Pear trellis rust is caused by the fungus Gymnosporangium sabinae. It was introduced into the United States from Europe in the 1990s and has been progressively spreading through states in both the East and West. The disease affects common pear, callery pear, and several species of juniper/eastern red cedar.
For the disease to complete development, both pear and juniper are needed. In early spring, round and orange gelatinous galls will form on juniper twigs during cool, moist conditions. Spores are produced on these galls, which are then disseminated to pears by wind and rain. Impacted pear trees will display conspicuous yellow to orange leafspots and spindle-shaped lesions on petioles and twigs. By late summer, fruiting structures resembling trellises will have formed on the undersides of diseased leaf tissue and on lesions on petioles and twigs. Spores from these fruiting structures will then further disseminate by wind and rain to juniper where infection occurs on twigs and shoots of these trees.
Pear trellis rust is a serious disease that can significantly impact the health and survival of pear trees. Reduced growth, crown thinning, and branch dieback will occur after several consecutive years of infection. In areas where the disease is prolific, it can eventually cause death of the pear host. Though juniper trees are part of the disease cycle, there is typically no serious effect on these plants.
Fungicide spray treatments applied in spring when new growth emerges are highly effective in preventing new infections on pear. Treatments to protect juniper are seldom needed because of the tolerance of this host to the disease.