It requires both an apple and cedar or juniper to complete its life cycle. During wet weather these galls swell and begin to push out bright orange gelatinous tubular structures. Wind carries fungal spores from these gelatinous structures to susceptible apple or crabapple cultivars. Infection occurs when these spores land on a susceptible apple cultivar and moist conditions prevail. Small, yellow spots begin to appear on the upper leaf surface shortly after bloom.
Spots gradually enlarge and become a bright yellow-orange color.
These brightly colored spots make the disease easy to identify on leaves. Heavily infected leaves may drop prematurely. In late summer small tube-like structures develop on the underside of the apple leaves. Spores are released from these structures and are blown by wind back to susceptible cedars or junipers, completing the disease cycle. If you have followed along with this cycle, you might immediately think that a good way to break it would be to avoid planting susceptible hosts next to each other.