First author: University of Maryland, 27664 Nanticoke Road, Salisbury 21801; and second author: University of Maryland, Salisbury, with joint appointment with the University of Delaware, 16684 County Seat Highway, Georgetown 19947
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 102, 103, and 104 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, R2 = 0.85) or lower stems (Y = 17.3 ln [X + 1] - 78.6, R2 = 0.86). Percent wilt (Y) also was correlated with the ratio (Xr, 0 to 1 values) of lower stem to root colonization (Y = 34 ln Xr + 112, R2 = 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) (R2 ≥ 0.58). The greenhouse seedling stem colonization assay described may be utilized as a collaborative method to quantify Fusarium wilt resistance in watermelon.