Issue 15, August 28, 2009

Pine Gall Rusts

Here is problem that has been popping up in nurseries and some landscape plantings. It is a gall that forms on pine stems. The galls are rust galls. Although much less common than the galls caused by cedar apple and related rusts, these galls also appear on stems, but on pines. The galls form as a swelling in the stem. You cannot cut out the gall without cutting out the stem. The image shows one of these galls before spores are evident. Eventually rust pustules and spores will be evident on the gall. The disease is not a widespread problem, but one that you should know about in order to stop spread of the disease.

The two rusts of pine are pine-oak gall rust and pine-pine gall rust. The fungus causing pine-oak gall rust (Cronartium quercuum) requires two different hosts to complete its life cycle (as the name implies). 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 measuring up to four inches in diameter, and slowed growth. Mature galls often have white to yellow, blister-like ridges (fruiting bodies) that rupture through the bark and produce yellowish spores like most other rusts you have seen. 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 the symptoms of rust on crabapple but much smaller. Small dark brown spots with yellow borders are visible on the upper leaf surfaces and fruiting structures and spores develop on the underside of infected leaves.

In the spring, mature galls on the pine host release wind-blown 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 three weeks later, hair-like structures are produced on the underside of infected leaves and different spores are released which infect pine needles, succulent stems, and expanding candles. New pine infections take two to four 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. You may also see the fungus as Peridermium 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 in the formation of galls. However, pine-pine gall rust does not infect oaks and does not need two hosts in order to complete its life cycle. The galls of pine-pine rust appear covered in a continuum of fissures and spores.

To avoid the pine rust galls, purchase seedlings and young pines from a reputable source and inspect the trees prior to planting. However, keep in mind that even close inspection is not foolproof since you may not be able to detect an infection until 2 years after it is initiated. Although the field symptoms of these two rusts are virtually indistinguishable on pine, there are microscopic differences in the spores from the pine galls. Examine nearby oak hosts for evidence of rust lesions, indicating the presence of pine-oak rust.

Infected pine branches, galls, and/or whole trees should be removed before spring because the rust galls will release infectious yellow-orange spores each spring. Start looking for these galls to remove now. There are two fungicide active ingredients available to protect pines from infection. These are triadimefon and mancozeb. Triadimefon is sold as Bayleton or Strike. Strike is labeled for production nursery use only. Mancozeb is another active ingredient sold as Dithane, Fore, Lesco Mancozeb, Pentathlon, or Protect.

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. The literature indicates that the pine-infecting spores are released 2-3 weeks after the first orange spores develop on the undersides of oak leaves.--Nancy Pataky

Nancy Pataky

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