Western Gall Rust
Cronartium harknessii (J.P. Moore) E. Meinecke (= Endocronartium harknessii)
Host(s) in Alaska:
Shore pine (Pinus contorta subsp. contorta)
Habitat(s): branches and boles
Current Status & Distribution in Alaska (2023 Update)
Western gall rust is prevalent throughout the range of shore pine in Southeast Alaska (see Detection Map). In monitoring plots established across Southeast Alaska, almost ninety percent of pines had western gall rust and the incidence of the perennial galls and diamond-shaped stem cankers (hip cankers) does not vary much between years. Gall invasion by secondary insects and fungi can girdle affected stems. Since 2021, there has been increased branch dieback, topkill, and tree mortality associated with Nectria cinnabarina infection of galls, likely linked to consistent summer precipitation. In 2023, aerial surveyors mapped active branch flagging symptoms (new dieback associated with western gall rust) at 240 locations. On the ground, western gall rust was detected at 19 additional locations, and seven research grade observations were contributed through iNaturalist. Crown dieback associated with galls was distributed throughout the range of shore pine, but was concentrated near Hoonah, and on Mitkof, Gravina, and Prince of Wales Islands in 2023.
In 2017, western gall rust was observed sporulating at the edge of a large, diamond-shaped canker on a shore pine tree bole in Gustavus, suggesting that it likely causes this common type of bole canker/wound. Another stem rust, stalactiform blister rust caused by Cronartium coleosporioides, was recently detected on shore pine near Haines (molecularly confirmed) and Gustavus (suspected). An additional suspected stalactiform blister rust observation was recorded in iNaturalist near Hoonah. Cronartium coleosporioides completes part of its lifecycle on pines and another part on plants in the family Scrophulariaceae/Orobanchaceae, especially paintbrush in the genus Castilleja.
The incidence of western gall rust changes little over time, though conditions in some years may be particularly favorable to infection. In British Columbia and other parts of the Pacific Northwest, gall rust infection has been documented to occur sporadically in years (‘wave years’) with weather conditions conducive to infection (cool and wet during spring sporulation). The wave year phenomenon has not been evaluated in Alaska, but it is thought that ideal infection conditions occur more regularly in the coastal rainforest of Southeast Alaska compared to other regions.
Symptoms, Biology & Impacts
Western gall rust infection causes spherical galls to develop on branches and main boles of 2- and 3-needled pines. Unlike many other rust fungi (e.g., spruce broom rust and spruce needle rust), western gall rust spreads from pine to pine and does not require an alternate host to complete its lifecycle. This disease is most damaging when it infects young pines on or near the main tree bole. In addition to contributing to topkill, main stem infections can also be infection courts for decay fungi and are common points of stem breakage.
In spring, conspicuous orange spores are released from galls to infect pines through newly-emerged foliage. The fungus moves from the vascular tissue in the leaf to the branch, where it causes swelling and develops spores for reproduction. Western gall rust does not usually kill branches directly, but infections facilitate secondary insects (e.g., Pityophthorus twig beetles and the Douglas-fir pitch moth) and fungi (e.g., Nectria) that can invade and girdle branches or boles.
Permanent plots (46) were established at five locales in Southeast Alaska in 2012 and 2013. In plots, 85% of live pines were infected with western gall rust, 34% had at least one gall on the main stem (bole galls) that could lead to top kill or whole tree mortality, and 25% had dead tops associated with bole galls.
Shore pine is not commercially managed for timber in Alaska; therefore, this disease is not actively managed. Homeowners wishing to treat landscape trees can prune affected branches to remove inoculum and prevent spread.
Western gall rust severity and distribution information comes from a plot network established in 2012 and 2013 to evaluate the health status of shore pine in Southeast Alaska. These plots were established with competitive funds from the U.S. Forest Service Forest Health Monitoring Program. Analysis of Forest Inventory and Analysis data has emphasized the need to gain more information about damage agents and mortality rates of shore pine.
Links to Resources & Publications
Hoffman, J.; S. K. Hagle. 2011. Western gall rust management. Chapter 14.1 Forest insect and disease management guide for the northern and central Rocky Mountains. USDA Forest Service, Northern Region, State and Private Forestry. 3 pp. Available here.
Mulvey, R. L.; Barrett, T.; Bisbing, S. M. 2015. Factors contributing to Shore Pine (Pinus contorta subsp contorta) mortality and damage in southeast Alaska (Project WC–EM–B–12–03). [Chapter 14]. In: Potter, Kevin M.; Conkling, Barbara L., eds. 2015. Forest Health Monitoring: national status, trends, and analysis 2014. Gen. Tech. Rep. SRS-209. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station: 151-160. Available here.
Peterson, R. S. 1960. Western Gall Rust on Hard Pines. Forest Pest Leaflet 50. U.S. Department of Agriculture Forest Service, Rocky Mountain Forest and Range Experiement Station. 8 pp. Available here.
Content prepared by Robin Mulvey, Forest Health Protection, firstname.lastname@example.org.