- Cedar and apple trees host this fungus. In fact, the fungus can only persist if both hosts are present, according to the Oklahoma State University Extension. Provided the hosts are within several hundred feet of each other, the fungal spores can reach their destination, but hosts that are separated by a mile or more aren't able to support its development, as the spores aren't able to travel that far.
- The orange goo is actually the third stage of development for this fruiting body, which is preceded -- and perhaps goes unnoticed, due to its color -- by brown, kidney-shaped galls on the branches of the tree. Galls usually form in the summer. The fungus overwinters in the galls, which are covered in very small depressions that resemble those of a golf ball.
- Once spring rains and wet weather arrive, the galls take on a completely different appearance. The small depressions begin to swell with the arrival of moisture and develop into horns. The entire entity turns a bright orange color and is filled with bodies called teliospores. As the telial horns swell, they release the spores and the entire process starts all over, perhaps on the host plant, perhaps on new plants. The telial horns shrink again with the return of dry weather and no more spores are released until they reappear, which they do, once rain returns. According to Cornell University, the swelling and drying may occur 810 times during the season.
- If your cedar trees only have a few galls that you can see, prune them off the tree and destroy the twigs. It's best to destroy them before they swell and develop telial horns, as the horns immediately release spores so removal may not do much. Your local county Extension office can recommend a fungicide for use on cedar apple rust, and it will have to be applied every two weeks from about June through September.