The Effect of Paclobutrazol Treatment on Starch Content, Mycorrhizal Colonization, and Fine Root Density of White Oaks (Quercus alba L.)

Mature white oaks (Quercus alba L.) averaging 61 cm (24.4 in) dbh and in a moderate state of decline were treated with 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.

Pin Oak Root Injury from Soil Acidification with Sulfuric Acid

Sulfuric acid soil treatments that have been used to reduce soil pH in the landscape can cause root damage. When 5 cm (2 in) diameter holes were filled with 33% (4 N) sulfuric acid, pH reduction of one-half unit or greater was limited to soil within 5 cm (2 in) of the treatment hole and persisted less than 2 years. Fine root density was significantly reduced and root tip injury was significantly increased within 15 cm (6 in) of the treatment hole. Cambium of woody roots greater than 3 mm (0.1 in) diameter was killed up to 25 cm (10 in) from the application hole. Destruction of the cambium would result in the death of the roots beyond the point of cambium injury and may be responsible for crown dieback which sometimes develops following acid treatments in the landscape.