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Mistletoe in Oaks

      Mistletoe is a parasite in both the Blue Oak (Quercus douglasii) and the Valley Oak (Q. lobata). Our oak mistletoe is Phoradendron villosum, found from California, Arizona, to West Texas. Mistletoe has both male plants that produce only pollen, and female plants (see photo) that produce flowers and fleshy, white seed pods. Each pod is filled with a slimy and sticky clear fluid (ochra anyone?) and one seed covered with a tough greenish membrane (see photos below). Seeds mature in the winter.

 


       Various extracts from mistletoe are being investigated for treating cancer in humans, including ovarian cancer, lymphoma, and others. However, our mistletoe is very poisonous and should not be eaten or even nibbled.

 

 

 


       Mistletoe fruits are attractive to wintering birds in the Santa Lucias. Birds can digest most of the outer white shell of the flower and some of the sticky, clear liquid. When a bird ingests the seed, the tough outer membrane stays intact.

 

 

 

 

 

      Seeds are rapidly defecated by birds (Western Bluebirds, Robins, Cedar Waxwings, among others) and they still have their slimy, sticky coating. This allows the seeds to cling to a branch, sprout and insert its root-like "haustoria" into the water-conducting system of the tree. It takes many years for the mistletoe to grow large enough to produce flowers and seeds. The haustoria in mistletoe both penetrate the water-conducting tissue of the trees (water transport) and infiltrate in between the cells where they absorb most nutrients. In many populations of mistletoe, the number of male plants (or female plants) is higher than one would expect (50:50).


       Mistletoe can grow into very large, heavy clumps. But, does it damage the tree? Certainly it makes some branches more likely to be broken in winter storms, just due to the increased weight load. In Spain, trees grew at about the same rate with and without mistletoe. Such experiments have not been done here in California, yet.

      Why are some trees infected with mistletoe, and others lack mistletoe? In Ponderosa pine, there appear to be chemical differences in the trees that are infected, suggesting that some may be simply more resistant. If nothing else, it seems that the chances of being infected with mistletoe are greatest if a tree is near another tree previously infected.

       Do wildlife use the big clumps of mistletoe? Yes- there appears to be a greater diversity of birds seen in trees with mistletoe, and some raptors (birds of prey; Mexican Spotted Owl and Goshawk) nest preferentially in trees with mistletoe. Some species of "possums" in Australia are specialists on mistletoe and require mistletoe, just as pandas require bamboo. Here in the western United State, martens (native in the weasel family) select mistletoe clumps, often as shelter from radiation heat loss on clear, cold winter nights.

       What controls mistletoe? Apparently fire can control mistletoe, and I often wonder if the continuous grazing (fuel removal) and subsequent almost complete lack of fires in the oak woodlands of California is responsible for the abundance of mistletoe in many places. Again, experimental fires, repeated frequently, might tell. Given the new acceptance of fire by the California ranching community, an opportunity for such a study might arise.

       If you had a big oak in your back yard (or you built your house under a big Blue Oak or Valley Oak) and you were concerned about a massive infection of mistletoe, what should you do? Well, there are control options and the UC Integrated Pest Management Project has some suggestions on a website but who knows? Several studies suggest that the massive pruning required to remove the mistletoe will harm the tree. Pruning your oak might make it more susceptable to harmful insects or fungus. But, if the infestation is small, you can cut out the mistletoe and then cover the entry point of the mistletoe with black plastic to kill (no light) the re-emerging mistletoe stems.

        Mistletoe is probably just another one of the things on a oak that takes 300 years to kill a big tree.



References (In no Particular Order)


Linhart, Yan B.; Snyder, Marc A.; Gibson, J. Phil. Differential host utilization by two parasites in a population of ponderosa pine. In: Oecologia (Berlin) 1994. 98 (1): 117-120.

