Mistletoes in Colorado Conifers Fact Sheet No. 2.925 Gardening Series|Diseases by R.D. Koski , W.R. Jacobi and C.E. Swift* Dwarf mistletoes (Arceuthobium species) Quick Facts are leafless parasitic plants that infect several species of conifers in Colorado forests. Dwarf • Mistletoes are parasitic mistletoes produce root-like structures that flowering plants that can grow in the living tissue just under the bark infect and damage many (phloem) and in the wood (xylem), where tree species. they extract both nutrients and water from their host plants. Germinating seeds of • Dwarf mistletoes mistletoes produce specialized structures (Arceuthobium spp.) are called holdfasts that allow newly emerged leafless parasitic plants that parasitic plants to penetrate the tissues of host infect several coniferous or plants, thus infecting the host plant. evergreen tree species in There are five species of dwarf mistletoes Figure 1: Lodgepole pine infected with many western states. that infect conifers in Colorado (Table 1). dwarf mistletoe display witches’ brooms and While some dwarf mistletoes are relatively dead branches. • Ponderosa, lodgepole, host-specific and generally do not infect limber, and pinyon pines other tree species, other species infect a wide and Douglas-fir are the most range of coniferous tree species (Table 1). In common trees affected by addition, juniper mistletoe (Phoradendron dwarf mistletoes in Colorado. juniperinum) is also present in Colorado and is a different type of mistletoe that is less • Juniper mistletoe damaging compared to dwarf mistletoes, in (Phoradendron juniperinum) that it primarily acquires only water from is another type of mistletoe the host tree. Juniper mistletoe is found in that can infect several western Colorado and can infect several juniper species in many juniper species (Juniperus species) in many western states. western states (Table 1). Figure 2: Witches’ brooms – dense, multiple The dwarf mistletoes and juniper branches on lodgepole pine infected with mistletoe are dioecious plants, meaning male dwarf mistletoe. flowers and female flowers are produced on separate plants. The flowers produced by Closer examination of branches of these mistletoes are small and inconspicuous. affected trees will reveal the yellowish green, olive green, or reddish brown segmented Symptoms and Signs shoots of the parasitic plant (Figures 4 and 5). The first symptom of dwarf mistletoe When viewed from a distance, coniferous infection is a slight swelling of the bark at trees infected with dwarf mistletoes may the infection site. The parasite is identifiable appear to have yellow foliage, reduced foliage, when shoots protrude two to three years after abnormally dense green and distorted foliage infection. Dwarf mistletoe shoots are 0.7 to or witches’ brooms, and mortality of the 15 cm (1/2 to 6 inches) long and 2-4mm in upper portion of the affected tree (Figures 1, diameter (Figures 4 and 5). Douglas-fir dwarf 2, and 3). mistletoe shoots are hard to see because they ©Colorado State University are only about 7 mm (1/2 inch) long. When Extension. 2/99. Revised 12/13. *R.D. Koski, Colorado State University research shoots have fallen off, look for the remnants www.ext.colostate.edu associate, horticulture and landscape architecture; of basal cups on branches. W.R. Jacobi, professor, bioagricultural sciences and pest management; C.E. Swift, retired Extension horticulture agent, Tri-River Area, Grand Junction. 12/2013 Table 1. Mistletoes of Colorado conifers. Common names Mistletoe species Common name of host Host plant species Description Lodgepole pine Arceuthobium Lodgepole pine Primary Host: Pinus contorta var. Shoots are yellowish to olive green, dwarf mistletoe americanum ponderosa pine, whitebark pine, latifolia mean shoot height 5-9 cm, with bristlecone pine, limber pine, Occasional hosts: Pinus whorl-like branching; fruit is an ovoid mugo or mountain pine, Scots or ponderosa var. scopulorum, berry, olive green and 3.5-4.5 mm Scotch pine, Engelmann spruce, Pinus albicaulis, Pinus aristata, long and 1.5-2.5 mm wide blue spruce, subalpine fir Pinus flexilis, Pinus mugo, Pinus