An Evaluation of Pruning Programs to Manage Shoot Blight, Caused by the Bacterium Erwinia amylovora

Fire blight is an economically devastating disease caused by the bacterium Erwinia amylovora. Blossom infections can lead to shoot blight and, when unmanaged, become systemic and can quickly cause tree death and spread through an orchard via active infections sites producing bacterial ooze. With climate change, increasingly popular high-density training systems, and the susceptibility of many consumers' desired apple cultivars, shoot blight management has become exceptionally challenging despite the diverse management tactics available. To better understand pruning as a management practice, we evaluated 10 pruning programs in two different orchards over the course of 2 years. The pruning programs in this study encompass extension recommendations and grower preferences and include a variety of supplemental chemical applications and sanitation practices to answer three primary questions: (i) How do the impacts of pruning on the management of shoot blight compare between a vertical-axis orchard with mature trees and a high-density planting with young trees? (ii) Do sanitation and ancillary chemical management have a significant impact on shoot blight control achieved through pruning? (iii) How do a variety of pruning programs, including those preferred by researchers, growers, and extension agents, compare in terms of shoot blight management? The impacts of pruning programs were more pronounced in the vertical-axis orchard in terms of reducing infection spread and overall shoot blight incidence. However, in the high-density plantings, pruning programs were mostly ineffective and fire blight spread quickly throughout the trees and planting. Overall, the most aggressive pruning programs, which removed tissue of several seasons of growth, had the greatest impact on fire blight in both plantings. In line with the findings of many previous studies, sanitation or chemical management supplementation was not found to be necessarily beneficial.