Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/27/2002
Publication Date: 8/20/2002
Citation: Bull, C.T., Shetty, K.G., Subbarao, K.V. 2002. Interactions between myxobacteria, plant pathogenic fungi and biological control agents. Plant Disease. v. 86. p. 889-896. Interpretive Summary: Myxobacteria are soil dwelling gram-negative gliding bacteria, which form fruiting bodies containing resistant myxospores. Fumigation with methyl bromide and chloropicrin (MBC) is currently used in many cropping ststems to control soilborne plant pathogens. Pathogens as well as beneficial orgaisms are killed when they are in contact with the fumigant. It is unknown if current fumigation techniques destroy resistant myxospores. Moreover, methyl bromide will no longer be available for use in the U.S. as soil fumigant for most purposes after 2005 due to its ozone depleting characteristics. As methyl bromide and other chemical tools for controlling plant diseases are removed from the market, understanding the interactions among soil microbes will become more important for managing soilborne diseases. Myxobacterial were found in soils in organic and conventionally managed strawberry production and transplant fields in the absence of soil fumigation. Fumigation eliminated these organisms from soil, however, it had no effect on isolation from strawberry roots. Economically important fungal plant pathogen and biological control agents were inhibited by myxobacteria in laboratory studies. In contrast bacteria used to control fungal diseases were not inhibited by the myxobacteria. Determining how microorganisms interact and influence plant health without soil fumigation is the first step in providing biologically based alternatives to soil fumigation for crop production. These studies will lead to management strategies that will help maintain an uninterrupted supply of fresh fruit and vegetables to the American consumer as chemicals including fumigants are removed from grower's tools for disease management.
Technical Abstract: Myxobacteria are soil dwelling gram-negative gliding bacteria, which form fruiting bodies containing resistant myxospores. Although they produce a wide range of antibiotics and lytic enzymes that assist in their ability to prey on microorganisms, their role in plant disease control 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; in most cases they were still undetectable after fumigation. 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 8 soilborne plant pathogenic fungi (Cylindrocarpon spp., Fusarium oxysporum f. sp. apii, Phytopthora capsici, Pythium ultimum, Rhizoctonia spp., Sclerotinia mino, verticillium albo-atrum, and V. dahliae) and against two fungal biological control agents (Gliocladium virens, & Trichoderma viride). P. capsici, P. ultimum, Rhizoctonia spp., S. minor, and T. viride were completely inhibited by all the Myxococcus spp. tested. F. oxysporus f.sp. apii was the least sensitive to myxobacteria and no inhibition occurred in some cases. The inhibition of the other fungi was intermediate to the reactions by Fo apii and fungi completely inhibited. Myxococcus coralloides inhibited all the fungi tested in virtually all experiments. The ability of bacterial biological control agents to produce antibiotics and other secondary metabolites had an effect on their interactions with myxobacteria. Phenazine production by Pseudomonas aureofaciens 30-84 and secondary metabolite production by P. fluorescens CHAO protected them from lysis by myxobacteria.