sylvestris, Picea engelmanii, Picea pungens, Abies lasiocarpa Limber pine Arceuthobium Limber pine Primary Host: Pinus flexilis Shoots are yellowish green, mean dwarf mistletoe cyanocarpum whitebark pine, bristlecone pine, Occasional hosts: Pinus shoot height 3 cm, with fan-like lodgepole pine, ponderosa pine, albicaulis, Pinus aristata, Pinus branching; fruit is a bluish (cyan)- southwestern white pine contorta var. latifolia, Pinus ovoid berry and 3.5 mm long and 2.0 ponderosa, Pinus strobiformis mm wide Pinyon dwarf Arceuthobium Pinyon pine Primary Host: Pinus edulis Shoots are olive green to brown, mistletoe divaricatum mean shoot height 8 cm, with fan- like branching; fruit is an ovoid berry, green/white and 3.5 mm long and 2.0 mm wide Douglas-fir Arceuthobium Douglas-fir Primary Host: Pseudotsuga Shoots are olive green, mean shoot dwarf mistletoe douglasii white fir, subalpine fir, menzeisii height 2 cm, with fan-like branching, Engelmann spruce, blue spruce Occasional Hosts: Abies fruit is an ovoid berry, olive green/ concolor, Abies lasiocarpa, white, and 3.5-4.5 mm long and 1.5- Picea engelmannii, Picea 2.0 mm wide pungens Southwest Arceuthobium Ponderosa pine Primary Host: Pinus ponderosa Shoots are orange to reddish brown dwarf mistletoe vaginatum subsp. Occasional Host: Pinus aristata, (sometimes very dark brown), mean cryptopodum Pinus contorta var. latifolia, Pinus shoot height 10 cm, with extensive flexilis, Pinus strobiformis, Pinus branching; fruit is an ovoid berry, sylvestris bi-colored, and 4.5-5.5 mm long and 2.0-3.0 mm wide Juniper Phoradendron Rocky Mountain juniper, Utah Primary Hosts: Juniperus Shoots are green to yellow green, mistletoe juniperinum juniper, oneseed juniper scopulorum, Juniperus leafless, mean shoot height 20-40 osteosperma, Juniperus cm; plants globose, with extensive monosperma branching; fruit is an ovoid berry, pinkish-white, and 4 mm in diameter

After initial infection, mistletoes can Dwarf mistletoe infested trees decline and cause distorted branching or witches’ die from the top down as witches’ brooms brooms in the host tree (Figure 2). When on lower branches extract more nutrients dwarf mistletoes infect occasional hosts and water (Figures 1 and 3). Death of the – hosts other than the primary host – host tree occurs slowly in most cases and different and unique symptoms may depends on the severity of infection and on occur. For example, lodgepole pine dwarf the vigor and size of the tree. mistletoe causes very large and dense A measure of dwarf mistletoe infection witches’ brooms when it infects ponderosa severity is based on a two-part rating pine (Figure 7). Lodgepole pine dwarf system (Tables 2a and 2b). First, a tree’s mistletoe also induces on limber pine, crown is divided into thirds, and each third large elongate galls with rarely any shoots is rated. If there are no visible infections, present. that third of the crown is rated 0; if 1 to Juniper mistletoe plants are usually 50 percent of the branches are infected seen as large yellowish-green round in that third, the rating is 1; and if more masses of small branches in the crown than 50 percent of the branches are of junipers (Figure 6). Juniper mistletoe infected, the rating is 2 (Table 2a). Add the does not induce the juniper to make ratings of each third to get a total dwarf witches’ brooms. mistletoe rating (Table 2b). Life expectancy information for trees afflicted with dwarf Figure 3: Lodgepole pine with dead top and mistletoe is provided in Table 3. Damage to Host Trees dense dwarf mistletoe-induced brooms on the Witches’ brooms develop over many lower stem. Dwarf mistletoe witches’ brooms extract years. Dwarf mistletoe witches’ brooms nutrients from uninfected parts of the extract nutrients and water from uninfected tree, gradually reducing host tree vigor parts of the tree, gradually reducing and eventually causing premature death. host tree vigor and eventually causing Table 2a. Rating scale for dwarf mistletoe infection severity. Percent of Tree Crown Section Infected with Tree Crown Section Rating Dwarf Mistletoe1 0 0 1-50 1 51-100 2 1 Top third, middle third, and bottom third of tree.

