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Cedar-apple rust, caused by the fungus Gymnosporangium juniperi-virginianae, is a distinctive disease that is indigenous and widespread throughout the Northeast in areas where apples, crabapples, and other related members of the Rose family grow in close proximity to Juniperus species, such as eastern red cedars and junipers. This fungus requires these two different hosts in order to complete its two-year life cycle.

The symptoms of this rust disease on red cedar and juniper are inconspicuous during the winter and appear as green to greenish-brown, kidney-shaped galls that vary in size from ¼-2" in diameter. During cool, rainy periods in the spring, distinctive bright orange, gelatinous telial horns (also called "spore horns") up to 4" long protrude from the surface of these galls. Tiny spores are released from these spore-horns and are carried by wind and driving rain to infect the alternate apple and crabapple hosts. As many as 7.5 million spores may be produced in a single gall and these spores have been known to be carried as far as 6 miles.

Symptoms of infection on the apple and crabapple hosts are also quite colorful. Lesions first appear in early June as greenish-yellow spots that increase in size and develop a characteristic bright yellowish-orange color and are visible on both the upper and lower leaf surfaces. Occasionally, symptoms develop on fruit and twigs. By midsummer, minute "spore cups" called aecia develop at the edge of the lesions on the lower leaf surface. The spores produced in these cups are released from midsummer into autumn and are carried by wind and rain back to the juniper and red cedar hosts where they complete the disease cycle. When these spores land, they germinate and stimulate the formation of galls, a process that takes from 19-22 months for completion.

DISEASE MANAGEMENT:
Cedar-apple rust is not considered a life-threatening disease to either type of host, so control measures are usually not necessary in most cases. However, if significant defoliation and/or fruit loss is experienced on apple or crabapple hosts, control measures may be necessary. This disease can be effectively managed through the combined use of culture, sanitation, resistance, and fungicide sprays. Cultural methods involve removal of either host within ½-1 mile from the other although, in most cases, this is not feasible. Sanitation involves pruning and removing galls from the red cedar and juniper hosts during the dormant season. Once again, this is practical in limited situations where only a few trees are involved and only a few galls are present. Selection and planting of resistant cultivars or varieties is the most effective means of control since this effectively reduces or eliminates the occurrence of the disease. Examples of resistant junipers are Juniperus chinensis var. sargentii, J. communis cv. Aureospica, and J. virginiana cv. Tripartita. Resistant apple cultivars include Delicious, Empire, Jonamac, McIntosh, and Paulared. Resistant crabapples include Ellwangerina, Henry Kohankie, Ormiston Roy, and Red Baron.

The final strategy for disease control involves the proper selection, timing, and application of fungicide sprays. Thorough coverage of all parts of the tree is necessary and the sprays should be applied until runoff. The fungicide label will contain information on plant hosts and diseases, dosage rates, days-to-harvest intervals, and safety precautions. Among the fungicides registered for use in Connecticut are chlorothalonil, ferbam, mancozeb, triadimefon, triforine, and myclobutanil and should be applied as necessary. If harvesting fruit for consumption, please consult the fact sheet Disease Control for Home Apple Orchards. This guide contains information on fungicides registered for use on edible fruit. Use of fungicides to protect Juniperus species has yielded disappointing results due to the difficulty in determining the timing of the applications since this midsummer-through-fall infection period remains poorly understood. The only fungicide registered for use on junipers in the landscape is triadimefon.

Summary
Cedar-apple rust, caused by the fungus Gymnosporangium juniperi-virginianae, is an indigenous and widespread disease throughout the Northeast. It occurs in areas where apples and crabapples grow in close proximity to eastern red cedar and other species of juniper. The fungus requires these two different hosts in order to complete its life cycle. Symptoms are colorful on both types of hosts and can result in significant defoliation of crabapples and apples. Methods for minimizing the impact of this disease are discussed.

https://portal.ct.gov/caes/fact-sheets/plant-pathology/cedar-apple-rust