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Interactions Between Myxobacteria, Plant Pathogenic Fungi, and Biocontrol Agents

Myxobacteria are soil dwelling gram-negative gliding bacteria that form fruiting bodies containing myxospores. Although myxobacteria produce a wide range of antibiotics and lytic enzymes that assist in their ability to prey on other microorganisms, their role in agriculture has received little attention. Myxococcus spp. were isolated from soils in organic and conventionally managed strawberry production and transplant fields in the absence of soil fumigation. Fumigation with methyl bromide and chloropicrin virtually eliminated these organisms from soil. However, soil fumigation had no effect on the frequency of isolation of Myxococcus spp. from strawberry roots. Six Myxococcus spp. were tested in vitro against eight soilborne plant pathogenic fungi (Cylindrocarpon spp., Fusarium oxysporum f. sp. apii, Phytophthora capsici, Pythium ultimum, Rhizoctonia spp., Sclerotinia minor, Verticillium albo-atrum, and V. dahliae) and against two fungal biological control agents (Gliocladium virens and Trichoderma viride). Phytophthora capsici, Pythium ultimum, Rhizoctonia spp., S. minor, and T. viride were completely inhibited by all of the Myxococcus spp. tested. F. oxysporum f. sp. apii was the least sensitive to the myxobacteria, and no inhibition occurred with some Myxococcus spp. Inhibition of the other fungi tested was variable. Myxococcus coralloides inhibited nearly all the fungi tested. The ability of bacterial biological control agents to produce antibiotics and other secondary metabolites determined whether or not they were lysed by myxobacteria. Secondary metabolite production regulated by gacS protected Pseudomonas fluorescens strain CHA0 from lysis by myxobacteria. More specifically, phenazine antibiotics produced by Pseudomonas aureofaciens strain 30--84 protected it from lysis.