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Disease: Cedar-Apple
Rust
Pathogen: Gymnosporangium juniperi-virginianae
Host: Cedar and Apple
This is a common fungal disease of red cedars (junipers) and apples (and crabapples) in North America and Europe. Cedar-apple rust is caused by a rust fungus that requires two hosts, apples and cedars, to complete its entire life cycle. In the spring, jelly-like orange horns form on small galls on the cedars and produce spores that can infect apples. Spores produced on these "horns" can only infect apples. Infection of apples results in the formation of yellow to orange-colored spots that are found mostly on the upper surfaces of the leaves. Later, rust-colored spores form in pustules on the lower surface of the leaves (pictured here). These spores infect the cedar trees (but not apple trees) in late summer or early fall. Young galls develop on cedars the next summer, mature during the summer and fall, and serve as a source of spores to infect the apples the following spring.
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