Hawthorn rust is caused by the fungus Gymnosporangium globosum.
Signs and Symptoms
The rust fungus causes small yellow spots on hawthorn leaves. These spots eventually enlarge and often have a reddish border. The centers of these yellow spots later show small black spots. On the lower leaf surface, directly above the spots, small brownish tube-shaped structures form. Infected leaves may eventually yellow and drop prematurely from the tree.
This disease is also referred to as cedar-hawthorn rust because the fungus requires two hosts to complete its life cycle, a rosaceous host (such as hawthorn) and a juniper host. The most common juniper hosts are eastern red cedar and Rocky Mounta in juniper. Apple, crabapple, mountain ash, and pear are also susceptible to infection.
The cedar-hawthorn rust fungus overwinters on junipers or red cedars in round, brown galls on leaves and twigs. During rainy spring weather, these galls produce bright orange, gelatinous tendrils that contain fungal spores. These spores are then blown to nearby hawthorn trees. Spores may be blown several miles. Fungal spores produced in infected hawthorn leaves in late summer are then blown back to the juniper host, completing the life cycle.
If desired, fungicides can be used to protect hawthorn leaves from infection. Labeled products inlcude chlorothalonil, Bayleton, Duosan, Fore, mancozeb, Rubigan, Strike, sulfur, Systhane, and Zyban. Fungicides should be applied as new growth app ears and flower buds start to open. Repeat 3 or 4 times as directed by the product label. Control on the juniper host is usually not necessary.
See this article for more information about cedar rusts.