Pruning date affects bacterial canker of sweet cherry

The effect of pruning date on the development of canker lesions caused by Pseudomonas syringae pv syringae (Pss) and Ps pv morsprunorum (Psm) was investigated in a field trial using three sweet cherry cultivars with or without Pss and Psm inoculation Four weeks after treatment the percentage of twigs showing bacterial canker lesions and the size of lesions were recorded Overall the percentage of twigs showing lesions did not vary between cultivars but did vary with the pruning date and between pathovars Uninoculated branches pruned in February 2014 did not develop canker lesions A moderate proportion (40) showed lesions when inoculated with Psm and 100 of twigs showed necrosis when inoculated with Pss However for later pruning dates (April and July 2014) the percentage of twigs showing lesions declined progressively for twigs inoculated with both pathovars up to the September pruning Thereafter a sharp increase was observed with nearly 100 of twigs showing necrosis after the final January 2015 pruning For branches that showed necrosis lesion size varied strongly with cultivar pruning time and pathovar Lesion size tended to be smaller with later pruning but the pattern varied considerably with pathovar and cultivar The significance of these findings in relation to bacterial canker management is discussed