The disease cycle of cedar apple rust is complex. In warm, wet springs, each spore horn produces a fantastic number of spores. The wind carries them to apple leaves just about the time when apple buds are in the pink to early blossom stage. Upon reaching apple leaves, the spores attach themselves, germinate and enter the leaf tissues. They infect within four hours under favorable conditions.
Yellow lesions develop in one to three weeks.
The wind carries them back to cedar trees, completing the cycle. The spores land on cedar needle bases or in cracks or crevices of twigs. There, they germinate and produce small, green-brown swellings about the size of a pea. Galls do not produce spores until the second spring. However, many mature galls usually are available every year.