THE IMPORTANCE OF THE MAIN COLONIZATION AND PENETRATION SITES OF PSEUDOMONAS SYRINGAE pv. ACTINIDIAE AND PREVAILING WEATHER CONDITIONS IN THE DEVELOPMENT OF EPIDEMICS IN YELLOW KIWIFRUIT, RECENTLY OBSERVED IN CENTRAL ITALY

Abstract

SUMMARY During the years 2008-2011 repeated isolations from symptomatic organs and plant parts of Actinidia chinensis cvs Hort16A and Jintao, grown in the provinces of Latina and Rome (Latium, central Italy), were done with the aim of determining the main colonization and penetration sites exploited by Pseudomonas syringae pv. actinidiae (Psa). Isolations were also made from exudates, pruned twigs and from tissues surrounding elastic laces used to tie young shoots. Artificial inoculations were performed to identify the main avenues for pathogen colonization. Climatic data (maximum, minimum and average temperature as well as rainfall) were recorded by weather stations established in the areas of yellow-fleshed kiwifruit cultivation. The bacterium appeared to be capable of colonizing the host all year round and to systemically migrate from young leaves to twigs. Unprotected lenticels, fruit stalks and leaf scars, as well as pruning wounds and tissues surrounding the elastic laces were easily colonized by the pathogen. The whitish exudates yielded, in many cases, pure cultures of the bacterium. A clear-cut relationship was found between the occurrence of frost events during winter 2007-2008 and the first outbreaks of bacterial canker on A. chinensis in the following spring-autumn. Moreover, in 2008, in the infected area, the rainfall was 30-35% higher than the average, which may have promoted the massive spread of the pathogen on the very susceptible yellow-fleshed cultivars. The disease cycle of Psa on A. chinensis in central Italy is postulated and described.