Arboriculture & Urban Forestry 32(2): March 2006 71 6). There was no significant difference in disease incidence between whips treated with one-time foliar applications of PBZ (treatments 4 and 5) and whips treated every 2 weeks with propiconazole (treatment 3) at the time this evaluation was made in mid-June. Significant growth reduction in these PBZ-treated whips shows rapid absorption of the compound by the plants (Figure 5). This strongly suggests that PBZ, in comparison to propiconizole, is either more persistent on the leaf surface (PBZ residue was still visible on the plant surface in October) and is more effective in preventing infection, has greater eradicative ability because of its systemic properties, or both. Another possibility is that PBZ-induced effects on plant growth and development affect the host–parasite inter- action, reducing disease incidence in treated plants. CONCLUSIONS No difference was observed in disease incidence that would suggest that PBZ, even at the 2 × rate, was successful in controlling apple scab during the year of treatment in ‘Snow Drift’ or ‘Hopa’ crabapple cultivars (Figures 3A and 4A, respectively). In the years after treatment, substantial reduc- tions in shoot growth were accompanied by small and spo- radic reductions in disease incidence in PBZ-treated trees. Even when sufficient time was given for compound translo- cation and subsequent growth reduction in the crown, apple scab occurrence was not sufficiently reduced in PBZ-treated trees to prevent the unacceptable levels of defoliation that are characteristic of this disease. Near-total defoliation by mid- summer occurred in all treatments throughout the study, sug- gesting that at the rates tested, PBZ applied as a basal drench was not an effective treatment to control apple scab on ‘Snow Drift’ or ‘Hopa’ crabapples. The superficial nature of this disease, and the fact that exposure of the fungus to PBZ is unlikely until after infection has occurred, may account for these observations. A basal drench applied to ‘Indian Magic’ saplings at the time of planting was equally ineffective in reducing apple scab in the year of treatment. However, a one-time foliar application of PBZ to ‘Indian Magic’ saplings (treatments 4 and 5) was very successful in controlling apple scab, while the one-time application of propiconazole did not effectively control apple scab (treatment 6). Reduction of disease inci- dence in saplings by treatment with a foliar application of PBZ was as effective as a standard spray program with propi- conazole, resulting in a greater than 90% reduction in disease incidence compared to the controls (Figure 6). Additional investigations of the mode of action and persistent effective- ness for the control of apple scab of a single foliar application of PBZ early in the growing season are needed. PBZ applied to the foliage for control of apple scab and other woody plant disease could be a unique tool in the management of land- scape or orchard trees where growth regulation is acceptable or desired. Acknowledgment. This research was funded in part by Rainbow Treecare Scientific Advancements, St. Louis Park, MN. LITERATURE CITED Ali, A., R. Hall, and R.A. Fletcher. 1979. Inhibition of fungal growth by plant growth retardants. Canadian Journal of Botany 57:458–460. Burden, R.S., G.A. Carter, T. Clark, et al. 1987. Comparative activity of the enantiomers of triadimenol and paclobut- razol as inhibitors of fungal growth and plant sterol and gibberellin biosynthesis. Pesticide Science 21:253–267. Figure 6. Average incidence of apple scab (%) on ‘Indian Magic’ crabapple sapling leaves 53 days after planting (11 June 2002). Treatments consisted of (1) control; (2) soil drench with PBZ (0.79 g a.i. diameter cm−1 ) at time of planting; (3) foliar application of propiconizole (0.016% by volume) every 2 weeks beginning 4 days after bud- break (2 May 2002); (4) foliar application of PBZ (0.02 g mL−1 plication of PBZ (0.02 g mL−1 ) 4 days after budbreak (2 May 2002); (5) foliar ap- ) 4 days after the first heavy infection period (15 May 2002); and (6) foliar application of propiconizole (0.016% by volume) 4 days after the first heavy infection period (15 May 2002). Lower-case letters indicate significant differences between means ( = 0.05). Baldwin, B.C., and T.E. Wiggins. 1984. Action of fungicidal triazoles of the diclobutrazol series on Ustilago maydis. Pesticide Science 15:156–166. Beckman, C.H. 1957. Growth inhibition as a mechanism in Dutch elm disease therapy. Phytopathology 48:172–176. Chaney, W.R. 2001. Effective plant growth regulators for right-of-ways and potential landscape use. In Proceedings of the 13th Annual Conference of the Western Plant Growth Regulator Society, Monterey, CA, Jan. 10–11, 2001, pp. 39–48. Copas, L., and R.R. Williams. 1987. Effect of paclobutrazol on the inoculum potential of Chondrostereum purpureum and silverleaf symptoms in cider apple cvs Michelin and ©2006 International Society of Arboriculture
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