sign in sign up
<<
Home , Arceuthobium abietinum, Mistletoe, Phoradendron, Phoradendron villosum, Viscum album

Growing Knowledge

Getting to know This Northwest-native parasitic can be viewed as a pest, as a resource, or both

By David Shaw Unbeknownst to most people, there are many throughout the Pacific Northwest, with the high- est diversity in Southern Oregon and Northern . There are 13 spe- cies of mistletoes in Oregon alone. Mistletoes are perennial, parasitic, flowering that attach to the above- ground stems of other plants. Some may be very specific, others not. There are more than 2,000 spe- cies of mistletoes in the large sandal- wood order (). The order includes five families, which have arisen independently. Mistletoes in the largest family, , have showy that are pollinated by . Different birds consume the and then disperse the . This family is mostly tropical and subtropical throughout the globe. Figure 1. True dwarf mistletoe ( abietinum) on white fir, Southern Oregon. In , the loranthaceous mistletoes are limited to Mexico. In the U.S., there are two genera both have separate male and female then disperse the seed. The dwarf in the family : Arceuthobium, plants. Their flowers are very small mistletoes, by contrast, discharge their the dwarf mistletoes (nine spe- and have nectar. However, the spa- explosively. It’s unusual for birds cies in Oregon; see Figure 1), and tial patterns of these mistletoes on the to consume the . , the leafy mistletoes landscape are driven by very different (four species in Oregon; see Figure 2). dispersal systems. Where mistletoes grow Dwarf mistletoes and leafy mistle- The leafy mistletoes produce a Mistletoes around the world are toes are dioecious, which means they that is consumed by birds, which mostly considered pests, especially ▲

An ongoing series provided by Oregon State University in collaboration with the United States Department of Agriculture and in partnership with OAN NOVember 2013 ▲ DIGGER 25 ▲ getting to know mistletoe

Figure 2. Dense mistletoe (Phoradendron densum) Figure 3. mistletoe () on Oregon white oak near Corvallis. on western , Southern Oregon.

on fruit and nut in tropical/sub- Oregon four species of Phoradendron seem to fit the same pattern; they tend tropical and India, and on pecan (Table 1). to occur mostly in the largest trees of and almond trees in the U.S. The oak mistletoe in Oregon their hosts and are clumped or aggre- The key principal hosts of dwarf white oak is rather poorly studied. The gated in patches. mistletoes in the Pacific Northwest northern range of oak mistletoe in the Mistletoe seeds are covered with include ponderosa , lodgepole Willamette Valley ends just south of a sticky substance called viscin. This pine, grand/white fir, western hemlock, Portland. For some reason, oak mistle- amazing substance allows the seed to and eastside and southwestern Oregon toe has not moved into the Columbia stick to its host or anything else. It is Douglas fir. The aerial shoots are small, River Gorge, nor has it crossed into also lubricated by wetting, allowing the perennial and leafless (Figure 2). They Washington. It has not colonized seed to slide down petioles, coni- can cause large deformations called in the eastern Cascade foothills of fer needles and twiglets, until it comes witch’s brooms, and have an impact Oregon, either. in contact with the branch, where it on growth, while the aerial shoots Like most mistletoes, the plant loves appears to “glue” the seed in place. remain diminutive. light and does very poorly in the shade, The seeds of Viscaceae are also For leafy mistletoes in the Pacific especially of conifers. The seed can only unique in that the endosperm has chloro- Northwest, the most common host is penetrate the bark of younger twigs, and phyll, unlike any other angiosperm seed. Oregon white oak. From the Willamette therefore colonizes the outer and upper Valley south through California oak canopy of trees. The perennial plant The mistletoe you know forests, large of oak mistletoe develops in place, eventually causing the Perhaps the best-known mistletoe is (Phoradendron villosum) grace the stem to swell at the infection site. the European mistletoe, album. canopies of many trees (Figure 3). The The mistletoe is initially in the It was used by pagans in , and juniper mistletoe (Phoradendron juni- outer canopy, but as the ages and today, it is commercially produced. It has perinum) shows up wherever juniper the branches continue to grow, it even- a host range of more than 400 mostly grows, particularly in the western juni- tually is shaded in the interior, unless hardwood tree species. V. album is com- per woodlands of eastern Oregon and the plant is in the upper crown of a posed of several subspecies which also south of Bend. mature tree. Often, one can observe a occur on pine and true fir in Europe. In Southern Oregon and Northern garden of mistletoe plants across the There are many folklore legends California, P. libocedri occurs on top of mature oak canopies. about mistletoe, from Frazer’s “golden incense cedar, and P. densum on Whenever the spatial patterns of bough,” which was a European mistle- juniper and cypress. In all, that gives mistletoe are studied, all mistletoes toe branch, to pagan winter fertility rites

26 NOVember 2013 ▲ DIGGER Figure 4. European mistletoe () on black locust near Sebastopol, Calif. that occurred under mistletoe-laden NURSERY tree crowns. And we all know about WHOLESALE kissing under the mistletoe during the season. Around the year 1900, the famous horticulturist Luther Burbank imported seed of the European mistletoe and introduced the plant to Sebastopol, Calif. on fruit trees. His idea was to 'JS#BSL/VHHFUTt)FNMPDLt4BXEVTUt8PPE$IJQTt$PNQPTU grow the mistletoe for the industry. The mistletoe per- FAST CUSTOM EXPERT sists to this day in Sonoma County, but DELIVERY NURSERY BLENDS ADVICE has only spread about 15 miles, occur- ring on horticultural trees such as black Delivery available throughout Western and Central Oregon 7 Days A Week. locust, poplar and (Figure 4). For Product Information call Mike Zentz or Tom Campbell: 888-345-9085 Luckily, the European mistletoe does not occur on most of the native trees in Sonoma County, such as red- woods, Douglas or native oaks. I personally have purchased boughs of V. album from Girl Scouts at the Safeway in Sebastopol. Apparently, when the idea of eradi- cation was proposed, the public was not supportive. The European mistletoe has also been reported in Vancouver, B.C.

