Chlamydospore Germination ofFusarium oxysporumf. sp.cucumerinumas Affected by Fluorescent and Lytic Bacteria from a Fusarium-Suppressive Soil

Abstract

Over 700 bacteria and actinomycetes were isolated from Fusarium-suppressive Metz fine sandy loam collected from the Salinas Valley in California [USA]. Isolates were screened for ability to produce the fungal cell-wall degrading enzyme, chitinase, to lyse dead and living germ tubes of chlamydospores, and to produce fluorescent compounds (siderophores) in iron (Fe)-deficient nutrient medium. Representative isolates from groups with or without these attributes were added to conducive soil where they were evaluated for influence on chlamydospore germination of Fusarium oxysporum f. sp. cucumerinum in rhizosphere and nonrhizosphere soil, and for ability to induce suppressiveness to Fusarium wilt of cucumber. Nonfluorescent bacteria generally induced little or no inhibition of chlamydospore germination except for Enterobacter cloacae and isolate 691 which induced lysis in vitro. Those isolates capable of inducing lysis in vitro did not induce significant germ-tube lysis in rhizosphere or nonrhizosphere soil. There was a direct correlation (r = 0.99) between siderophore production by various fluorescent pseudomonads and their inhibition of chlamydospore germination in soil. Adding Fe2+, Zn2+, Co2+, Mn2+ and MoO42- to soil partially counteracted inhibition induced by fluorescent pseudomonads. The same concentration of micronutrients did not inhibit the growth of pseudomonads in soil. A chelator, ferric ethylenediaminidei-O-hydroxyphenylacetic acid (FeEDDHA), previously reported to induce suppression of Fusarium in soil, inhibited chlamydospore germination in the rhizosphere but not in nonrhizosphere soil. This supported theory that Fe3+ is involved in the mechanism of competition. Disease was suppressed more strongly by the highly fluorescent siderophore-producing pseduomonad isolates than by other isolates; however, some nonfluorescent bacteria also suppressed disease. Combinations of lytic and fluorescent bacteria did not increase suppressiveness.