Forest & Grassland Health

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

Photos 

Click on the image for a larger version.

Hardwood leaf rust (M. betulinum) sporulating on birch near the Slana River.

An upclose view of hardwood leaf rust (M. betulinum) on a birch near the Slana River.

Hardwood leaf rust (M. betulinum) sporulating on birch near the Slana River.

Hardwood leaf rust (M. betulinum) on birch near the Slana River.

Hardwood leaf rust on willow, with spores on lower leaf surfaces and yellow-brown discoloration.

Hardwood leaf rust (M. epitea) on willow, with spores on lower leaf surfaces and yellow-brown discoloration above.

Hardwood leaf rust on willow, with spores on lower leaf surfaces and yellow-brown discoloration.

Hardwood leaf rust (M. epitea) on willow, with spores on lower leaf surfaces and yellow-brown discoloration above.

Melampsora leaf rust on willow near Juneau.

Hardwood leaf rust (M. epitea) on willow near Juneau.

Current Status & Distribution in Alaska (2020 Update)

Moderate hardwood leaf damage was common on birch and willow in Interior Alaska and on willow in Southeast Alaska in 2020, with 61 records on willow and 15 on birch. Near Salcha, birch leaf rust damage coincided with very heavy defoliation from birch aphid. Several hardwood leaf rust fungi infect the leaves of hardwood trees and shrubs throughout Alaska (See Distribution Map). Orange spores are produced on the undersides of leaves in late summer. Affected birch, cottonwood, and willow display yellow to brown discoloration of leaves or entire tree or shrub crowns.

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.

Distribution in Alaska

Melampsora spp. observations in Alaska as of 2020.
Georeferenced observations of Melampsora spp. in Alaska as of 2020. Host information is not shown since Melampsora spp. affects such a wide variety of tree and shrub species, including willow, birch and poplars, in forested and unforested areas.

 

Links to Resources & Publications-

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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