Annual Deblossoming Increases Fire Blight Susceptibility of ‘Golden Delicious’/M.9 Apple Trees

Abstract

A natural infection of fire blight occurred in 1987 in a mature planting of ‘Jonathan,’ ‘Paulared,’ ‘Delicious,’ and ‘Golden Delicious’ on M.9 rootstocks. The ‘Golden Delicious’/M.9 apple trees were: 1) mechanically root pruned on two sides at a distance of 50 cm from the trunk to a depth of 40 cm; or 2) manually deblossomed. The treatments were applied annually at full bloom and arranged as a 2 x 2 factorial. ‘Golden Delicious’ had the most severe injury followed by ‘Jonathan’ and ‘Paulared,’ while ‘Delicious’ had hardly any strikes. ‘Golden Delicious’ that were not deblossomed and carrying a full crop had very little infection, while trees which had been deblossomed had numerous strikes which penetrated into older wood, often including main scaffold branches. In addition to the strikes on the scion, 71% of the deblossomed trees had infected M.9 rootstocks, while the slight injury to cropping ‘Golden Delicious’ trees was confined to current season’s growth. Root pruning had no effect on fire blight susceptibility. Deblossomed trees had lower photosynthesis and transpiration rates than cropping trees both before and after the infection period, indicating that the physiological status of these trees was altered. The lower photosynthesis and transpiration in these trees has been previously linked to high foliar levels of starch, chloroplast disruption and lowered stomatal conductance. The susceptibility of ‘Golden Delicious’/M.9 apple trees to fire blight appears to be greatly dependent upon the cropping status of the tree.