Diversity and Plant Pathogenicity of Bursaphelenchus and Related Nematodes in Relation to Their Vector Bionomics

Abstract

The nematode genus Bursaphelenchus is a highly divergent group containing fungal feeders and obligate and facultative plant parasites. The genus is also known as the phoretic (and parasitic) associates of many groups of insects. Further, two major plant pathogens, B. xylophilus and B. cocophilus, are members of this genus, and several other species are suspected to be weak to moderate plant pathogens. Here, the diversity of vector insects and host/habitat/plant species interactions, as well as phylogenetic relationships, are summarized for Bursaphelenchus species. The hypothetical origins of plant pathogenicity for different species of Bursaphelenchus are then discussed in relation to the bionomics of their vector insects. Phylogenetic analyses suggest that the genus is separated into four clades, and the basal clade will be further separated into two new genera. Based on a literature survey, the phoretic host association patterns appear different among the other three true Bursaphelenchus clades, i.e., one clade is mostly associated with bark beetles that occupy relatively dry niches (under bark), whereas the other two clades are associated with various groups of insects inhabiting diverse niches in other relatively humid conditions. Plant pathogenicity for a few members is hypothetically derived from the nematodes’ tolerance to the static resistance of host plants, i.e., the nematode species vectored by the insects attacking or interfacing with live plant tissue were provided an opportunity and, in a few cases, possessed the means to manifest plant pathogenicity with serious consequences, i.e., B. xylophilus and B. cocophilus.