Apple, crabapple, eastern red cedar, southern red cedar, and juniper. This fungus can persist only where both hosts (apple or crabapple trees and red cedar or juniper trees) are present. The entire life cycle of the causal fungus for cedar-apple rust takes two years. Cedar-apple rust can be very harmful to the thriftiness of apple trees, however on the native red cedar and ornamental cedar, cedar-apple rust is not known to be extremely harmful.
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 (spermogonia). On the under side of the leaf, lesions are formed called "aecia". At the point where an aecium is formed, hairlike projections can be observed. The leaf thickens around these projections, causing the aecium to have a cup-like appearance. These cup-like 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 1/16 of an inch to over 2 inches across. The next spring, in moist weather, the pocket-like depressions in the galls put forth orange telial horns. These telial horns are a gelatinous material that swells immensely. A gall covered with telial horns may reach the size of a small orange. The spores produced by these horns may infect apple trees up to 4 miles away, causing the cycle of the fungus to begin again.
Control can be achieved by removing either host from the vicinity of the other; however, in Oklahoma removal of juniper hosts is not usually possible. In most urban settings, removal of a host is not feasible. If practical, in late winter remove all cedar-apple rust galls from cedar and juniper. This practice will help break the disease cycle; however, no benefit will be received from this method if your neighbor's tree has cedar-apple rust galls. Several varieties of apple and crabapple are resistant or more tolerant to cedar-apple rust. Apple and crabapple trees can be protected from cedar-apple rust infection by following a fungicide spray schedule starting at blossom time and continuing at seven-day intervals until the cedar galls have stopped spreading spore. Fungicide control on the cedar is not recommended since the injury to cedars is not significant.