266 Watson and Martinez-Trinidad: Leafy Mistletoe Suppression on Cedar Elm from the tree by pruning out the mistletoe shoots at the branch surface. Although the mistletoe might quickly resprout, some benefit is derived by annual mistletoe removal because it reduces seed production and spread of mistletoe within and among trees (Scharpf and Hawksworth 1974). Pruning of infected branches or covering affected areas with black plas- tic, paper, or aluminum foil has also been used with some success (Harris et al. 1999). These treatments work by pro- hibiting photosynthesis of the haustorium by opaque cover- ings. However, these treatments can be impractical in heavily infested trees as well as aesthetically objectionable. Several growth regulators and herbicides have been tested that kill mistletoe shoots, but mistletoe can resprout from the haustoria several months after application (Lichter et al. 1991; Wood and Reilly 2004). Some of these chemicals such as dicamba also cause damage to the host (Wood and Reilly 2004). Ethephon, an ethylene-type compound, is currently labeled for control of mistletoe (Monterey Chemical Co., Fresno, CA), but this growth regulator has shown limited effectiveness at labeled rates (Adams et al. 1993). Auxin-type compounds such as 2,4-D have been effective, but it is un- likely that a product, which is primarily used as an herbicide, will ever be labeled for aerial spray applications in trees. Other auxin-type growth regulators such as 1-naphtaleneace- tic acid, ethyl ester (NAA), that are labeled for control of sprouts and suckers in trees may provide useful control of leafy mistletoe in woody plants. As a result of the lack of effective and practical means for controlling mistletoe, alternative methods for effectively con- trolling leafy mistletoe in urban environments need to be developed. The purpose of this article is to test the efficacy of several current and potential methods for controlling P. to- mentosum in U. crassifolia. MATERIAL AND METHODS Experiment 1 Cedar elm trees infested with P. tomentosum served as hosts to evaluate the effect of different methods for controlling leafy mistletoe. In the first experiment in 2002, eight treat- ments were considered. Five replicate trees 20.3 to 30.5 cm (8.1 to 12.2 in) dbh per treatment were studied for a total of 40 trees. Within each tree, five mistletoe plants (>20.3 cm [8.1 in] canopy spread diameter) were randomly selected within the canopies, resulting in 200 mistletoe test subjects. Remaining mistletoe plants were removed to avoid seeds dropping and sprouting into the test areas. The treatments consisted of removing mistletoe from the branch; removing mistletoe and branch to the nearest union (>30.5 cm [12.2 in] below the mistletoe); spraying mistletoe with a 5% ethephon solution ([2-chloroethyl] phosphonic acid) until runoff (Flo- rel, Lawn and Garden Products, Fresno, CA); removing mistletoe and spraying with 1.15% solution of ethyl 1-naph- thaleneacetic acid (Sucker Stopper Concentrate, Monterey ©2006 International Society of Arboriculture Chemical Co., Fresno, CA); spraying mistletoe with 2.5% solution of iso-octyl (2-ethylhexyl) ester of 2,4-dichloro- phenoxyacetic acid and isooctyl (2-ethylexyl) ester of 2-(2,4- dichlorophenoxy propionic acid) solution (Brush Buster, Lawn and Garden Products, Fresno, CA); spraying mistletoe with 1.5% glyphosate solution (Roundup, Monsanto, MO); removing mistletoe and applying a latex based caulk NP-1™ (Sonneborn, Shakopee, MN); and the controls. Spray and caulk treatments were applied to the mistletoe shoots and/or the bark to cover the entire branch out to 15.2 cm (6.1 in) on each side of the mistletoe. Treatments were applied during the third and fourth week of February 2002 on dormant cedar elm trees located at the Texas A&M University campus (College Station, TX). Treated trees were assessed on April 2002 and July 2002. Treated plants exhibiting more than 80% mortality were evaluated again on July 2004. During the first two evalua- tions, the mistletoe condition was scored based on visual assessment. The scale used during the assessment consisted of: 0not present, 1dead, 2severely damaged, 3 curling, 4 yellowing, and 5 no apparent damage. Mistletoe regrowth was measured in July 2004. Experiment 2 Dormant cedar elms similar to those used in experiment 1 were used. Based on the results from experiment 1, the effi- cacy of a modified treatment from experiment 1 was evalu- ated. To improve the results achieved with NAA in experi- ment 1, the treatments consisted of controls and a treatment to assess the efficacy of NAA mixed with black latex paint to would provide added benefit by excluding light from the haustorium. The evaluated chemical compound consisted of a mixture of 1.15% NAA (Sucker Stopper Concentrate, Mon- terey Chemical Co., Fresno, CA), black latex-based paint (Benjamin Moore & Co., Montvale, NJ), and distilled water in a ratio 0.65:3.35:6.0, respectively. An average of 14.8 mL (0.44 fl oz) of the mixture was sprayed to cover the entire surface of the branch 15.2 cm (6.1 in) beyond either side of the cut mistletoe. The control treatment consisted of simply removing the mistletoe from the branch so that regrowth rates could be determined. Treatments were applied during the first week of March 2003 using cedar elms located at the Texas A&M University campus in College Station, Texas. The percentages of re- sprouted mistletoe were evaluated every month from April to July, in October 2003, and finally in July 2004. Mistletoe height was measured once during July 2004. The treatments were randomly assigned, and the data from both experiments were analyzed using SPSS. In the case of mortality, regrowth, and shoot length, the variables were transformed by adding 0.5 to the value and determining the square root. Mean com- parisons were determined using LSD with a P < 0.05. The condition values were analyzed using the Kruskal-Wallis test
November 2006
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