Homeowners and landscape designers in regions where eastern red cedar and hawthorn trees coexist often face the perplexing damage caused by cedar quince rust. This fungal disease, caused by the pathogen Gymnosporangium clavipes, requires two distinct hosts to complete its life cycle, moving between the evergreen cedar and the deciduous hawthorn. Unlike typical surface infections, the rust manifests in unusual places, such as the fruit of the hawthorn, making it a specific and frustrating issue for those monitoring tree health.
Understanding the Fungal Lifecycle
The severity of an outbreak hinges entirely on the weather, specifically the presence of cool, wet conditions during the spring. The cycle begins when rainwater splashes spores from galls on the cedar onto the emerging leaves and blossoms of the hawthorn. Once established on the hawthorn, the fungus manipulates the tree’s resources to produce abnormal, gelatinous growths known as telial horns. These structures release a new wave of spores that must find a compatible Eastern Red Cedar or other juniper species to survive the winter, ensuring the perpetuation of the disease cycle.
Visual Identification on Hawthorn
Identifying the infection on hawthorn trees is relatively straightforward for the observant gardener. The most obvious sign is the appearance of bright orange, rubbery, finger-like growths on the leaves, twigs, or fruit. While leaves may distort slightly, the most significant visual impact is on the hawthorn fruit. Instead of developing a uniform color, the berries become severely disfigured, covered in swollen, lumpy, and often cracked tissue that fails to ripen properly.
Impact on Fruit Production
For those growing hawthorn for ornamental or culinary purposes, the economic and aesthetic toll can be significant. The fungus diverts energy from the tree’s core functions to create its own reproductive structures, weakening the overall vigor of the plant. More directly, the fruit becomes inedible, covered in a black, powdery mass of spores. This not only ruins a potential harvest but also creates a messy environment as the diseased fruit eventually falls and decomposes.
Management and Cultural Practices
Because the disease requires two hosts, the most effective long-term strategy is to disrupt the proximity of the trees. Homeowners should avoid planting Eastern Red Cedar and hawthorn species within close proximity of each other. If the trees are already established, rigorous sanitation is crucial. Removing and destroying the galls from the cedar trees during the winter, before the spring rains, can significantly reduce the number of spores available to infect the hawthorn.
Chemical Control Considerations
In high-value landscape settings, chemical intervention may be necessary, though it is complex due to the timing requirements. Fungicides applied to the hawthorn in the spring—specifically when the flower buds are pink and just before bloom—can help protect the foliage and fruit. Conversely, fungicides on the cedar host are less common but can be used to protect the galls during the wet weather when spores are expelled. Homeowners should look for products containing active ingredients like chlorothalonil or mancozeb, following label instructions precisely.
