Cedar Apple Rust is a very unique type of fungus. It begins as a gull on an Eastern red cedar trees and then moves to an apple (or crab apple) tree. With slight exception, it must have these two specific types of trees to complete its life cycle. (Some types junipers can substitute as cedar and quince or hawthorn can substitute the apple.)
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On a cedar tree, the gulls look like a hard, hollow, brown golf ball with thick, hair-like protrusions. This is the dormant state. Springtime rains bring on a alien-like transformation. The whole things begins to swell and the protrusions grow into gelatinous, orange tentacles. It looks like an orange sea urchin hanging from a tree. The creepy tentacles produce millions of spores that can travel up to two miles looking for a host apple tree.
When an apple tree is infected the first sign will be bright orange spots on the leaves. As apples grow, slightly raised orange lesions will appear. The lesions will grow larger and turn brown as the infection progresses. Small, tube-like structures will grow in the bottom of leaves. This will produce the spores that will travel in search of another cedar to infect.....starting the whole process over.
Cedar apple rust generally causes little damage to the Eastern red cedar. There may be some slight die back of the twigs. When galls are seen they can be pruned out in the winter or early spring.
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The infection will cause extensive damage to apple crops. The fungus itself will not kill the tree, but it can damage leaves and weaken the tree, making it more susceptible to other types of illness. One hundred years ago, it was such a big problem that the state of Virginia enforced the Cedar Rust Act of 1914 saying that if any infected cedar found within one mile of an apple orchard must be cut down.
"Be it enacted by the general assembly of Virginia' that it shall hereafter be unlawful within this state for any person, firm or corporation to own, or keep alive and standing upon his or its premises, any red cedar tree, or trees (which are or may be) the source, harbor or host plant for the communicable plant disease commonly known as 'orange' or 'cedar rust', of the apple, and any such cedar trees when growing with a radius of one mile of any apple orchard in this state, are hereby declared a public nuisance and shall be destroyed as hereinafter provided, and it shall be the duty of the owner or owners of any such cedar trees to destroy the same as soon as they are directed to do so by the state entomologist, as hereinafter provided."
There are varieties of apple that are less prone to attacks and fungicides can be used to prevent cedar apple rust, but the most fool-proof way to avoid it is still not to grow cedar within a 2+ mile radius of apples.