In recent years, we have begun to see some rust diseases on pines that involve a gall formation on the stem. These rusts are not yet common in Illinois, but we have seen them each of the last three summers. The diseases are less likely to occur in home landscapes. Watch for them if you have a commercial tree nursery or Christmas tree farm.
There are many common rust diseases of trees in Illinois, including cedar-apple rust, cedar-hawthorn rust, and cedar-quince rust. These diseases require two hosts and cause most concern on the apple, crabapple, or hawthorn hosts. The two rusts of pine are pine-oak gall rust and pine-pine gall rust. Two hosts are necessary for pine-oak gall rust to occur. An alternate host is not needed for pine-pine gall rust.
The fungus causing pine-oak gall rust (Cronartium quercuum) requires two different hosts to complete its life cycle. In Illinois, the primary coniferous host is Scotch pine; but jack, Austrian, mugo, ponderosa, and red pines may also be infected. Deciduous hosts include red, pin, and bur oaks. This disease is also known as eastern gall rust.
Symptoms on pine include swollen areas on the branches, lumps or galls measuring up to 4 inches across, and slowed growth. Mature galls often have white to yellow, blisterlike ridges (fruiting bodies) that rupture through the bark and produce yellowish spores. Severe infections may result in witches' broom (multiple shoots growing from a gall), death of branches, and possibly death of the entire tree. Symptoms on oak leaves are similar to those of rust on crabapple. Small dark brown spots with yellow borders are visible 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. Two to 3 weeks later, hairlike structures are produced on the underside of infected leaves and different spores are released that infect pine needles, succulent stems, and expanding candles. New pine infections take 2 to 4 years to develop into mature galls that can release spores capable of infecting oak leaves.
Pine-pine gall rust (also called western gall rust) is caused by Endocronartium harknessii. In Illinois, the primary host is Scotch pine, but jack and ponderosa pines may also be infected. Pine-pine gall rust is very similar to pine-oak gall rust in severity, symptoms, and formation of galls. However, pine-pine gall rust does not infect oaks and does not need two hosts to complete its life cycle.
To avoid this disease, purchase seedlings and young pines from a reputable source and inspect the trees before planting. However, even close inspection is not foolproof because you will not be able to detect an infection until 1.5 to 2 years after infection. Although the field symptoms of these two rusts are virtually indistinguishable on pine, there are microscopic differences in the spores from the pine galls. While on site, it may be helpful to examine nearby oak hosts for evidence of rust lesions, which indicate the presence of pine-oak rust.
Infected pine branches and/or whole trees should be removed before spring because the rust galls release infectious yellow-orange spores each spring. To protect pines in nursery settings from new infections, apply Bayleton or mancozeb (several trade names available) every 7 to 14 days during pine shoot elongation. Fungicides are not recommended for landscape trees.
Because pine-oak gall rust has a few extra steps in the spring infection cycle, peak pine infection will likely be later than for pine-pine gall rust. Pine-infecting spores are released 2 to 3 weeks after the first orange spores develop on the underside of oak leaves.)