Latent infection by Pseudomonas syringae pv. actinidiae in Actinidia chinensis cv. Hort16A asymptomatic plants: five years of survival and colonization of a mutant virulent strain

The presence of a latent period during which symptoms are not evident is quite common in bacterial diseases; in some cases there may also be a greatly prolonged period of incubation due to host plant factors or to the presence of environmental factors unfavourable for the pathogen symptom expression. To tackle the critical epidemiological role of the latency phase of Pseudomonas syringae pv. actinidiae (Psa) in susceptible asymptomatic host as potential tool to spread the pathogen in the orchards, its survival and colonization were studied in seven-year-old plants of Actinidia chinensis cv. Hort16A inoculated five years before with a virulent Psa gfp-expressing/rifampicin-resistant strain (Psa::gfp-Rifres) at low inoculum dose. The endophitic presence of Psa was studied by microbiological analysis of the whole asymptomatic plants to re-isolate Psa::gfp-Rifres and by PCR analysis to confirm the identity of the re-isolated strains. Moreover, their ability to induce disease symptoms and HR respectively in host and non-host plants was also tested. During the five years after the inoculation, the establishment of the Psa::gfp-Rifres inside symptomless tissues was detected, however, the pathogen isolation time was considerably increasing during the last years. In particular, starting from the third year, the time required to isolate the pathogen was more than two months. Latent infection may imply low numbers of the pathogen and the presence of endophytic bacteria isolated during the microbiological assays from plant extracts that may interfere with the Psa virulence expression in susceptible host plants. The data obtained showed that systemic colonization of host tissues by Psa::gfp-Rifres took place for a long period of time. The epidemiological significance of this finding raises questions about the effectiveness of the control measures to prevent bacterial canker spreading primarily based on antimicrobial treatment by spraying on plant surfaces. In fact, if Psa is already within the host, the efficacy of the treatment could be reduced in inverse proportion to the degree of the latent systemic infection.