ECOLOGY OF FUSARIUM-OXYSPORUM F-SP NIVEUM IN SOILS SUPPRESSIVE AND CONDUCIVE TO FUSARIUM-WILT OF WATERMELON

Population dynamics and chlamydospore germination of Fusarium oxysporum f. sp. niveum, as well as colonization of watermelon roots by F. oxysporum, were monitored in relation to other microorganism populations and the incidence of Fusarium wilt in four soils representing different suppressive and conducive conditions. The soils consisted of an induced suppressive soil developed through monoculture to watermelon c cultivar Crimson Sweet, a nonsuppressive monoculture soil, a conducive fallow soil, and the suppressive soil rendered conducive by microwave treatment. An orange-colored mutant isolate of the pathogen, comparable to the wild-type in growth, pathogenicity, and root colonization, was used to differentiate the pathogen from indigenous populations of F. oxysporum in the field soils. Pathogen populations remained stable in the monoculture soils over a 6-mo period, but increased somewhat initially and remained at higher levels when added to conducive soils. Suppressiveness was not associated with inhibition of pathogen chlamydospore germination. There were no differences among the field soils in pathogen chlamydospore germination with glucose amendments of 0-1.0 mg/g of soil. Populations of general bacteria, actinomycetes, and fluorescent pseudomonads, both in soil and on watermelon roots, tended to be greater in suppressive than in conducive field soils, whereas fungal populations were greatest in conductive field soil. Root colonization by introduced F. a. niveum, indigenous F. oxysporum, or other microorganism groups was not consistently related to suppressiveness, suggesting that specific antagonistic strains rather than general populations of microorganisms may be involved in suppression.