Keeping it under control 2111 Prairie Rd. Eugene, OR. Should oak mistletoe be considered (888) 345-9085 www.laneforest.com a pest in Oregon? Well, that depends

NOVember 2013 ▲ DIGGER 27 ▲ getting to know mistletoe

on your perspective! Table 1. Leafy (Phoradendron spp.) mistletoes of Oregon, principal and other hosts, and general The general consensus is that a few geographic occurrence. plants in the crown won’t affect tree 1 2 3 4 vigor. Eventually, however, mistletoe can become well established enough in the host’s crown to cause decline and die- back. This can take decades to occur. The best-known control for oak mistletoe is pruning to remove and kill

it. Make the cut about 2 feet below the Principal Host Other Hosts Geographic distribution obvious swelling caused by the mistle- 1 Dense mistletoe Western juniper In Oregon, only Extreme Southern and toe plant. However, if an oak tree is (Phoradendron densum) Western juniper Central Oregon heavily infected, pruning out all the 2 Incense cedar mistletoe Incense cedar None Southern Oregon plants may remove all the branches, (P. libocedri) leading to decline of the host tree any- 3 Juniper mistletoe Western juniper In Oregon, only Central and Southern Oregon way. Therefore, it’s best to begin when (P. juniperinum) Western juniper there are few plants in a tree crown, 4 Oak mistletoe Oregon white oak, Manzanita, locust, alder, Western Oregon, I-5 corridor; before it becomes fully occupied. (P. villosum) California black oak, aspen, buckeye, European not in Portland city limits, There are also these lesser red oaks in cities birch, sycamore, walnut, willow but just south approaches: • Removing the aerial shoots. This and starlings are thought to feed on cially, but who is to say this couldn’t be will not kill the plant. The -like mistletoe berries and disperse the seed. done with our local species? endophytic system will stay alive in the Oak mistletoe infection introduces com- branch and can re-sprout. plexity to the tree crown, which diversi- David Shaw is an associate professor • Band-Aid approach. Some have fies the biota of oak canopies. of forestry and natural resources at tried removing shoots and wrapping The mistletoe infection site typically the OSU Extension Service. He can be the location with aluminum foil or duct becomes swollen. After many years, it reached at [email protected] tape to prevent re-sprouting. may produce a distinctive deformation or 541-737-2845. • Growth regulators. No herbi- that is colonized by decaying fungi, cides are registered for control of oak making habitat for cavity-nesting birds. References mistletoe, although the growth regu- Partially dead crowns with large defor- Hawksworth, F.G. and Wiens, lator Ethephon is registered for use. mations and many mistletoe plants D. (1996). Dwarf Mistletoes: , Ethephon causes the aerial shoots to make great wildlife habitat. Pathology, and Systematics. Agriculture fall off, but does not kill the plant. Handbook 709, U.S. Department of Therefore, the aerial shoots can come Markets for mistletoe Agriculture, Forest Service. back and require re-treatment. In Oregon, oak mistletoe is the Mallams, K.M. and R.L. Mathiasen. local mistletoe that is collected as a (2010). Mistletoes of hardwoods in Benefits to wildlife Christmas ornament. In California, oak the United States. Forest Insect and Oak mistletoe, a native plant, may mistletoe, European mistletoe and large- Disease Leaflet 147, U.S. Department of provide benefits to wildlife and other leaved mistletoe (P. macrophyllum) are Agriculture, Forest Service. biota in two ways. First, the plant itself collected and sold. The female plants Mathiasen, R.L., D.L. Nickrent, can provide them with resources. Second, with berries are the most sought-after D.C. Shaw, and D.M. Watson. (2008). the plant can improve tree habitat. plants for sale, as the white to pinkish- Mistletoes: Pathology, Systematics, Oak mistletoe flowers in mid-to- white berries add a delightful ornament Ecology, and Management. Plant late summer, providing a nectar/ to the evergreen plants. Disease 92:988–114. resource to pollinators at a time when If you purchase mistletoe plants in Shaw, D.C. and R.L. Mathiasen. most other native plants have already packaging, these may be P. serotinum (2013). Forest diseases caused by higher flowered out. The female plant may have from the eastern U.S., or the European parasitic plants: Mistletoes. Chapter many berries that mature in late fall and mistletoe that is collected by the ton in the 5. In: Gonthier, P. and G. Nicolotti. early winter, again providing a resource United Kingdom for . Infectious Forest Diseases. CABI, when other resources are scarce. Other than Luther Burbank’s Oxfordshire, UK. Birds such as American robins, attempt, I am not aware of anyone in The Connection: western , Townsend’s solitaire the U.S. who grows mistletoe commer- www.parasiticplants.siu.edu.

28 NOVember 2013 ▲ DIGGER