Hemlock Dwarf Mistletoe
Host(s) in Alaska:
western hemlock (Tsuga heterophylla) (primary host)
Sitka spruce (Piceae sitchensis), mountain hemlock (Tsuga mertensiana), true firs (Abies spp.), and shore pine (Pinus contorta subsp. contorta; never documented as host in AK)
Habitat(s): tree branches and boles
Current Status & Distribution in Alaska (2022 Update)
Hemlock dwarf mistletoe, a parasitic plant, is the leading disease of western hemlock in unmanaged old-growth stands in Southeast Alaska. Sitka spruce can be infected in areas with heavy disease pressure. Hemlock dwarf mistletoe brooms (prolific branching) provide important wildlife habitat and serve as infection courts for decay fungi, while tree mortality caused by severe infection creates canopy gaps. Overall, hemlock dwarf mistletoe is estimated to affect 12% of the forested land area in Southeast Alaska. Hemlock dwarf mistletoe brooms (prolific branching) provide important wildlife habitat and serve as infection courts for decay fungi, and tree mortality caused by severe infection creates canopy gaps.
The incidence of hemlock dwarf mistletoe does not vary noticeably between years, but 15 observations of the disease were made in Juneau, on Prince of Wales Island, and near Ketchikan. Additionally, three research grade observations were contributed through iNaturalist near Sitka and Tenakee Springs. Hemlock dwarf mistletoe is uncommon above 500 feet in elevation and 59°N latitude (Haines, AK) and is absent from Cross Sound to Prince William Sound despite the continued distribution of western hemlock (see Detection Map). Hemlock dwarf mistletoe is one of seven disease examples that we presented to demonstrate how a conceptual framework based on the plant disease triangle can be applied to better understand how climate change may influence tree disease behavior (Hennon et al. 2021, https://onlinelibrary.wiley.com/doi/10.1111/efp.12719). Other examples from Alaska include Dothistroma needle blight and yellow-cedar decline.
Hemlock dwarf mistletoe infection causes branch swellings and prolific branching called mistletoe brooms. Infection can also occur on or near the main tree bole. This parasitic flowering plant can only survive on live host tissue. It produces short male or female shoots (2-5 inches tall) on infected branches that have sufficient access to light. Dwarf mistletoe can survive long-term on shaded branches and boles without producing shoots, but significantly distorting tree form.
The pathogen is spread by explosively discharged seeds coated in a natural adhesive substance (viscin) that allows them to stick to host branches up to 50ft away. Mistletoe growing high in the overstory commonly infects understory trees. Infection occurs after seeds overwinter on tree bark, then germinate and penetrate the tree's vascular system to obtain water, nutrients and sugars. Successful infection, shoot production and seed dispersal takes several years, with the specific length of time dependent on climate, canopy position and other factors.
Dwarf mistletoe brooms increase habitat and species diversity by providing nesting platforms and structures for wildlife, creating canopy gaps, and serving as infection courts for decay fungi. Canopy gaps are created where overstory trees are killed, allowing mid-canopy trees (including those with mistletoe infection) to release to the overstory. Understory trees may have substantial mistletoe infection if they have been subjected to overhead inoculum (seeds) for many years or decades. Nevertheless, most infected trees will successfully grow into canopy gaps, where they may provide a source of inoculum to nearby trees. These dynamics allow for the perpetuation of dwarf mistletoe in mixed-age forests following its initial establishment. Short-distance spread is driven by seed dispersal between adjacent trees, while long-distance migration is attributed to birds (seeds sticking to feathers).
Clear-cutting virtually eliminates dwarf mistletoe in young-growth timber stands. Because the spread patterns of hemlock dwarf mistletoe and the severity of infection that causes growth impacts (dwarf mistletoe rating ≥3) and tree mortality (dwarf mistletoe rating of 5 or 6) have been quantified, managers can carefully retain some mistletoe-infected trees for wildlife benefits without significantly reducing timber yield (Trummer et al. 1998). The models developed by Trummer et al. show that greater size and infection levels of residual trees result in greater dwarf mistletoe ratings of regenerating hemlock crop trees (see Table 7 from Trummer et al. 1998). Hennon and McClellan (2003) found that snags had far greater incidence of mistletoe compared to live trees, which supports the observation that infection contributes to tree mortality.
See the synthesis report by Muir and Hennon (2007) for more detailed information.
