Epidemiology of Bacterical Canker of Kiwifruit. 3. The Seasonal Changes of Bacterial Population in Lesions and of Its Exudation from Lesion.

Abstract

The seasonal changes in bacterial population of Pseudomonas syringae pv. actinidiae in the infected leaves and of bacterial exudation from the lesions were investigated from May to December for kiwifruit (cv. Hayward) in the field. The following relations were observed between mean temperature per 10 days before isolation of the bacteria and the population in the lesion. The population was highest in early May (13 to 17°C), and it began to decrease from 106-7CFU/ml when the temperature was over 18°C (mid. to late May). It decreased rapidly from 104-5 to 102-3CFU/ml when the temperature rose to the range from 20 to 24°C (early June to early July). It decreased more after the rise in temperature and became 100-1CFU/ml in late August (25 to 26°C). The bacterium was not isolated in some lesions. However, the bacterial population began to recover when the temperature dropped gradually to the range from 22 to 20°C (late September to early October) in fall. It increased rapidly, when the temperature dropped further to the range from 17 to 15°C (late October to early November), and approached the population at 104-7CFU/ml in June. After that, a high population was maintained though the temperature dropped gradually from 13 to 10°C (late November to mid.-December). Bacterial exudation from lesions was observed immediately after floating them in 10ml of sterile distilled water in early May. The amount of exuded bacteria was 106-7CFU/ml. It decreased below 104-5CFU/ml in early June and took more time for exudation. The bacterial exudation was not observed during July to September. However, it occurred again in early October when the bacterial population in lesions began to recover. Both the percentage of lesions in which bacterial exudation was observed and the amount of exuded bacteria increased rapidly during early November to mid.-December. However, the amount of exuded bacteria did not become over 102-3CFU/ml. The above results indicate that the most suitable mean temperature per 10 days ranged from 10 to 20°C for the growth of pathogen in the leaf tissue. It suggests that the bacteria may attack canes in spring and in mid.-fall to early winter because the optimum temperature for growth occurs.