Evaluation of pruning therapies in apple trees with fire blight

Abstract

Ten experiments were conducted to evaluate the efficacy of methods to therapeutically remove fire blight cankers from apple trees with different age, vigor, training system and rootstock combinations in Washington, Oregon, Pennsylvania and New York, United States. Removal methods included pruning at varying distances from visible symptoms, utilization of a sanitizing agent and the use of a stub cut to minimize cankers on structural wood. Fire blight removal resulted in reductions of rootstock blight and tree death. Removal treatments resulted in fewer additional cankers compared to the no-treatment control in most trial sites and years. The standard best management practice (BMP) for removing fire blight was branch removal at 30 cm below the proximal edge of visibly cankered tissue with sanitized loppers. This practice significantly reduced the number of new systemically-caused symptoms compared to the no-treatment control in seven of nine experiments with significant reductions in five experiments. Aggressive removal, defined as BMP but branches removed approximately 76 cm from the proximal edge of visibly cankered tissue, generally did not improve the standard practice. Elimination of cutting tool sanitation did not result in more cankers than the standard practice. Breaking off diseased branches by hand provided a rapid removal method, but it resulted in a greater number of cankers in the orchard at the end of the season. In one of five experiments cutting which left a 13 cm stub distal to structural wood significantly reduced the number of cankers on structural wood compared to flush cut or 4 cm stubs.