Cedar apple rust
Gymnosporangium juniperi-virginianae Schwein
On leaves, the disease appears on the upper surface as small, faint, yellow spots (A) shortly after the appearance of active cedar galls (B) found on the alternate host for this fungus, the red cedar (Juniperus virginiana L.). Lesions eventually turn dark yellow to yellow-orange and develop a reddish border (C). Tiny pustules form in the center of these lesions, eventually turning black. Blisters form on the underside of lesions by midsummer and produce small, tubular projections (D). As lesions age, they split and the walls curve back, forming a cup with masses of powdery orange to brown spores.
On the fruit, lesions usually appear on the calyx end, similar to those on leaves, but only a bit larger, and are usually slightly raised. A dark-green border forms around the yellow to orange colored lesion as they age (E, F).
Primarily limited to eastern North America where Juniperus species occur. Cedar apple rust is the most common rust on apple; it does not affect pear.
Quince rust affects both apple and pear, but is a minor disease on pear. Hawthorn rust affects the leaves of both apple and pear. These diseases can be confused with each other, but can be differentiated in part based on the plant tissue infected.
Eliminating red cedar trees in the vicinity of orchards helps to reduce disease pressure. However, it is difficult to remove all sources of disease because the infective spores can travel on air currents for several miles. Pruning the "cedar apples" from cedar trees is an alternative to removing the tree itself. Where the disease is a problem, fungicides are applied from tight cluster through petal fall. The varieties Cortland, Crispin, Golden Delicious and Jonagold and are susceptible to both cedar apple and quince rust, Ida Red and Paula Red are susceptible to cedar apple rust, and Delicious and Empire are susceptible to quince rust.
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