Key Features
- Orange to yellow leaf spots
- Orange dust or horns on fruit
- Branch galls on juniper (red cedar)
Symptoms
Large galls or crack in the stem form on small diameter juniper branches. In the spring, stem cracks turn bright orange as they produce spores. Depending upon type of juniper rust (and there are at least 70 different species!) galls develop orange, gelatinous, horns. Galls and stem infections can girdle and kill the branches and stems they infect. Fruit of crabapple or hawthorn may be slightly stunted or discolored before bright white-orange tubes, less than 1 cm in length, emerge from them. Some fruit become covered in powdery orange spores without the presence of the tubes. Small yellow-orange spots appear on the upper and lower surfaces of hawthorn, crabapple or serviceberry leaves. Small, whisker-like, spore producing structures are formed on the underside of these spots. Spores produced on all hosts are rust colored and leave a red-orange smear if rubbed with a finger or light colored paper.
Biology
Rusts are unique plant diseases with complex life cycles. Juniper rusts require a member of the rose (Rosaceae) family, most often crabapple, hawthorn, and serviceberry for infection in the spring. These lesions then produce spores later in the summer that re-infect a cedar (Juniperus spp). The pathogen overwinters in the galls formed on juniper stems. The bright orange 'telial horns' that emerge from the galls in the spring produce wind borne spores that infect the rosaceous hosts. Infected trees are rarely killed, but may become unsightly as they develop an orange cast in summer. Infected juniper branches will defoliate and die once the canker or gall girdles the branch.
Management Recommendations
Avoid planting susceptible cedar and juniper varieties near apple or crabapple trees. If susceptible varieties of cedar or juniper are already planted near crabapple, hawthorn, or serviceberry trees. Removal of either host may be an effective form of control. Spores may, however, be blown from neighboring trees from great distances, allowing the disease to continue. Juniper galls should be pruned out as part of any management strategy. Planting resistant varieties will also help reduce the severity of these rusts. Ornamental varieties rarely require fungicide treatment.
Effective Pesticides
Active Ingredients include: Chlorothalonil, Myclobutanil, Propiconazole, Tebuconazole, Triforine
