Arboriculture & Urban Forestry 32(6): November 2006 Arboriculture & Urban Forestry 2006. 32(6):265–270. 265 Strategies and Treatments for Leafy Mistletoe (Phoradendron tomentosum [DC.] Engelm ex. Gray) Suppression on Cedar Elm (Ulmus crassifolia Nutt.) W. Todd Watson and Tomas Martinez-Trinidad Abstract. Leafy mistletoe, Phoradendron tomentosum (DC.) Engelm ex. Gray, can adversely affect trees growing in urban environments. The efficacy of several methods for controlling P. tomentosum in Ulmus crassifolia was evaluated in two separate trials. In the first experiment, eight treatments with five replicate trees, 20.3 to 30.5 cm (8.1 to 12.2 in) dbh, were evaluated. Removal of the branch 30.5 cm (12.2 in) below the mistletoe, removal of mistletoe, and treating the branch bark with naphthaleneacetic acid (NAA) or a caulking compound in which mistletoe was removed resulted in reduced regrowth of the ectophyte (>90%) after 5 months. The use of growth regulator and herbicides (ethephon, 2,4-D, and glyphosate) on intact mistletoe plants in experiment 1 did not provide acceptable control of mistletoe. After 29 months, only removal of the branch and caulking over the bark after mistletoe removal demonstrated a significant long-term effect on mistletoe mortality (40% and 57%, respectively). In the second experiment, a new treatment was evaluated based on the results from the first experiment. The use of NAA and light exclusion (black latex paint) reduced the resprouting of mistletoe by 50% after 8 months, but this effect diminished over time. However, 16 months after application, NAA and paint significantly reduced regrowth compared with removing mistletoe alone. This study provides strategies to achieve acceptable short-term control and long-term management of leafy mistletoe in urban trees. Key Words. Auxin; Christmas mistletoe; herbicide; parasitic plant; pest. Mistletoe is a parasitic plant that grows on conifer and hard- wood trees in forest and urban environments around the world (Michailides et al. 1987; Lichter et al. 1991; Geils et al. 2002). Although mistletoe is photosynthetic, it is an obligate, semiparasitic evergreen plant that infects host plants to derive support, water, and essential elements (Scharpf and Hawks- worth 1974; Coder 2004). In urban environments, they are considered a nuisance because of their appearance in decidu- ous trees during the winter. When colonization is extensive in individual trees, mistletoe can adversely affect tree health and cause decline and death of trees (Paine and Harrison 1992). Phoradendron tomentosum (DC.) Englem. ex Gray, a leafy mistletoe species, primarily infests broad-leaved tree species such as hackberry, mesquite, oak, and elm in USDA zone 6 and warmer in the United States (Scharpf and Hawksworth 1974); however, it also commonly infests hickory, beech, poplar, walnut, pecans, cherry, and other tree species (Paine and Harrison 1992; Coder 2004; Wood and Reilly 2004). Although some mistletoe species show host specialization (Norton and Carpenter 1998), new sites (Melgar 2002) and new host species (Gilbertson and Mathiasen 2001) have been reported for broadleaf mistletoe. Therefore, studying control methods for mistletoe in urban forests is important as a result of the particular interactions of different tree species in sev- eral environments. The primary mode of dispersion of mistletoe is by birds, which complicates the control of this parasitic plant. Birds deposit mistletoe seeds in a heterogeneous pattern creating a clumped distribution among hosts (Aukema and Martinez del Rio 2002). After seeds germinate, they produce an hausto- rium or root-like structures that penetrate the host to extract water and minerals (Paine and Harrison 1992). This endo- phytic portion results in a challenging control problem be- cause treatments must kill the outer ectophytic portion of the plant as well as the endophytic portion without damaging the host. This is analogous to management of fungal diseases in plants. Although the control of broadleaf mistletoe in urban forests can be difficult, several control methods have been tested, including branch removal, the use of ethylene, 2,4-D, glypho- sate, shoot removal, and sunlight exclusion (Joyce et al. 1984, 1990; Lichter et al. 1991; Wood and Reilly 2004). Tradition- ally, arborists have controlled leafy mistletoe by using several different mechanical strategies. Mistletoe can be removed ©2006 International Society of Arboriculture
November 2006
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