EFFECT OF SUCCESSIVE WATERMELON PLANTINGS ON FUSARIUM-OXYSPORUM AND OTHER MICROORGANISMS IN SOILS SUPPRESSIVE AND CONDUCIVE TO FUSARIUM-WILT OF WATERMELON

Five successive greenhouse plantings of watermelon cultivars Florida Giant (susceptible to Fusarium wilt) and Crimson Sweet (moderately resistant and associated with soil suppressiveness) had different effects on the populations of Fusarium oxysporum f. sp. niveum, indigenous F. oxysporum, and various microorganism groups in the soil and on watermelon roots within four soils representing different suppressive and conducive conditions to Fusarium wilt. Pathogen populations were not affected by planting either cultivar in an induced suppressive soil developed by monoculture of Crimson Sweet or in a nonsuppressive Florida Giant monoculture soil. In a previously fallow, conducive soil and in a suppressive soil rendered conducive by microwave treatment, successive plantings of Florida Giant, but not Crimson Sweet, resulted in increasing populations of F. o. niveum. Indigenous populations of F. oxysporum showed no overall change in soil successively planted to Florida Giant, whereas planting Crimson Sweet resulted in increased populations in all field soils. Successive planting of Florida Giant also resulted in an increase in incidence of wilt, whereas planting Crimson Sweet maintained low wilt incidence throughout the study. Colonization of roots by F. o. niveum and other F. oxysporum was similar in both suppressive and nonsuppressive monoculture soils, indicating that suppression was not directly related to the degree of root colonization. Higher populations of actinomycetes, fluorescent pseudomonads, and overall bacteria occurred with successive plantings of Crimson Sweet than in nonplanted soil or most soils planted to Florida Giant. These results suggest that cultivar differences are responsible for the promotion of differences in rhizosphere microflora populations that are associated with soil suppressiveness.