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

Population dynamics and chlamydospore germination of Fusarium oxysporu m f. sp. niveum, as well as colonization of watermelon roots by F. oxy sporum, were monitored in relation to other microorganism populations and the incidence of Fusarium wilt in four soils representing differen t suppressive and conducive conditions. The soils consisted of an indu ced suppressive soil developed through monoculture to watermelon c cul tivar Crimson Sweet, a nonsuppressive monoculture soil, a conducive fa llow soil, and the suppressive soil rendered conducive by microwave tr eatment. 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. o xysporum in the field soils. Pathogen populations remained stable in t he monoculture soils over a 6-mo period, but increased somewhat initia lly and remained at higher levels when added to conducive soils. Suppr essiveness was not associated with inhibition of pathogen chlamydospor e germination. There were no differences among the field soils in path ogen chlamydospore germination with glucose amendments of 0-1.0 mg/g o f soil. Populations of general bacteria, actinomycetes, and fluorescen t pseudomonads, both in soil and on watermelon roots, tended to be gre ater in suppressive than in conducive field soils, whereas fungal popu lations were greatest in conductive field soil. Root colonization by i ntroduced F. a. niveum, indigenous F. oxysporum, or other microorganis m groups was not consistently related to suppressiveness, suggesting t hat specific antagonistic strains rather than general populations of m icroorganisms may be involved in suppression.