Hardwood Leaf Rusts

Melampsora epitea Thuem.
Melampsora medusae Thuem.
Melampsoridium betulinum Kleb

Host(s) in Alaska:

Birch species (Betula spp.)
Black cottonwood (Populus trichocarpa)
Willow species (Salix spp.)

Habitat(s): hardwood leaves and conifer needles

Current Status & Distribution in Alaska (2023 Update)

In 2023, 47 observations of hardwood leaf rust were made during ground detection surveys (see Detection Map). In Southcentral and Interior Alaska, hardwood leaf rusts were recorded at 30 locations on willow, nine on dwarf birch, five on paper birch, two on trembling aspen, and one on balsam poplar. In Southeast Alaska, willow leaf rust was recorded at one location near Juneau. Scattered leaf rust was aerially mapped but not ground-checked on willow and black cottonwood west of Haines along the Chilkat River. There were 14 research grade willow leaf rust observations in iNaturalist in Utqiagvik, Fairbanks, and around Denali Park in the Interior; Anchorage and Anchor Point on the Kenai Peninsula in Southcentral Alaska; and Haines and Sitka in Southeast Alaska. Distinguishing among the species that cause hardwood leaf rusts depends on the host plant: Melampsora epitea mainly occurs on willow, M. medusae on poplars, including aspen, and Melampsoridium betulinum on birch. However, there is enough molecular and pathogenic variation in Melampsora epitea of willow that it is now considered a species complex rather than a single species (Bennett et al. 2011, Mycologia,103(5), 2011, pp. 1004–1018. DOI: 10.3852/10-289). Several samples were submitted to Oregon State University Plant Clinic for DNA identification.  

Historic Activity

This disease rarely occurs at severe enough levels to warrant mention in Alaska Forest Health Conditions Reports. The most recent pronounced activity occurred in 2014, when leaf discoloration symptoms were common on various hardwood hosts, especially birch trees, throughout Alaska in late-summer. Symptoms ranging from scattered infected leaves to entirely yellow crowns were widespread across much of Southcentral and Interior Alaska (Matanuska-Susitna Valley, Glenn Highway, and Upper Copper River Valley). Individual heavily-infected birch trees were also observed scattered near Glacier Bay National Park. Along the western coast of Alaska (near Kotzebue), there were several reports of rusted willow leaves. Many residents in Southeast Alaska complained of pollen allergy-like symptoms coinciding with rust spore production on willow and cottonwood. Prior to 2014, the only other year with significant reported hardwood leaf rust activity was 1998. At this time, Melampsora epitea caused moderate to severe damage to willows in the Interior, especially on the Upper Yukon River flats, coinciding with a large outbreak of willow leaf blotch miner.

Symptoms, Biology & Impacts

Orange spores are produced on the undersides of leaves in late summer, causing yellow, mottled leaf discoloration and blight symptoms on various hardwood hosts. Affected birch, cottonwood and willow display yellow to brown discoloration of leaves or entire tree or shrub crowns.

Melampsora epitea alternates between willow and hemlock (or cycles on willow alone), M. medusae alternates between poplars and conifers (or on poplars alone), and Melampsoridium betulinum occurs on birch alone, but is known to alternate on larch in Europe. For some species and races of hardwood leaf rust, spores produced on hardwoods infect needles of susceptible conifers in late summer. Damage to hardwood and conifer hosts is usually ephemeral, since infected leaves are replaced the following year and weather conditions are not normally conducive to widespread or severe disease across consecutive years. This disease is not considered serious in Alaska and is not actively managed.

Survey Method

Severe outbreaks (or severely infected individual trees) can be mapped by aerial survey. Most observations of this disease are informal.

Detection in Alaska

Links to Resources & Publications-

Bennett, C.; Aime, M.C.; Newcombe, G. Molecular and pathogenic variation within Melampsora on Salix in western North America reveals numerous cryptic species. Mycologia. 2011 Sep-Oct;103(5):1004-18. doi: 10.3852/10-289. Epub 2011 May 10. PMID: 21558505. Available here.

Melampsora Rusts: Common leaf rusts of poplars and willows. U.S. Forest Service Rocky Mountain Region. 2011. Available here.

Content prepared by Robin Mulvey, Forest Health Protection, robin.mulvey@usda.gov

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