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 G iant (susceptible to Fusarium wilt) and Crimson Sweet (moderately resi stant 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 waterme lon roots within four soils representing different suppressive and con ducive conditions to Fusarium wilt. Pathogen populations were not affe cted by planting either cultivar in an induced suppressive soil develo ped by monoculture of Crimson Sweet or in a nonsuppressive Florida Gia nt monoculture soil. In a previously fallow, conducive soil and in a s uppressive soil rendered conducive by microwave treatment, successive plantings of Florida Giant, but not Crimson Sweet, resulted in increas ing populations of F. o. niveum. Indigenous populations of F. oxysporu m showed no overall change in soil successively planted to Florida Gia nt, whereas planting Crimson Sweet resulted in increased populations i n all field soils. Successive planting of Florida Giant also resulted in an increase in incidence of wilt, whereas planting Crimson Sweet ma intained low wilt incidence throughout the study. Colonization of root s by F. o. niveum and other F. oxysporum was similar in both suppressi ve and nonsuppressive monoculture soils, indicating that suppression w as not directly related to the degree of root colonization. Higher pop ulations of actinomycetes, fluorescent pseudomonads, and overall bacte ria occurred with successive plantings of Crimson Sweet than in nonpla nted soil or most soils planted to Florida Giant. These results sugges t that cultivar differences are responsible for the promotion of diffe rences in rhizosphere microflora populations that are associated with soil suppressiveness.