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Races and Inoculum Density of Fusarium oxysporum f. sp. niveum in Commercial Watermelon Fields in Maryland and Delaware

A survey was conducted to determine races and inoculum density of Fusarium oxysporum f. sp. niveum, the causal agent of Fusarium wilt of watermelon in Maryland and Delaware. Virulence on six differential cultivars was tested for each of 63 isolates of F. oxysporum f. sp. niveum, obtained from 25 commercial watermelon fields. Thirteen isolates (21%) were identified as race 0, 36 isolates (57%) as race 1, and 14 isolates (22%) as race 2. Races 0 and 1 were present in 12 (48%) and 10 (40%) of the fields, respectively. The highly aggressive race 2 was identified from five fields in two counties in Maryland and from one field in Delaware, representing 24% of the fields. Race 2 was copresent with one or two other races. Race 2 (19 isolates) predominated among the 25 isolates obtained from a research field in Maryland. Nineteen commercial fields had inoculum densities of F. oxysporum f. sp. niveum ranging from 100 to 1,200 CFU/g of soil at harvest. Within this range of inoculum densities, >20% incidence of wilt was observed when the susceptible watermelon cv. Sugar Baby was planted in samples of soil collected from these fields. The relationship (P < 0.0001) between inoculum density of F. oxysporum f. sp. niveum (X) and incidence of Fusarium wilt (Y) on Sugar Baby was best described using the monomolecular equation, Y = 1 - exp^{[-0.0013 (X + 166)]}. The ratio of pathogenic to total population of F. oxysporum in the fields linearly increased with increasing inoculum density of F. oxysporum f. sp. niveum (R ² = 0.4; P < 0.0009).