The cycle of disease and population structure of Pseudomonas syringae pv. actinidiae
Recent studies aiming at elucidating the epidemiology of Pseudomonas syringae pv. actinidia (Psa) in different continents allowed us to better define its disease cycle on cultivated Actinidia species. Some climate factors appear to play a fundamental role in favoring the virulence and the spreading of the pathogen in an area. Early and late frost event promote the rapid multiplication of the bacterium and act as a predisposing factors both in central-northern Italy and central-southern Chile, whereas typhoons are responsible for a large dissemination of the pathogen in eastern Asia (Japan, South Korea). A relatively uniform rainfall distribution through the year coupled with air temperatures largely favorable to Psa multiplication seem to positively correlate with the appearance and spreading of the disease in New Zealand. Agronomical techniques can greatly contribute to the further spreading of the pathogen within the orchard. The endophytic phase of the bacterium still poses problems in effectively controlling the disease through the year. The determination of a pathogen population structure is fundamental for predicting the genetic diversity of the pathogen, the origin of an epidemic, the spread of clonal lineages, the emergence or re-emergence of new populations, the impact and effectiveness of control measures and the suitability of current phytosanitary pathogen detection protocols. So far, Psa population structure has been studied by analyzing many strains of the current pandemia and very few from the past outbreaks. By assessing also strains isolated in Japan from past epidemics of kiwifruit bacterial canker, it has been ascertained the occurrence of other genetically distinct lineages which resulted different from the Psa 1, Psa 2, Psa 3 and Psa 5 and not related to the presence/absence of the phaseolotoxin gene cluster. Consequently, the basis of the current naming system of Psa populations on numbering and the presence or absence of phytotoxins should be reconsidered. In addition, all the different lineages are capable to effectively colonize and infect kiwifruit at a very similar rate. Because all major areas of kiwifruit cultivation of the world are currently affected by Psa 3, we can continue to refer to this by using this acronym or pandemic Psa and refer to the others by quoting the geographic origin, host and year of isolation and some basic (molecular and/or phenotypic) characteristics rather than resorting to a numbering system. Finally, because all Psa populations are virulent, the definition of Psa 3 as Psa-V (i.e., virulent) is unnecessary.
Scortichini, M. (2019). The cycle of disease and population structure of Pseudomonas syringae pv. actinidiae. Acta Hortic. 1243, 55-58
epidemiology, predisposing factors, clonal lineages, detection