The effect of paclobutrazol treatment on starch content, mycorrhizal colonization, and fine root density of white oaks (Quercus alba L.)

Abstract

Mature white oaks (Quercus alba L.) averaging 61 cm (24.4 in) dbh and in a moderate state of decline were treated wit 0.8 g active ingredient (a.i.)/cm (2 g/in) paclobutrazol (PBZ) on 11 April 1995, and again with 1.2 g a.i./cm (3 g/in) on 8 October 1998, as a basal drench. PBZ produced no reduction in leaf size or twig growth of the white oaks at any time during the 7 years of the study. Rapidly growing plants may be more effectively growth-regulated by PBZ. Fine root density, starch content of the woody roots, and percentage of mycorrhizal root tips were unaffected by PBZ treatment. This is the first direct evidence that mycorrhizae are not reduced by the fungicidal properties of PBZ. The root zone of these trees was mulched but laterally restricted. Root density and mycorrhizae may have already been high under these conditions, limiting the ability of PBZ to improve them further. © 2006 International Society of Arboriculture.