PACLOBUTRAZOL PROTECTS BLACK SPRUCE SEEDLINGS AGAINST PREDISPOSITION TO GRAY MOLD INDUCED BY HIGH-TEMPERATURE AND DROUGHT

Effects of paclobutrazol on the ability of high temperature and drough t to predispose black spruce (Picea mariana (Mill.) B.S.P.) seedlings to infection and sporulation of Botrytis cinerea Pers.:Fr. were examin ed. Paclobutrazol was applied as a soil drench at doses of 0, 20, and 40 mg/seedling pair on the first and third days of 2- and 4-week perio ds before the seedlings were subjected to darkness at 35 and 45-degree s-C, or to drought, for various periods and inoculated with the pathog en. The paclobutrazol treatments markedly reduced sporulation incidenc e and the number of spores produced by the pathogen on needles of seed lings subjected to the environmental stresses for periods that margina lly or moderately exceeded those needed for predisposition to B. ciner ea. Sporulation in needles of paclobutrazol-treated seedlings increase d substantially only when the stress treatments were relatively prolon ged. The activity of paclobutrazol in suppressing effects of environme ntal predisposition of the seedlings to infection and sporulation of B . cinerea was considered to be mediated through stress protection as o pposed to fungicidal action of the triazole.