Evaluation of Pruning Techniques and Bactericides to Manage Bacterial Canker of Sweet Cherry

Abstract

Bacterial canker of stone fruit occurs worldwide and is caused by Pseudomonas syringae for which three distinct pathovars are known that are associated with host range (Kennel ly et a l 2007, Hattingh and Roos 1995). We have focused on pathovar Syringae, which has been the most commonly isolated P. syringae pathovar from sweet cherry orchards (Renick et al. 2008, Foulkes and Lloyd 1980). The bac ter ium colonizes leaf surfaces of many plant species , including weeds. They can invade cherry buds, twigs, and branches systemically without causing symptoms, can incite mild symptoms including dead buds, twig cankers, leaf spots, flower and fruit lesions, or can cause severe symptoms of collapse and death of the above-ground parts of the tree, including the suckers (Figure 1). Disease severity and symptom expression is favored by factors including poor nutrition, plant parasitic nematodes, sandy soils, and by climatic factors such as late spring frosts and cool, wet growing conditions. The pathogen invades the tree through leaf stomates from which it moves into buds and twigs. It may also enter through pruning wounds and through frost-damaged tissues (Renick et al. 2008, Sule and Seemuller 1987). Leaf surface populations increase during cool, wet weather and are highest during fall, late winter and early spring (Latorre et al 1985). Copper sprays are aimed at reducing these epiphytic populations and are timed to correspond with 20% (Figure 2) and 80% leaf drop and up to three late dormant applications (Wimalajeewa et al 1991, Olson and Jones 1983). Best management relies on maintaining optimal growing conditions for the trees. “our studies have shown that copper or phosphite sprays applied on pruning wounds of sweet cherry provide minimal to no protection against bacterial canker infection. we have found that leaving an 8” pruning stub when removing a limb may protect against pseudomonasincited cankers by ‘distancing’ the main trunk from invasion. Field experiments on pruning date have provided evidence that the best time to prune sweet cherry in terms of limiting bacterial canker infection may be shortly after harvest in late July. However, this timing results in fewer renewal shoots from the pruning stubs.” Figure 1. Bacterial canker of sweet cherry below a pruning cut and progression down the branch (Sunken area on branch). Replacement side shoot has also developed bacterial canker.

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