Effects of Different Light Conditions on Anatomical and Histological Features of Galls in Bacterial Gall Disease of Cerasus × yedoensis.

Cerasus × yedoensis (cherry 'Somei-yoshino' Fujino) is affected by bacterial gall disease caused by Pseudomonas syringae pv. cerasicola (PSC). C. × yedoensis is often infected with PSC under weak light intensity, which indicates that susceptibility of C. × yedoensis to PSC is affected by light. To evaluate the effects of white light intensity and different light qualities, white or blue, on bacterial gall disease development, we quantitatively assessed the anatomical and histological features of bacterial-inoculated sites on branches of 2-year-old potted C. × yedoensis seedlings grown under different light intensities and qualities. The stronger the white light intensity, the less severe the gall symptoms. Gall formation was suppressed more by blue than white light of the same intensity. The validity of a simple gall index for assessing gall development with the naked eye, via quantitative evaluation of gall shape by measuring gall height, width, and volume, showed that the gall index could be used as a practical method for on-site assessments of gall development. The ratio of degeneration area in the gall remained constant, suggesting the presence of some regulatory mechanism preventing PSC from affecting the entire gall within the plant. Microscopy showed that the gall tissue is composed primarily of callus cells and has voids containing gummy material that is exuded from cracks in the gall, and the periderm develops at the gall foot but not at the gall apex, so the cells at the gall apex were necrotic or collapsed.