Cedar-apple rust is caused by the fungus Gymnosporangium jumiperi-virginianae. The fungus needs two hosts to complete its life cycle, cedar and apple or crabapple.
On apples, small yellow spots develop shortly after bloom on the upper leaf surface. These yellow spots gradually enlarge and become orange. Later black spots (pycnia) appear on the upper leaf surface. In late summer small tubes develop on the lower leaf surface below the yellow/orange spots. Superficial brown spots may develop on fruit.
Since the disease does require two hosts to complete its life cycle, removal of one of the hosts can control the disease. This is not always practical, however, since the rust fungus can be spread over long distances. (Fungal spores are carried from host to host by air currents.) If feasible, avoid planting rust-susceptible varieties of cedars. The susceptible cedars and junipers include eastern red cedar and its varieties, and Rocky Mountain juniper and its varieties. Apple varieties differ in their resistance to and susceptibility to rust. The most susceptible apples are Beacon, Jonathon, Lakeland, and Wealthy. Apples resistant to the disease include Delicious, Liberty, and McIntosh.
Fungicide sprays can be used to control the disease on apples. Fungicides available to the homeowner are limited. Funginex is labeled for control of rust on apple. A Benlate/Captan tank mix applied every 10 days from the pink stage (blossoms begin to show pink before opening) to second cover (3 weeks after all petals fall) is an option. Also refer to Pm-175 Home Fruit Insect and Disease Management. Additional products, including Daconil 2787 and ClearyÕs 3336, are available for control of rust on crabapples.
Fungicide sprays are usually not warranted or practical on cedars. If there are only a few small cedars present, galls can be hand-picked and destroyed by April 1.
This article originally appeared in the May 17, 1996 issue, p. 78.
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