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Quantification of Root and Stem Colonization of Watermelon by Fusarium oxysporum f. sp. niveum and Its Use in Evaluating Resistance

August 2004 , Volume 94 , Number  8
Pages  832 - 841

X. G. Zhou and K. L. Everts

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

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Accepted for publication 26 March 2004.

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.

Additional keyword: Citrullus lanatus.

© 2004 The American Phytopathological Society