Wood Microbiome Variation and Interactions with Fungal Symbionts of Invasive Ambrosia Beetles

The microbiomes of plants can modulate the impacts of pests, including through interactions with the microbiomes of pathogen vectors, such as ambrosia beetles. Although physical and chemical traits of plant hosts are known to affect beetle-carried microbes, how beetle and host microbiomes interact is seldom explored. We aimed to determine whether wood-inhabiting endophytes mediate host susceptibility to Fusarium dieback, an emergent tree disease complex that includes ambrosia beetle vectors. We studied three competent host tree species (Persea americana, Salix spp., and Platanus racemosa) common in disease hot spots in agricultural and wildland habitats. Using culturing methods, we compared the wood microbiomes of 319 attacked and 133 nonattacked trees across a network of 47 beetle-infested and 41 noninfested plots in Southern California, United States. We conducted 1,148 in vitro assays to evaluate antagonism by wood-inhabiting endophytic fungi (60 species) and bacteria (40 species) to the Fusarium pathogens and found 15 fungal and 11 bacterial species with clear antagonism to the pathogen. Such wood endophytes may have potential to protect tree hosts as biological control agents. However, antagonistic microbes were more common in attacked trees than in nonattacked trees, suggesting that either the abundance of antagonistic fungi and bacteria in the wood microbiome is insufficient to determine the host susceptibility to attack or antagonistic strains could be enriched in attacked trees in response to the pathogen invasion. Wood-inhabiting microbial communities were consistently different between cultivated Persea americana and wildland tree species, as there were some differences based on host attack status. Differences between attacked and nonattacked trees were reflected in different microbial consortia rather than the abundance of individual, antagonistic microbial species.