116 Watson: Effects of PBZ Treatment on White Oaks growth regulation with PBZ are on moderate-sized, heavily pruned trees with rapid sprouting (utility and orchard), or small vigorous plants. Annual twig growth on these large mature trees was typically only 5 to 8 cm (2 to 3.2 in) per year. Growth regulation by PBZ may be less effective on slowly growing plants. More research is needed on the use of PBZ on large trees. Except for one year, 1998, fine root density was not in- creased in this study (Table 3) as it had been in others, in- cluding one study using similar size and species trees treated with a similar rate. Greening was present, indicating the growth regulator was present in the leaves, especially after the second application, but there was no overregulation of top growth that could have reduced root growth. The 1 year in- crease is probably not horticulturally significant, and the rea- son for it is unclear. Starch content of the woody roots was not affected by PBZ in either season. Visual assessment of starch is not as precise as chemical methods used to detect carbohydrate changes in previous studies but can detect biologically important differ- ences that could affect tree health and vitality (Wargo 1975). In June, starch levels were low but not depleted. It is possible that reserves started to rebuild from the lowest levels by that time, as they did for sugar maple in the northeast (Wargo 1979). Stored starch levels did not increase by October, but the reason for this is not clear. There was no unusual weather pattern or defoliation that may have reduced photosynthesis, nor was there any unusual crown growth that might have depleted reserves. Mycorrhizal colonization of roots also was not affected by PBZ (Table 4). Without an increase in carbohydrates avail- able in the roots, and/or changes in the soil environment, a lack of change in mycorrhizal colonization of root tips is understandable. This is the first direct evidence that mycor- rhizae are not reduced by PBZ and is in agreement with previous reports that basally applied PBZ is not transported to the root tips, where the fungicidal properties could affect the mycorrhizal fungi (Couture 1982). The percentage of mycor- rhizal root tips appears to be lower in June than October, but Table 3. Root density (cm/cc soil) of white oaks following PBZ treatments in April 1995 and October 1998. Year 1996 1997 1998 1999 2000 2001 2002 PBZ 1.23 1.40 2.31* 3.71 3.32 1.64 1.92 *Significantly different from control (P 0.05) ©2006 International Society of Arboriculture Control 1.46 1.76 1.26 3.55 2.35 1.68 1.75 Table 4. Stored starch and mycorrhizal colonization of roots of white oaks in the third season after treatment with PBZ. PBZ Starch June October Ectomycorrhizae (% of root tips) June October 2.0 2.4 38% 76% Control 2.5 2.6 33% 71% There was no significant difference between treated and control trees in any year. this may simply have been the result of a different technician performing the visual color assessment. The root environment for the majority of these trees was of high quality, with a well-established mulch layer, but limited in quantity, with lateral spread restricted by pavements and structures. Root density and mycorrhizae may have already been maximized under these conditions before the PBZ treat- ments. Mulching with wood chips has been shown to increase root density and mycorrhizal colonization of the roots of white oak trees (Himelick and Watson 1990). It is possible that in a very favorable root environment, fine root density and mycorrhizal colonization were already high and PBZ was not able to improve them further. Though in some situations PBZ can improve the root growth and crown appearance of declining trees (Watson 1996), this study indicates that the conditions under which PBZ can be effective may be quite specific. More research is needed to understand under what conditions PBZ treatments can be used by arborists to im- prove fine root development and help to stabilize declining trees. Acknowledgments. This study was funded in part by the Care of Trees, Wheeling, Illinois; Rainbow Treecare Scientific Advance- ments, St. Louis Park, Minnesota; and Dow AgroSciences, India- napolis, Indiana. LITERATURE CITED Burch, P.L., R.H. Wells, and W.N. Kline III. 1996. Red maple and silver maple growth evaluated 10 years after application of paclobutrazol tree growth regulator. Journal of Arboriculture 22:61–66. Couture, R. 1982. A new experimental growth regulator from ICI. Proceedings of the Growth Regulation Society of America 9:59. Himelick, E.B., and G.W. Watson. 1990. Reduction of oak chlorosis with wood chip mulch treatments. Journal of Arboriculture 16:275–278.
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