Sterba, H.; Andrae, F.; Pambudhi, F.. Crown efficiency of oak standards as affected by mistletoe and coppice removal. In: Forest Ecology and Management . 62 (1-4): 39-49.

Elsaesser-Beile, U.; Voss, M.; Schuehle, R.; Wetterauer, U.. Biological effects of natural and recombinant mistletoe lectin and an aqueous mistletoe extract on human monocytes and lymphocytes in vitro. In: Journal of Clinical Laboratory Analysis 2000. 14 (6): 255-259.

Buchanan, Joseph B.; Irwin, Larry L.; McCutchen, Edwin L.. Characteristics of spotted owl nest trees in the Wenatchee National Forest. In: Journal of Raptor Research 1993. 27 (1): 1-7.

Reid, Nick. Control of the mistletoes by possums and fire: A review of the evidence. In: Victorian Naturalist (Blackburn) 1997. 114 (3): 149-158

Elias, Pavol. A male-based sex ratio in mistletoes. In: Biologia (Bratislava) 1997. 52 (1): 49-51.

Hadfield, James S.; Flanagan, Paul T.. Dwarf mistletoe pruning may induce Douglas-fir beetle attacks. In: Western Journal of Applied Forestry Jan., 2000. 15 (1): 34-36.

Overton, Jacob M.. Spatial autocorrelation and dispersal in mistletoes: Field and simulation results. In: Vegetatio 1996. 125 (1): 83-98.

Lopez-de Buen, Lorena; Ornelas, Juan Francisco. Frugivorous birds, host selection and the mistletoe Psittacanthus schiedeanus, in central Veracruz, Mexico. In: Journal of Tropical Ecology May, 1999.

McNamee, D.. Mistletoe extract ineffective in melanoma. In: Lancet (North American Edition) Sept. 25, 1999. 354 (9184): 1101.

Zarkovic, N.; Zarkovic, K.; Grainca, S.; Kissel, D.; Jurin, M.. The Viscum album preparation Isorel inhibits the growth of melanoma B16F10 by influencing the tumor-host relationship. In: Anti-Cancer Drugs 1997. 8 (SUPPL. 1): S17-S22.

Parks, Catherine G.; Bull, Evelyn L.; Tinnin, Robert O.; Shepherd, Jay F.; Blumton, Arlene K.. Wildlife use of dwarf mistletoe brooms in Douglas-fir in northeast Oregon. In: Western Journal of Applied Forestry April, 1999. 14 (2): 100-105.

Hadfield, James S.. Douglas-fir dwarf mistletoe infection contributes to branch breakage. In: Western Journal of Applied Forestry Jan., 1999. 14 (1): 5-6.

Parks, Catherine G.; Bull, Evelyn L.. American marten use of rust and dwarf mistletoe brooms in northeastern Oregon. In: Western Journal of Applied Forestry 1997. 12 (4): 131-133.

Krenzelok, E. P.; Jacobsen, T. D.; Aronis, J. M. (North American Congress of Clinical Toxicology Annual Meeting Rochester, New York, USA September 16-19, 1995). Mistletoe exposures: The kiss of death? In: Journal of Toxicology Clinical Toxicology 1995. 33 (5): 543

Ashworth, Vanessa E. (Annual Meeting of the Botanical Society of America held with the American Institute of Biological Sciences Seattle, Washington, USA August 3-8, 1996). Phylogenetic relationships within the mistletoe genus Phoradendron (Viscaceae). In: American Journal of Botany 1996. 83 (6 SUPPL.): 137-138.

Seamans, Mark E.; Gutierrez, R. J.. Breeding habitat of the Mexican Spotted Owl in the Tularosa Mountains, New Mexico. In: Condor 1995. 97 (4): 944-952.

Calvin, Clyde L.; Wilson, Carol A.. Relationship of the mistletoe Phoradendron macrophyllum (Viscaceae) to the wood of its host. In: IAWA Journal 1995. 16 (1): 33-45.