Quantification of Root and Stem Colonization of Watermelon byFusarium oxysporumf. sp.niveumand Its Use in Evaluating Resistance

Colonization of watermelon root and stem tissues by Fusarium oxysporum f. sp. niveum race 1 and its relationship to the apparent resistance to Fusarium wilt was investigated. In each of 2 years, 17 differentially susceptible watermelon cultivars and one accession were tested in the greenhouse, and 7 cultivars also were tested in the field. Colonization by a chlorate-resistant marked isolate of the fungus was assayed by plating homogenized tissue samples on a selective medium. Six days after inoculation, seedlings of highly resistant, moderately resistant, and susceptible cultivars had F. oxysporum f. sp. niveum race 1 CFU counts in the lower stems of 10(2), 10(3), and 10(4) CFU/g of fresh tissue, respectively. Percent wilt (Y) of the seedlings was positively correlated with colonization (X) by F. oxysporum f. sp. niveum race 1 in roots (Y = 21.2 ln [X + 1] - 140.7, R(2) = 0.85) or lower stems (Y = 17.3 ln [X + 1] - 78.6, R(2) = 0.86). Percent wilt (Y) also was correlated with the ratio (X(r), 0 to 1 values) of lower stem to root colonization (Y = 34 ln X(r) + 112, R(2) = 0.36). Field evaluations confirmed these relationships, and a link between cultivar resistance and a reduced rate of spread of the fungus in primary stems during a season was observed. Fruit yield decreased with increased tissue colonization at linear rates of 9.9 to 12.7 t/ha per ln (CFU/g + 1) (R(2) >/= 0.58). The greenhouse seedling stem colonization assay described may be utilized as a collaborative method to quantify Fusarium wilt resistance in watermelon.