Response of shade trees to root collar drenches of inhibitors flurprimidol and paclobutrazol

Abstract

Root collars of white ash (Fraxinus americana L.) and black walnut (Juglans nigra L.) were drenched in April with wettable powder formulations containing 2, 4, or 12 g of flurprimidol {α-(1-methylethyl)-[4-(trifluoromethoxy)phenyl]-5-pyrimidinemethanol} or paclobutrazol [(2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-pentan-3-ol)] to determine shoot growth inhibition and the soil location of residual activity. By the end of the first growing season, black walnut shoots were inhibited by all doses of flurprimidol or paclobutrazol. Growth inhibition of black walnut was dramatic (<50% the growth of control) during the second season with a linear positive response to increasing flurprimidol doses. White ash growth was inhibited by flurprimidol but not paclobutrazol. Flurprimidol activity was concentrated within 10 cm of the treated root collar 1 1/2 years after treatment, which is evidence that the root collar and adjacent roots were the major avenues to the vascular system.