Two-year endophytic colonization of Pseudomonas syringae pv. actinidiae in adult plants of Actinidia chinensis cv. Hort16A

A prolonged permanence of a plant pathogenic bacterium as an active endophyte without affecting the host plant survival is a sign of pathoadaptation. At low inoculum dose, Pseudomonas syringae pv. actinidiae (Psa) could colonize susceptible plants for a long time, causing local lesions at the inoculation site concurrent with its systemic presence in asymptomatic organs. To test this hypothesis, two years-old plants of Actinidia chinensis cv. Hort16A grown outdoors were inoculated in October 2010 with Psa DiSTA8404 (gfp espressing/Rif-resistant-strain). From May 2011 until June 2012, the canker lengths on infected and control shoots were measured every 15 days and, for each date, the average lengths were calculated. The comparison between the two data sets showed statistical differences. The canker progression has been compared with the variation in daily temperatures. In the Psa infected shoots, the canker length increased linearly from May to November 2011, with no or minimal increase between November 2011 and January 2012 and with parabolic rising trend between late January and early April 2012. Our findings indicate that Psa, inoculated at low dose through autumnal wound can survive: 1) during the first winter and actively colonizing the tissues during the next growing season causing local basipetal infections visible at the inoculation point and systemic infection in parts of the trunk remaining asymptomatic; 2) during the second winter and reactivating much earlier than the vegetative growth of the host plant. The canker progression on shoots indicates that Psa remains active even during winter and can colonize cortical tissue with higher aggressiveness after frost periods.