Table 2b. Dwarf mistletoe rating system (DMR). Rating1 Infection Severity 2-3 Light 4-5 Moderate 6 Heavy Figure 4: Ponderosa pine dwarf mistletoe plants. 1 Sum of rating of top third, middle third, and bottom third of tree.

Table 3: Approximate number of years needed to kill 50 percent of Ponderosa pine trees infected with dwarf mistletoe.1 Tree Diameter Infection Severity 2 Light Moderate Heavy 4-9 inches 30 17 7 Greater than 9 inches 60 25 10 1 Based on ponderosa pine in open, multi-aged stands. 2 Infection severity based on the following: light = a rating of 2 or 3; moderate = a rating of 4 or 5; heavy = a rating of 6.

premature death. Junipers infested with (six to eight years). The long life cycle Figure 5: Lodgepole pine dwarf mistletoe plants. juniper mistletoe will have clusters of the allows for long-term disease management. Note thin green-yellow shoots. yellowish green parasitic plant growing Mistletoes are not common in nursery and amongst the juniper foliage (Figure 6). ornamental plantings, but the parasites infected trees may become sufficiently Juniper mistletoe witches’ brooms can be introduced into an area by planting stressed as to attract mountain pine bark extract primarily water from the host trees unknowingly infected with mistletoe. beetle (MPBB), Ips bark beetles, and twig plant. On heavily infested trees, the Dwarf mistletoes can greatly impact the beetles that may breed and kill parts (twig parasite can cause death of portions of the forest structure and appearance. Areas of and Ips) of or the whole tree (MPBB, Ips). tree during prolonged periods of below expanding numbers of infected trees can be Mistletoe management options include normal precipitation. found in the forest where the center of the branch pruning, tree removal, and planting area or “donut hole” has no large trees left resistant tree species. since they were infected years ago and died, Spread and there might be some heavily infected Dwarf mistletoes spread slowly from understory trees in the center and a ring of Management Options: tree to tree. In closely spaced trees of heavily infected trees at the margin. about the same height, this spread is 0.3 to The fruits and seeds ofPhoradendron 1. Pruning infected lower branches 0.6 m per year. The spread from large to juniperinum are spread by birds and and removing trees is the small trees can extend 19 m (60 ft), but the consequently can be spread a great distance best management measure average usually is less than 9 m. in a single season. A number of bird available to reduce or eliminate The sticky seeds ofArceuthobium species feed on the juniper mistletoe fruits dwarf mistletoe infestations in species are explosively discharged from the and disperse the seeds by excreting or ornamental trees or urban forests. fruit at almost 60 miles per hour, adhering regurgitating them. Seeds are deposited on to any surface they strike. Seeds that the top side of branches of juniper hosts. 2. Plant resistant trees under adhere to young branches of susceptible Germinating seeds produce a holdfast that infected trees to replace trees trees germinate, and the mistletoe rootlet penetrates the host plant. when infected ones are removed. penetrates the bark. Dwarf mistletoe seeds generally are dispersed in August 3. Use ethephon sprays in high- and early September. Birds and other Management value areas where planting with animals can occasionally spread the seeds Mistletoes cause a gradual decline the same species under infected some distance to uninfected trees. Dwarf of plant health and do not quickly cause trees is the only option. mistletoes have a relatively long life cycle serious injury, thus long-term management between infection and seed production options are feasible. However, mistletoe- Branch Pruning and Tree Removal Planting Resistant Tree Species Pruning out the witches’ brooms Planting resistant or non-host tree and removing infected trees is the best species in areas with infected