Description: Cedar-apple rust can be very harmful to the health of apple trees. If severe infections of cedar-apple rust continue for several seasons, apple tree death can be the result. This is mainly due to premature defoliation. The affected fruit are smaller and deformed. On the native red-cedar, cedar-apple rust is not known to be extremely harmful, although it can become so covered in galls that it becomes unattractive. And, under some backyard conditions where both hosts (cedar and apple) are in close enough proximity to each other, both trees have been killed by this disease.
Symptoms: On apple trees the first signs of the disease appear on the leaves as small greenish yellow spots which gradually get larger and change to orange yellow. The spots are then surrounded by red bands. On the upper leaf surface the spots develop black stipling, which is specialized fruiting structures. On the underside of the leaf, lesions form and hair-like projections can be seen. The leaf thickens around these projections causing the lesion to have a cuplike appearance. These cuplike lesions also appear on immature fruit, causing dwarfing and malformation.
Cedar needles are infected during the summer and by June of the next year small greenish brown swellings appear on the foliage. By fall they have enlarged and appear as a chocolate brown, kidney shaped gall between 1/16th of an inch to over two inches across. Each gall has small circular depressions. The next spring, in moist weather, the depressions in the galls produce orange gelatinous material that swells immensely. A gall with this orange material may reach the size of a small orange.
The disease cycle of cedar-apple rust is one of the most complex of any plant diseases, and the fungus that causes cedarapple rust spends almost two years of its life cycle on the cedar tree.
Action: Control can be accomplished by removing either host from the vicinity of the other. In a backyard situation, it is possible to remove all cedar-apple rust galls by pruning them out of the tree. This must be done in late winter.
Apple trees can be protected by following a fungicide spray schedule starting at blossom time and continuing every seven days until the cedar trees have stopped spreading spores.
Control on cedars can be obtained with a fungicide spray schedule, every two weeks, from June through September.
There are several apple varieties that do carry resistance to cedar-apple rust.
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