Cedar apple rust is a common fungal disease that affects apple, crabapple, and cedar trees. Unfortunately, many gardeners don’t realize they need to treat for this disease until it’s too late. By the time symptoms appear in summer—yellow-orange leaf spots on apple and crabapple trees or gelatinous orange growths on cedar trees—the damage has already been done. The best way to protect your trees is to take preventive action now, before the disease has a chance to develop.

Cedar apple rust is caused by the fungus Gymnosporangium juniperi-virginianae. It requires both apple (or crabapple) and eastern red cedar (or other juniper species) to complete its life cycle. The fungus overwinters on cedar trees and releases spores in the spring, which infect apple and crabapple trees just as they begin to leaf out.

The key to preventing cedar apple rust is applying fungicide at the right time. Preventive fungicide treatments should begin in early spring, just as apple and crabapple trees start breaking dormancy and new leaves emerge. Repeat applications may be necessary every 7–10 days until petal drop, depending on weather conditions.