Wednesday afternoon, this sample of Virgina Pine came into the Master Gardener Info line. The gall on this branch is caused by a disease known as Pine-Oak Gall Rust. This fungal disease, caused by Cronartium quercuum, requires two different hosts to complete its life cycle. It lives part of its life-cycle on pines and part of its life-cycle on oaks.
Symptoms on pine include swollen areas on the branches, lumps or galls measuring up to 4 inches across, and slowed growth. Trunk galls on young trees may result in death or breakage of the trunk or branches at gall location. Galls on branches may girdle and kill tissue beyond the infection. Symptoms on oak leaves include small dark brown spots with yellow borders on the upper leaf surfaces, and reproductive structures develop on the underside of infected leaves.
In the spring, mature galls on the pine host release windblown spores, which infect expanding oak leaves. About one week after infection, orange spores are released from the underside of infected oak leaves, causing additional oak leaf infections.
The above photograph shows the orange spores produced by the pathogen and being released on the surface of the gall.
Control measures include removal of infected branches on pines to remove the source of inoculum (spores) available for infecting oak trees. No control measures are feasible on oak trees.