Arboriculture & Urban Forestry 32(6): November 2006 267 for determining significant differences among the treatments and the Wilcoxon test for determining significant differences (P < 0.05) of particular pairs of treatments. RESULTS AND DISCUSSION Experiment 1 None of the treatments resulted in visible short-term or long- term phytotoxicity to the host. Where the mistletoe was re- moved, all treatments (mistletoe removal, removal of the branch below the mistletoe, spraying the cut surface with NAA, or applying NP-1 to the branch) exhibited significantly higher mortality of the ectophyte at 5 months after treatment than other treatments in which the mistletoe was left intact (Figure 1A). These results mirrored other studies that dem- onstrated that mechanical controls (pruning out infested branches and light exclusion) can be highly effective at con- trolling mistletoe regrowth (Lichter et al. 1991). However, pruning may be impractical for heavily infested trees as a result of the negative impact on tree aesthetics (Paine and Harrison 1992), structure, or health. No prior studies have documented the impact of dark-colored caulking compounds to prevent photosynthesis of the haustoria. However, Lichter et al. (1991) observed similar results when using black plastic or pruning paint instead of caulk. Although caulking may be easier to maintain and may be aesthetically less obvious than black plastic, we found the treatment to be physically cum- bersome and less uniform. There are no prior studies that can be used for comparison in which NAA was used to control leafy mistletoe. However, Wood and Reilly (2004) used an auxin-type compound, 2,4- D, on broadleaf mistletoe (Phoradendron flavescens) in pe- can trees (Carya illinoinensis). They achieved 90% and 100% mortality and no resprouting 1 year after application with three applications of 2,4-D applied at concentrations of 0.23% and 1.01% per application. Although we achieved excellent short- term results with NAA, we found 2,4-D to be ineffective. The difference in efficacy may be the result of formulation, tim- ing, number of applications, or environmental factors. For example, Michailides et al. (1987) reported effective control 1 year after treatment when applying a mixture of 2,4-D and dicamba on leafy mistletoe, but only a 58% and 60% control after 1.5 years on white alder (Alnus rhombifolia) and Chi- nese hackberry (Celtis sinensis), respectively. Regardless, it is unlikely that 2,4-D will ever be labeled for mistletoe con- trol because of environmental and regulation concerns. Michailides et al. (1987) found that growth regulators can affect the growth rate of mistletoe for 1 year after application. We compared the condition of treated mistletoe that had not been removed (spraying ethephon, 2,4-D, or glyphosate). These growth regulators, applied at labeled rates to control mistletoe or broadleaf weeds, did not significantly affect the health or condition of the mistletoes (Figure 1B). In most cases, the mistletoe exhibited chlorotic leaves. These data are supported by other studies on mistletoe (Joyce et al. 1984; Lichter et al. 1991; Wood and Reilly 2004). Lichter et al. (1991) were unable to achieve acceptable results with glypho- sate (5%) when used against Phoradendron villosum on blue oak (Quercus douglasii) or P. macrophyllum on honeylocusts Figure 1. Ectophyte mortality (A) and condition (B) of P. tomentosum at 2 and 5 months after application of different control methods in experiment 1 on U. crassifolia at College Station, Texas (0 = not present, 1 = dead, 2 = severely damaged, 3 = curling, 4 = yellowing,5=no apparent damage). Mortality data showed without transformation. Different letters between treatments within the same date indicate significant differences (P < 0.05) using LSD after transformation for mortality and the Wilcoxon test for condition. ©2006 International Society of Arboriculture
November 2006
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