PINE-OAK AND PINE-PINE GALL RUSTS
Pine trees in Minnesota are infected by several species of rust fungi. Two common rust fungi include pine-oak gall rust and pine-pine gall rust. Pine-oak gall rust requires two different hosts to complete its life cycle, while pine-pine gall rust is able to reinfect pine, therefore requiring only one host.
Pine-Oak Gall Rust
Pine-Oak Gall Rust is a native fungal disease, caused by Cronartium quercuum, which occurs on Austrian (Pinus nigra), jack (P. banksiana), mugo (P. mugo), red (P. resinosa), ponderosa (P. ponderosa), and Scots (P. sylvestris) pines. Oak (Quercus spp.) is the alternate host in Minnesota. The severity of this disease depends on several factors including species, age, and portion of the tree infected. In Minnesota, jack pines are particularly susceptible to infection. If galls develop on the main stem, young trees can be killed. Gall development on well-established trees causes disfigurement, but has little effect on overall growth of the tree.
Reproductive structures (basidia) on infected oak leaves release spores in the spring and early summer. These spores are wind blown to nearby pine needles. The fungus grows within infected needles and then into branch and stem tissue. Growth of the rust fungus stimulates cell enlargement and an increase in the number of cells within the branch or stem. As a result, galls are formed, which inhibit the movement of water and nutrients. Galls which encircle the branch may cause death of the branch from the point of infection outward. Galls typically appear on pines one year after infection. Orange colored spores produced on galls the following spring are wind dispersed to the underside of oak leaves.
The health of infected oak trees is not seriously impaired, but the aesthetic appearance of oak leaves is diminished. Oak leaves less than three weeks old are most susceptible to infection. Small dark brown spots with yellow borders are visible on the upper leaf surfaces, while reproductive structures (uredia) develop on the underside of infected leaves. In the spring, these structures produce spores which infect other oak leaves. Shortly thereafter, hair-like reproductive structures (telia and basidia) develop. These structures produce spores in late spring or early summer which infect pine needles. The fungus survives winter in galls on infected pine branches.
Control measures should include inspecting pine seedlings for galls when purchasing, to reduce the chances that an infected tree would be planted. Seedlings with galls should not be planted. Removal of infected branches on pines may improve the aesthetic appearance of infected trees, as well as remove the source of inoculum (spores) available for infecting oak trees. No control measures are feasible on oak trees.
Pine-Pine Gall Rust
Fig. 1. Pine-pine gall rust.
Photo: Chad Behrendt
Pine-pine gall rust is caused by Endocronartium harknessii. In Minnesota, pine-pine gall rust infects the stems of jack (Pinus banksiana) and Scots (P. sylvestris) pines. This fungus does not require two hosts in order to complete its life cycle, as does pine- oak gall rust. Instead, pine-pine gall rust is able to reinfect pine needles continually.
Pine-pine gall rust is very similar to pine-oak gall rust in severity, symptomology, and in the formation of galls (Fig. 1). However, pine-pine gall rust does not infect oaks. The same control measures used for pine-oak gall rust apply to pine-pine gall rust.
Chad Behrendt, Crystal Floyd