Induction of Systemic Acquired Resistance Aids Restoration of Tree Health in Field-Grown Pear and Apple Diseased with Fire Blight.

Induction of systemic acquired resistance as a therapeutic aid to restoration of tree health was evaluated in 3-to 14-year-old pear and apple trees diseased with fire blight. Acibenzolar-S-methyl (ASM) was applied to diseased trees in late spring near the time of removal of primary fire blight cankers, which had originated from floral infection. Suspensions of ASM (7.5 to 22.5 g of active ingredient per liter plus silicone surfactant) were painted onto a 30-to 45-cm length of branch tissue immediately below primary pruning cuts or sprayed onto an 80-to 100-cm length of central trunk. In some experiments, a second ASM treatment was made in late June to early July in conjunction with secondary pruning of redeveloped cankers. After pruning primary cankers, effects of ASM were measured by assessing weight and length of secondary cankers that were the result of fire blight redevelopment. Over 5 years of field experiments, trees that received an ASM treatments yielded 62% less diseased wood at the time of secondary and tertiary canker removal compared with non-ASM-treated trees. Moreover, tree mortality and proportion of pruning cuts where fire blight redeveloped were reduced by ASM. Induction of systemic acquired resistance could prove practical as an aid to pruning therapy in young, fire-blight-susceptible pear and apple trees where, after canker removal, disease symptoms frequently redevelop owing to residual cells of the pathogen distributed within symptomless portions of the tree.