FACTORS AFFECTING THE SEVERITY OF BACTERIAL CANKER OF PEAR CAUSED BY PSEUDOMONAS-SYRINGAE PV SYRINGAE

Several factors affecting the severity of bacterial canker of pear wer e studied. In the orchard, infection of shoots by Pseudomonas syringae pv. syringae occurred only when the inoculum dose exceeded 10(6) colo ny-forming units/shoot. However, under favourable conditions in a grow th chamber, cankers formed on detached shoots inoculated with 5 cfu/sh oot. A second-order polynomial relationship was established between lo g(10) transformed canker length and log(10) transformed inoculum dose. In orchard and growth chamber experiments, shoots were susceptible fr om the time of bud swell until after fruit harvest. The severity of Ps eudomonas canker of detached shoots increased if they were frozen at - 10 degrees C for 24 h before inoculation. Shoots were most susceptible when inoculated immediately after wounding, and no cankers developed in the orchard when 3-day-old wounds were inoculated. Additionally, no cankers resulted from inoculation of leaf scars at leaf drop. Activel y growing, current-season shoots were more susceptible than shoots tha t had set a terminal bud. The practical implications of these results are discussed as a basis for control of bacterial canker of pear.