Innovative approaches to manage Pseudomonas syringae pv. actinidiae by manipulating the disease triangle

Kiwifruit (Actinidia sp.) production has being severely affected by Pseudomonas syringae pv. actinidiae, the etiological agent of the bacterial canker of Actinidia. The present work aimed at manipulating/exploring features of the three vertex of the disease triangle (host, pathogen and environment) to explore novel strategies to prevent disease development. Chapter 2 includes the study of the influence of red (R):blue (B) light ratio and green (G) light supplementation on plant physiology and disease development. Our study shows that 50%R-50%B and 25%R-75%B lead to the highest PSII efficiency and photosynthetic rate, but monochromatic blue light was the most efficient in reducing disease symptoms, possibly by modulating Psa virulence more than host plant defenses. Chapter 3 explores the potential of kiwifruit phyllosphere as a reservoir of microbes with potential biological control. Two strains stood out as great flower and leaf colonisers with good capacity to inhibit Psa growth. Their genomes were shotgun sequenced and the strains were taxonomically affiliated to Pseudomonas azotoformans and Pseudomonas putida species. The genome analysis revealed that both strains may produce the siderophore pyoverdine, which may contribute to limit iron availability and subsequently reduce pathogen growth. Chapter 4 explores the hypothesis that plant-derived factors recognized by Psa could be uniquely present, or more abundant, in susceptible species, thus increasing bacterial virulence traits. Leaf extracts, produced from five Actinidia species displaying different degrees of tolerance to Psa, were screened for their ability to induce Psa virulence and the results showed that all crude extracts tested enhance Psa virulence, independently of the susceptibility of the plant. Nevertheless, extract fractionation revealed that the molecule(s) responsible for Psa virulence induction by the different plant species could differ, suggesting that leaf metabolite composition may play a role in regulating Psa virulence and thus contribute to the outcome of the interaction.