Abstract
Bacterial canker of kiwifruit caused by Pseudomonas syringae pv. actinidiae is a cyclic disease in which a spring to early summer phase in leaves and other green tissues alternates with a late winter to early spring phase in branches and trunk. A close association between the number of diseased leaves on vines of kiwifruit cv. “Hayward” in spring and the severity of disease development on those branches and trunk after winter was confirmed by investigation in the field, although some exceptions to this trend were observed. The host resistant reaction in fall to early winter to growth of the bacterium varied equally in spring to early summer when the mean temperature per 10 days was approximately 20°C. When the temperature was higher than 22°C, growth of the bacterium was abruptry inhibited by the appearance of wound-healing tissue surrounding the infected area, and the bacterial population declined rapidly. A few bacteria remained alive during the summer in diseased tissue. In many cases, however, recurrence of the disease on these branches after winter was prevented by wound-healing tissue surrounding the affected parts. Formation of wound-healing tissue on the affected parts gradually ceased when the temperature dropped gradually from 22 to 20°C. No healing of the affected tissue was observed after the temperature dropped to 15°C or below. When pieces of absorbent cotton saturated with bacterial suspensions were applied to the axillary buds and lenticells on branches from summer to winter, disease development in these specimens did not occur in the following early spring. The branches were attacked through damaged areas by bacteria dispersed from lesions formed in the spring on leaves from fall to early winter. The bacterium was latent in the cortex tissue of the branches, and grew and moved in the tissue from winter to early spring. It was clarified from the above results that the disease development in branches and trunk which appeard in late winter to early spring depends primarily on the degree of infection in fall to early winter.
