Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/13/2009
Publication Date: 1/1/2010
Citation: Zhou, X.G., Everts, K.L., Bruton, B.D. 2010. Race 3, a new and highly virulent race of Fusarium oxysporum f. sp. niveum causing Fusarium wilt in watermelon. Plant Disease. 94(1):92-98. Interpretive Summary: Fusarium wilt remains one of the greatest yield-limiting diseases of watermelon, worldwide. In 1894, Fusarium wilt was first described as a disease of watermelon in the southern United States. It was more than 50 years after the first description of Fusarium wilt on watermelon that differences in virulence of the fungus were first recognized. In 1972, two races were discovered in Italy and designated as race 0 and race 1. By 1973, scientists reported a highly aggressive isolate in Israel and designated it race 2. The results of this study demonstrated the existence of the fourth race (race 3) that is more aggressive than race 2. The existence of race 3 represents an additional threat to commercial production of watermelon and increases the difficulty of managing Fusarium wilt. Race 2, to which no resistance exists in current watermelon cultivars, is already distributed in many watermelon-growing areas in the United States. The presence of race 3 worsens this situation, especially for watermelon production areas like Maryland and Delaware where most farmers plant triploid (seedless) watermelon. Triploid watermelon cultivars are generally more susceptible to Fusarium wilt than diploid cultivars, and almost all currently available triploid watermelon cultivars (with a few exceptions such as Seedless Sangria and Revolution) are susceptible to race 1. The identification of this extremely aggressive strain necessitates the development of integrated productions systems to minimize losses along with genetic manipulation to increase disease resistance.
Technical Abstract: Three races (0, 1, and 2) of Fusarium oxysporum f. sp. niveum have been previously described in watermelon (Citrullus lanatus) based on their ability to cause disease on differential watermelon genotypes. Four isolates of F. oxysporum f. sp. niveum collected from wilted watermelon plants or infested soil in Maryland, along with reference isolates of races 0, 1, and 2, were compared for virulence, host range, and vegetative compatibility. Race identification was made on the differential watermelon genotypes Sugar Baby, Charleston Gray, Dixielee, Calhoun Gray, and PI-296341-FR using root-dip, tray-dip or pipette inoculation method. All four Maryland isolates were highly virulent, causing 78 to 100% wilt on all differentials, one of which was PI-296341-FR, considered highly resistant to race 2. The isolates also produced significantly greater colonization in the lower stems of PI-296341-FR than a standard race 2 reference isolate. In field microplots, two of the isolates caused over 90% wilt on PI-296341-FR in comparisons with no disease caused by a race 2 control isolate. All four isolates were non-pathogenic on muskmelon, cucumber, pumpkin, and squash, confirming their host-specific pathogenicity to watermelon. The Maryland isolates were vegetatively compatible to each other but not compatible with the race 2 isolates evaluated, indicating their genetic difference from race 2. This study demonstrated that the four Maryland isolates tested were more virulent than race 2 isolates and are proposed as a new race 3, the most virulent race of F. oxysporum f. sp. niveum described to date.