If you examine Scotch pine, particularly in Christmas tree nurseries, you may notice swollen areas or golf ball–sized lumps on the branches or trunks. We receive calls and samples each year about these symptoms, so I’d like to to discuss the two most likely culprits, pine–oak gall rust and pine–pine gall rust.
Pine–oak gall rust. As with cedar–apple 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.
Symptoms on pine include swollen areas on the branches, lumps or galls 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. Se-vere infections may result in witches’-broom (multi-ple shoots growing from a gall), death of branches, and possibly death of the entire tree. Symptoms on oak leaves are similar to those on crabapple infected with cedar–apple rust. Small, dark brown spots with yellow borders are visible on the upper surface, and reproductive structures develop on the underside.
In spring, mature galls on pine release windblown spores that infect expanding oak leaves. About a week after infection, orange spores are released from the underside of infected leaves, causing more 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 galls that release spores capable of infecting oak leaves.
Pine–pine gall rust is caused by Endocronartium harknessii. In Illinois, the primary host is Scotch pine, but jack and ponderosa pines may be infected. Pine–pine gall rust is similar to pine–oak gall rust in sever-ity, symptoms, and formation of galls. Pine–pine gall rust does NOT infect oaks or need two hosts.
Management. Purchase seedlings and young pines from a reputable source, and inspect the trees before planting. Keep in mind that you cannot detect an infection until 1.5 to 2 years later. Although the field symptoms of these two rusts are virtually indistinguishable, there are microscopic differences in spores from the pine hosts. While on-site, examine nearby oak hosts for rust lesions indicating pine–oak rust.
Infected pine branches and/or whole trees should be removed before spring because the rust galls re-lease infectious yellow-orange spores each spring. To protect pines in nurseries from new infections, apply Bayleton or mancozeb (several trade names) 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 is likely later than for pine–pine gall rust. The literature indi-cates that the pine-infecting spores are released 2 to 3 weeks after the first orange spores develop on the underside of oak leaves.