trees will management measure available to reduce ensure that trees will be in the area even or eliminate dwarf mistletoe infestations after the infected trees are removed in stands of high-value trees. First, remove (Table 4). severely infected trees (trees rated 5 and 6) NOTE: Scotch or Scots pine (Pinus or those with only a few live branches. sylvestris) is susceptible to both southwest Trees with high, unreachable mistletoe and lodgepole pine dwarf mistletoes. infections will continue to shower seeds Deciduous trees and shrubs, such as Figure 6: Juniper mistletoe. Note the globose on nearby trees if not cut down. However, birch, peashrub, ash, aspen, cottonwoods, growth in this juniper. it is not necessary to completely eradicate and Gambel oak also can be planted in the mistletoe, as this may require removal affected areas because dwarf and juniper of all trees. Pruning infected branches and mistletoe do not attack these plants. Keep removal of a few heavily infected trees can in mind that site conditions and moisture keep a green forest on the property. availability will determine what trees and Pruning off the lower and the largest shrubs can be planted in a particular area. witches’ brooms from lightly to moderately infected trees (trees rated 1 to 4) can Chemical Sprays improve the health and allow these trees Ethephon (2-chloroethanephosphonic to survive for decades. When removing a acid) is a growth regulating chemical that witches’ broom, prune the entire branch at can be used to remove mistletoe shoots the branch collar near the trunk. Examine and reduce seed production. Ethephon trees every two or three years, and remove is usually only used in high value areas any newly infected branches. When where young trees should be protected until pruning, keep 30 percent to 40 percent infected overstory trees are removed. This of the branches on the tree (from the top treatment does not kill the entire mistletoe down), even if that means leaving some plant, just the shoot. Retreatment is infected branches. Mistletoe shoots die as necessary until infected trees are removed, soon as the tree branch is cut, so no special mistletoe infections are pruned from the disposal of pruning debris is needed. tree, or new non-host trees are planted. Figure 7: Dense dark green brooms on Trunk infections are not as detrimental ponderosa pine infected with lodgepole pine as branch infections, so their removal is dwarf mistletoe. not necessary. If space allows, create 18 m Additional Information buffer zones between infected trees and Colorado Master Gardener Garden healthy trees by cutting or by planting Notes (available from the Colorado Master resistant trees. Contact a professional Gardener Program website, www.cmg. forester, the Colorado State Forest Service, colostate.edu/gardennotes.shtml or other professionals to obtain help in • GardenNotes #618, these decisions. Pruning Evergreens • GardenNotes #633, Table 4. Recommended species to replant in mistletoe affected areas. The Science of Planting Trees Mistletoe-infected Species Recommended Species Fact Sheets (available from the CSU ponderosa pine white fir (Abies concolor), blue spruce (Picea pungens), Extension website www.ext.colostate.edu/ bristlecone pine (Pinus aristata), pinon pine (Pinus edulis), pubs/pubs.html limber pine (Pinus flexilis), Douglas-fir (Pseudotsuga menziesii), • Fact Sheet 5.558, Ips Beetles and/or Rocky Mountain juniper (Juniperus scopulorum) • Fact Sheet 5.528, Mountain Pine Beetle lodgepole pine subalpine fir (Abies lasiocarpa), Engelmann Spruce (Picea engelmannii), bristlecone pine, limber pine and/or Douglas-fir Geils, B.W., Tovar, J.C., and Moody, B. 2002. Mistletoes of North American Douglas-fir Engelmann spruce, lodgepole pine, and/or ponderosa pine Conifers. Gen. Tech. Rep. RMRS-GTR-98. juniper Ponderosa pine and/or pinyon pine Ogden, UT: USDA, Forest Service, Rocky Mountain Research Station. 123p.

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