The incidence of hemlock dwarf mistletoe changes little from year to year without active management. On the Tongass National Forest, removal of mistletoe-infected residual saplings (mistletoe sanitation) after clearcut harvest and prior to pre-commercial thinning was a common treatment in the 1970s and 80s to prevent infection in developing young-growth stands. This practice was discontinued because, without treatment, trees were shown to outgrow infection (height growth of trees surpassed the vertical spread of dwarf mistletoe). See the synthesis report by Muir and Hennon (2007) for more detailed information.
Our knowledge of hemlock dwarf mistletoe distribution comes from the U.S. Forest Service Forest Inventory and Analysis plot network, as well as informal ground observations. Forest Inventory and Analysis plot data may disregard hemlock dwarf mistletoe when present at low levels or when other damages are present, so extent of the disease is likely underestimated.
Trummer et al. (1998) installed research plots on Kuiu Island (78) and Chichagof (18) Islands to evaluate mistletoe spread patterns following partial disturbance from a high-severity wind event in the 1880s. Stem maps were produced in these stands depicting the size, species, live-dead status and dwarf-mistletoe rating of trees to determine how far mistletoe had spread into stands from relic infected western hemlock trees. This work produced and validated a model that predicts the dwarf mistletoe rating in the mature stand based on the basal area and dwarf mistletoe ratings of large and small residual trees. Hennon and McClellan (2003) evaluated the incidence of hemlock dwarf mistletoe on live and dead trees in 27 old-growth forests at three locales in Southeast Alaska (Hanus Bay on Baranof Island, Portage Bay on Kupreanof Island, and Lancaster Cove on Prince of Wales Island).
Dwarf mistletoe infection severity is quantified using a visual rating system (Hawksworth 1977). The live tree crown is visually divided into three equal vertical sections, with each section assigned a rating of 0 (no visible infection), 1 (one-half or fewer of the branches are infected), or 2 (more than one-half of the branches are infected). Inspection often requires the aid of binoculars. The scores for each one-third of live crown are then summed for an overall dwarf mistletoe rating (DMR) of 0-6. For hemlock dwarf mistletoe, detectable growth loss is observed when infected trees receive moderate ratings of 3 or greater. The risk of tree mortality increases with severe DMRs of 5 and 6.
Bakker, V. J.; K. Hastings. 2002. Den trees used by northern flying squirrels (Glaucomys sabrinus) in southeastern Alaska. Canadian Journal of Zoology 80(9): 1623-1633. Available here.
Barrett, T. M.; Latta, G.; Hennon, P. E.; Eskelson, B. N. I.; H. Temesgen. 2012. Host-parasite distributions under changing climate: Tsuga heterophylla and Arceuthobium tsugense in Alaska. Canadian Journal of Forestry Research. 42: 642-656. Available here.
Hawksworth, F. G. 1977. The 6-class dwarf mistletoe rating system. General Technical Report RM-48. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station 7 p.
Hennon, P. E.; Barrett, T. M.; D. Wittwer. 2011. The Distribution of Hemlock Dwarf Mistletoe Suggests Influences of Climate (Ch4 pp.74-78). In Forests of southeast and south-central Alaska, 2004–2008: five-year forest inventory and analysis report. Barrett, T. M.; Christensen, G. A., (tech. eds). Gen. Tech. Rep. PNW-GTR-835. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 156 p. Available here.
Hennon, P. E.; M. H. McClellan, 2003. Tree mortality and forest structure in the temperate rain forests of southeast Alaska. Canadian Journal of Forest Research, 33, 1621-1634. Available here.
Hennon, P. E.; Beatty, J. S.; D. Hildebrand. 2001. Hemlock Dwarf Mistletoe (FIDL). Forest Insect & Disease Leaflet 135. [Portland, OR:] U.S. Dept. of Agriculture, Forest Service, Forest Health Protection, Pacific Northwest Region. Available here.
Muir, J. A.; P. E. Hennon. 2007. A synthesis of the literature on the biology, ecology, and management of western hemlock dwarf mistletoe. Gen.Tech. Rep. PNW-GTR-718. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 142 p. Available here.
Shaw, D. C.; M. C. Agne, 2017. Fire and dwarf mistletoe (Viscaceae: Arceuthobium species) in western North America: contrasting Arceuthobium tsugense and Arceuthobium americanum. Botany 95(3): 231-246. Available here.
Trummer, L. M.; Hennon, P. E.; Hansen, E. M.; P. S. Muir. 1998: Modeling the incidence and severity of hemlock dwarf mistletoe in 110-year-old wind-disturbed forests in Southeast Alaska. Canadian Journal of Forest Research, 28, 1501-1508. Available here.
Content prepared by Robin Mulvey, Forest Health Protection, firstname.lastname@example.org.