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Cedar-Apple Rust disease is a very common disease which affects cedar trees, junipers and apple trees. It is unique because in order for it to perpetuate itself, it must alternate between an apple tree and a cedar or juniper tree. It is therefore important to treat all apple trees, junipers and cedar trees in an area, regardless of whether they are yet showing symptoms.
The symptoms of Cedar-apple Rust are most identifiable on apple trees. In the mid to late spring small pale yellow spots appear on the upper surfaces of the leaves. As the season progresses the spots grow in size and small black or brown spots develop in the centre of the larger yellowish spot. As the spots develop and the tree comes under more stress, leaves and fruit will drop prematurely. Cedar-apple Rust is common on both fruit-producing apple trees as well as ornamental flowering apples.
In the spring the fungal spores from the brown and red galls on juniper or red cedar trees are blown by the wind on to the leaves of apple trees where the spores infect the apple tree and rapidly multiply during wet periods. As the season progresses, the fungal spores from the apple tree are blown back on to juniper and red cedar trees where a new infection takes place.
Cedar-Apple Rust cannot be controlled during the active growing season after it has already infected the tree. It is, however, a good idea to apply a fungicide spray during the growing season on the infected tree, as well as surrounding cedar and juniper trees, so as to reduce the degree of infection and re-propagation. During the growing season spray all of the infected and uninfected cedar, juniper and apple trees with Monterey Complete Disease Control. Repeat the spray a second time in about 14 to 21 days.
The following spring spray the trees (especially the apple tree) with Monterey Complete Disease Control. The first spraying should occur when the tree flowers; the second spray should be applied when about 75% of the flower petals have dropped; and the third spraying should occur about 10 to 14 days later.