Cedar Apple Rust | Tree and Shrub Insect Library

Identification

Cedar apple rust presents in distinct ways on its two hosts. On cedar trees, the most noticeable sign is the appearance of rust galls, which are large and woody abnormal growths that produce bright orange, gelatinous tendrils in the wet weather of early spring. On apple trees, the disease shows up on the leaves and the fruit. Yellow spots will develop on the upper surfaces of the leaves, eventually turning orange-red in a concentric ring pattern. The infected leaves may also have raised, orange spots on their underside. These spots release spores that will go on to infect cedar trees.

Life Cycle

Cedar apple rust alternates between cedar and apple trees over a two-year period. It begins with the fungus overwintering as galls on cedar trees. With the arrival of spring rains, these galls swell and produce orange, jelly-like horns full of spores. These spores are carried by the wind and insects to nearby apple trees, which then become infected. In the fall, spores make their way from apple trees to the cedar trees that are closeby and the cycle repeats. This entire process creates a continuous pattern of infection and re-infection that can be difficult to break without intervention.

Distribution & Habitat

Cedar apple rust is predominantly found in regions where eastern red cedars and apple tree varieties coexist, thriving in areas with high humidity and rainfall conducive to the disease’s spread and development. It is particularly prevalent in the eastern United States.

Damage & Symptoms

The damage caused by cedar apple rust can be significant, leading to defoliation, reduced fruit yield, and in severe cases, tree death. On apple trees, the disease weakens trees by attacking the leaves, causing spots and eventual leaf drop, which affects the tree’s overall health and productivity. The quality of the fruit itself can also suffer, marked by disfiguring spots. On cedar trees, although less harmful, the galls can disfigure the tree, reduce its aesthetic value and continue the cycle of infection.

Management & Control

Effective control of cedar apple rust involves an integrated pest management (IPM) approach. Planting resistant varieties of apple trees and cedar trees can significantly mitigate risk. Where possible, ensure a suitable distance between cedar and apple trees.

Removing and destroying infected galls and leaves will also help to reduce the spread of the disease. Chemical controls, like fungicide sprays, may be necessary for high-value trees or severe infections.

Prevention

Despite the repetitive pattern of the cedar apple rust life cycle, preventive measures can work to avoid disease spread. Start by selecting resistant varieties of apple and cedar trees. Reduce the fungal spores’ ability to spread by making sure there is ample air circulation around trees and the area around them is free of fallen leaves and debris. Regular monitoring of both cedar and apple trees during the vulnerable periods can help in early detection and management.

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