Pluriannual latency of a gfp-marked Pseudomonas syringae pv. actinidiae in Actinidia chinensis adult plants

In Italy, leading producer of kiwifruit in the world (excluding China), the recent outbreaks caused by Pseudomonas syringae pv. actinidiae (Psa) in both green (Actinidia deliciosa) and yellow-fleshed (Actinidia chinensis) cultivars and the pollinators of these crops gave rise to serious production losses. To overcome the current phytosanitary emergency it is now required to further investigate some critical epidemiological aspects. More specifically, the information presently available on the life-cycle of Psa are still inadequate with regard to its “latency” within the susceptible host plant. Indeed, a feature of the species to which Psa belongs is to survive endophytically in adult plants without inducing symptoms. Recent epidemiological studies showed that Psa can be isolated from trunks and branches of asymptomatic kiwifruit plants. Similarly to P. syringae pv. syringae, the causal agent of the bacterial canker of stone fruits, Psa may cause systemic infections which are completely asymptomatic or with delayed expression. To shed light on this dangerous “latent phase”, the duration of which is still unknown, the survival of Psa at low inoculum dose was studied in susceptible plants over three years. In 2010, shoots of two years-old plants of Actinidia chinensis cv. Hort16A grown outdoors were inoculated with a virulent Psa gfp-expressing/Rif-resistant-strain. The microbiological (re-isolation on selective media), biological (pathogenicity/HR assay) and molecular (PCR) analyses of the whole plants were carried out from May 2011 at the appearance of the cankers in the inoculated shoots. The results obtained in the next three years have highlighted the ability of Psa to survive within asymptomatic plants.