The ISU Plant and Insect Diagnostic Clinic recently received a Scots (Scotch) pine sample with galls typical of pine gall rust. Although this disease is not especially common in Iowa, it appears occasionally, especially in Christmas tree plantations growing susceptible Scots pines.
There are two different pine gall rust diseases, Eastern pine gall rust and Western pine gall rust. In general, these two diseases cause identical symptoms, but Western is more destructive and more common in Iowa. Both diseases can affect two and three needle pines, such as Scots pines, Jack pines, and sometimes Austrian pines.
Symptoms of pine gall rust appear as round, swollen galls on the branches or trunks of susceptible pine trees. Galls may range from an inch or less in diameter to several inches across, and they are perennial, persisting for many years. Sometimes, a gall girdles a branch, causing needles to turn yellow and brown as the branch dies.
Eastern pine gall rust is sometimes called 'oak-pine rust' because the fungus that causes it spends part of its life cycle on pine and part on oak, its alternate host. On the oak, the fungus forms small hair-like projections on the undersides of leaves. Western gall rust does not have an alternate host, so it is sometimes called 'pine-pine rust' because the spores produced on pine are able to directly infect other pines.
The fungus that causes Western pine gall rust infects newly emerging shoots of susceptible trees in the spring. About a year or two after infection, the branch begins to swell to form a round gall. Galls that are about three years old or older produce yellow spores in blisters underneath the bark in the spring, and these spores travel through air currents to infect other nearby trees. There are other, more serious disease problems of Austrian and Scots pine that prevent them from being good choices for Iowa landscapes. Scots pines are still a good choice for Christmas tree plantations, though. Always carefully inspect nursery stock before purchase. However, remember that an infected plant may not show symptoms for at least a year. If galls are found on a tree in the landscape or plantation, the galled branches should be pruned out before the fungus has a chance to produce spores in the spring. In some cases, a heavily galled tree may need to be removed. The fungus will not survive once the plant is dead, so infected branches may be saved for firewood or other uses.
A Scots pine branch with galls caused by Western pine gall rust. (Photo by Christine Engelbrecht)