Cedarapple

Cedarapple, commonly cedar-apple rust, is a fungal disease that requires two different plant hosts to complete its life cycle: junipers (cedars) of the genus Juniperus and apples or crabapples (Malus). The disease is caused by fungi in the genus Gymnosporangium, most often Gymnosporangium juniperi-virginianae, with related species affecting other Rosaceae hosts. The interaction between these hosts enables recurring infections across seasons.

The life cycle involves galls on cedar trees that, in wet conditions, produce orange gelatinous tendrils called telial horns. These horns release spores that infect apple or crabapple leaves and fruit. Infected apples develop spots and lesions on the leaves, which can cause premature defoliation and reduced fruit quality. The fungus then completes its cycle by forming new spores on the apple host that reinfect the cedar host, where new galls form and the cycle continues.

Symptoms on apples include bright orange spots on the upper surface of leaves with yellow halos, which may turn brown and crack; fruit lesions are typically small, dark, and sunken, and severe infections can lead to leaf drop. On cedars, galls appear as hard, brownish growths on branches or trunks, which release orange horns during wet weather.

Management focuses on reducing inoculum and protecting susceptible trees. This can involve removing cedar galls or limiting juniper proximity to apple orchards, selecting resistant apple varieties, and applying fungicides at timing windows such as green tip or petal fall, following label instructions. Good orchard hygiene and pruning to improve air circulation also help minimize spread. Cedarapple rust is most common in regions where both hosts coexist, particularly in eastern North America.