Forest & Grassland Health

Spruce Broom Rust

Chrysomyxa arctostaphyli Diet.

Host(s) in Alaska:

Sitka spruce (Picea sitchensis)
white spruce (P. glauca)
black spruce (P. mariana)
bearberry/kinnikinnick (Arctostaphylos uva-ursi); alternate host

Habitat(s): branches or main boles of spruce trees, bearberry/kinnikinnick leaves

Photos

Click on the image for a larger version.

Yellow-green foliage of a broom pre-sporulation in early-July.

Yellow-green foliage of a spruce broom rust broom pre-sporulation in early-July.

An orange-tinged broom in September.

An orange-tinged spruce broom rust broom in September.

Spruce broom rust in the upper and lower crowns of adjacent spruce trees in September.

Spruce broom rust in the upper and lower crowns of adjacent spruce trees in September.

An up-close view of rust pustules on infection foliage of a broom.

Close-up view of rust pustules on infected foliage of a broom.

Spruce broom rust sporulating on bearberry/kinnikinnik in early-July.

Spruce broom rust sporulating on bearberry/kinnikinnick in early-July.

 

Current Status & Distribution in Alaska (2020 Update)

In 2020, more than 125 spruce broom rust observations were made during ground surveys, all within the previously known disease range (see Map). Only three research grade observations were reported to iNaturalist. The incidence of perennial brooms in spruce crowns changes little over time, although aerial detection varies by surveyor, locations flown, and timing of color changes. In 2018, an observation was made on the Seward Peninsula, over 100 miles west of previous detections and west of the proposed range of Arctostaphylos uva-ursi, the alternate host plant (based on Hulten, 1968, Flora of Alaska). This part of the state has not since been flown to confirm the record. Broom rust is common and widespread on white and black spruce branches and stems throughout Southcentral and Interior Alaska. It is absent from most of Southeast Alaska aside from Glacier Bay, northern Lynn Canal, and Halleck Harbor on Kuiu Island.

Historic Activity

The incidence of the perennial brooms changes little over time, although conditions for infection may be particularly favorable in some years.

Symptoms, Biology & Impacts

Spruce broom rust is one of the most conspicuous diseases of white, black, and Sitka spruce trees in Alaska. Infected trees have dense clusters of branches with a yellow or orange appearance on the branches or the main tree bole. These branch clusters are sometimes referred to as witches'-brooms or brooms.

During spring and summer the brooms have a yellow or orange appearance because of the short, yellowish needles they contain. In midsummer, yellow or orange spores erupt from pustules on the needles. A distinct sweet but earthy odor is often noted from sporulating brooms. Needles are shed in the fall, giving the broom a bare, dead appearance, but the twigs in brooms do not normally die. The following spring, brooms produce new, yellowish needles.

Many rust fungi must infect two different types of plants. Spruce broom rust infects kinnikinnick, sometimes called bearberry (Arctostaphylos uva-ursi (L.) Spreng). This low-growing, trailing plant has flexible stems, round and leathery perennial leaves, and red berries. Although spruce broom rust requires the alternate host kinnikinnick/bearberry to complete its lifecycle, infected spruce are sometimes observed where the alternate host is locally uncommon in the understory. Overall, the rust on spruce closely follows the distribution of kinnikinnick/bearberry, which is absent from most portions of Southeast Alaska, but can be found in many areas of South-central and Interior Alaska. Close relatives of kinnikinnik e.g., A. alpina (L.) Spreng and A. rubra (Rehd. & Wilson) Fern. may also be susceptible to the fungus. These host plants may extend the range of the disease to spruce growing in northern and northwestern Alaska.

The lifecycle of spruce broom rust.

The lifecycle of spruce broom rust (Chrysomyxa arctostaphyli). Diagram by Paul Hennon.

The distribution of Arctostaphylos uva-ursi in Alaska.

The distribution of Arctostaphylos uva-ursi in Alaska (adapted from E. Hulten, 1968, Flora of Alaska). Because the pathogen requires both hosts, this map roughly shows the range of spruce broom rust in Alaska. Note that it is missing from most of Southeast Alaska.

Many infected spruce trees suffer no apparent consequences. Other trees, however, may suffer some damage including reduced growth, top-kill, and infection by wood decay-causing fungi, or tree death.

  • Reduced tree growth. Because the rust fungus causes extra nutrients to be channeled to the brooms, fewer nutrients are available to spruce trees for height and radial growth.
  • Top kill. When brooms occur on the main bole of spruce trees, the top portion of the tree, above the broom, sometimes dies.
  • Wood decay fungi. The dead portions of trees infected by spruce broom rust (e.g., tops, large limbs, and large scars) may serve as entrance courts for wood decay fungi. These fungi remove structural compounds from wood, which reduces usable wood volume for foresters and make trees vulnearble to bole snap. Trees with internal decay can become hazardous if located near homes, or in parks and recreation areas.
  • Tree death. Spruce trees with several large brooms sometimes die, although this does not typically occur. It is possible that severely affected trees are weakened, allowing other fungi, bark beetles, or abiotic factors to finally kill these trees.

Ecologically, the brooms appear to serve as important, perhaps critical, winter habitat for birds and mammals. Studies on flying squirrels in Interior Alaska indicate that squirrels create a cavity or hollow within the brooms and utilize the site for dens.

Management

Brooms offer refuge for many birds and small mammals, perhaps a desirable feature for some homeowners or woodlot owners. For managers or homeowners hoping to reduce disease, removal of the alternate host (all kinnikinnick within 1,000 feet of spruce), pruning of affected branches, or removal or affected trees during thinning operations are all strategies to reduce the incidence and severity of spruce broom rust. Unfortunately, many brooms occur on the main bole, where they cannot be easily removed by pruning; for effective control of bole brooms, the top of the tree (including the broom) would have to be removed.

Survey Method

Spruce broom rust can be mapped by aerial survey when brooms occur in the upper crown but is often more easily observed from the ground.

Distribution Map 

Locations with spruce broom rust in Alaska as of 2020.

The distribution of Chrysomyxa arctostaphyli in Alaska as of 2020 based on plot data, informal observations, and aerial survey with the distribution of spruce (Picea spp.) hosts in Alaska. Modeled host tree layers were developed by the Forest Health Technology Enterprise Team in 2011 (240m-resolution, presence based on dominant tree species by tree diameter). Click image for larger version.

Links to Resources & Publications

Hennon, P. E. and L. Trummer. 2001. Spruce Broom Rust. USDA Forest Service Leaflet R10-TP-100. USDA Forest Service Region 10. Available here

Schwandt, J. 2006. Management Guide for Spruce Broom Rust. USDA Forest Service. Available here.

Ziller, W. G.1974. The tree rusts of western Canada. Canadian Forestry Service, Dept. of the Environment, Victoria, B.C. Publication No. 1329.

 

Content prepared by Robin Mulvey, Forest Pathologist, Forest Health Protection, robin.mulvey@usda.gov, adapted from Hennon and Trummer 2001

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