January 2015 Plant Disease Update
Rust diseases are common on fruit trees and other plants in Virginia, but the diagnosis of pear trellis rust, caused by the fungus Gymnosporangium sabinae, is a new report for the state. Pear trellis rust causes serious damage to fruiting pear (Pyrus communis) in Europe and has been present in the United States since the 1990’s. Since then it has gradually moved eastward. It was diagnosed on a sample submitted to the VT Plant Disease Clinic for the first time in November.
The fungus causes conspicuous yellow to orange leaf spots and can also infect small twigs and fruit, causing fruit to mummify. Fruiting bodies called “aecia” form on lower leaf surfaces. They are similar to aecia of the more familiar cedar-apple rust fungus, but the aecia of pear trellis rust have conspicuous acorn-like swellings at the base and can produce spores for two years in a row. Like many other rust fungi, pear trellis rust requires two alternate hosts to complete its life cycle. Many species of juniper are alternate hosts for pear trellis rust. Spindle-shaped galls form on branches of the juniper host, followed by development of orange, gelatinous spore masses the second spring after infection. Galls can produce spores for several years in a row. The spores are wind-blown to pear in the spring where they initiate new infections.
Pruning galls from junipers in early spring before production of the orange, gelatinous spore masses helps to prevent new infections. When planting new junipers near pear trees, choose resistant species, such as Juniperus horizontalis, J. communis, and J. squamata. Immunox fungicide (active ingredient = myclobutanil) is registered for preventative control of rusts on ornamental pears, but it may not be used on fruit-producing pears.