crab apples.
Scott Hininger is with the Sheridan County Extension office.