Distribution, causal agents, and infection dynamic of emerging ink disease of sweet chestnut in Southern Switzerland.

Emerging diseases caused by both native and exotic pathogens represent a main threat to forest ecosystems worldwide. The two invasive soilborne pathogens Phytophthora cinnamomi and Phytophthora × cambivora are the causal agents of ink disease, which has been threatening Castanea sativa in Europe for several centuries and seems to be re-emerging in recent years. Here, we investigated the distribution, causal agents, and infection dynamics of ink disease in southern Switzerland. A total of 25 outbreaks were identified, 19 with only P. cinnamomi, 5 with only P. × cambivora, and 1 with both species. Dendrochronological analyses showed that the disease emerged in the last 20-30 years. Infected trees either died rapidly within 5-15 years post-infection or showed a prolonged state of general decline until death. Based on a generalized linear model, the local risk of occurrence of ink disease was increased by an S-SE aspect of the chestnut stand, the presence of a pure chestnut stand, management activities, the proximity of roads and buildings, and increasing annual mean temperature and precipitation. The genetic structure of the local P. cinnamomi population suggests independent introductions and local spread of the pathogen.

Phylogenetically closely related pseudomonads isolated from arthropods exhibit differential insect-killing abilities and genetic variations in insecticidal factors.

Strains belonging to the Pseudomonas protegens and Pseudomonas chlororaphis species are able to control soilborne plant pathogens and to kill pest insects by producing virulence factors such as toxins, chitinases, antimicrobials or two-partner secretion systems. Most insecticidal Pseudomonas described so far were isolated from roots or soil. It is unknown whether these bacteria naturally occur in arthropods and how they interact with them. Therefore, we isolated P. protegens and P. chlororaphis from various healthy insects and myriapods, roots and soil collected in an agricultural field and a neighbouring grassland. The isolates were compared for insect killing, pathogen suppression and host colonization abilities. Our results indicate that neither the origin of isolation nor the phylogenetic position mirror the degree of insecticidal activity. Pseudomonas protegens strains appeared homogeneous regarding phylogeny, biocontrol and insecticidal capabilities, whereas P. chlororaphis strains were phylogenetically and phenotypically more heterogenous. A phenotypic and genomic analysis of five closely related P. chlororaphis isolates displaying varying levels of insecticidal activity revealed variations in genes encoding insecticidal factors that may account for the reduced insecticidal activity of certain isolates. Our findings point towards an adaption to insects within closely related pseudomonads and contribute to understand the ecology of insecticidal Pseudomonas.