If you have both Eastern Red Cedars (Juniperus virginiana) and apple (Malus spp.) trees, you may be privy to the show of a lifetime on your cedar tree one of these days. There is a fungus, specifically a rust fungus, that needs two hosts, Eastern Red Cedar and apple trees, to complete its life cycle. In order to survive, the fungus must “move” from one host species to another.
If contaminated by the cedar-apple rust, an Eastern Red Cedar tree will have small, woody brown galls on its twigs for the better part of a year. Following a warm spring/early summer rain, these brown galls transform into orange, gelatinous growths the size of a golf ball, adorned with “telial horns” that point in all directions. The function of these horns is to disperse spores. If the spores happen to land on the leaf of an apple or crabapple tree, and conditions are just right, galls will result. These galls look very different from the cedar’s galls. Small, yellow spots on the upper surface of the leaves appear after affected apple trees bloom. The spots gradually enlarge and become yellow-orange-red. Small, raised, black dots form in the center of the leaf spots on the upper surface of the leaves as the leaf spots mature. (Apple trees may defoliate early or spots may develop on the surface of the apple as a result of this rust.) Very short, finger-like, fungal tubes protrude from the lower surface of the leaf directly below the spot which release yellow to orange powdery spores. If the wind carries them to an Eastern Red Cedar, the cycle continues. The complete cycle of cedar-apple rust takes 24 months to complete and requires infection of two different hosts.
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