Resistance to race 1 of Fusarium oxysporum f. sp. niveum (E.F. Sm.) W.C. Snyder & H.N. Hans) is the most widely used tool for management of Fusarium wilt of watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai). However, this resistance is ineffective against the more aggressive F. oxysporum f. sp. niveum race 2. Race 2 was first identified in Israel in 1973 (2) and was subsequently reported in Texas (1981), Oklahoma (1988), and Florida (1989) (1). From July through September 2000 in Maryland and Delaware, 123 isolates of F. oxysporum f. sp. niveum were collected from wilted plants or plants with vascular discoloration from commercial production fields with moderate to severe wilt severity. Race determination was performed in the greenhouse on five race-differential cultivars. Differential cvs. Sugar Baby, Crimson Sweet, Charleston Gray, Allsweet, and Dixielee range from susceptible to highly resistant (in the order listed) to race 1, but all are susceptible to race 2. Seeds were planted in pots containing pasteurized vermiculite and peat moss (2:3 mixture) growth medium infested with 1 × 105 conidia per g of growth medium. Conidia were harvested from 5- to 6-day-old cultures of F. oxysporum f. sp. niveum grown in a liquid mineral salts medium (2). Control pots were treated with water or diluted liquid medium. Pots were maintained in a greenhouse at 19 to 28°C. Three replicate pots containing seven plants each were used for each isolate-cultivar combination. Race 0 obtained from B. D. Bruton (USDA-ARS, Lane, OK), and races 1 and 2 obtained from the American Type Culture Collection (Manassas, VA) were included in the tests for comparative purposes. Eleven isolates from Maryland and three isolates from Delaware consistently caused 60 to 100% wilt or mortality of all cultivars, with a mean of more than 75% wilt. Percent wilt of cvs. Sugar Baby and Dixielee to races 0, 1, and 2, and the 14 isolates from Maryland and Delaware was 78, 76, 100, and 95 to 100% and 0, 14, 59, and 63 to 93%, respectively. All control plants remained healthy. In each of the three replicated tests, these 14 isolates caused an equal or greater incidence of wilt as the reference race 2 isolate, and cvs. Dixielee and Allsweet were susceptible to these isolates. Therefore, these isolates were identified as F. oxysporum f. sp. niveum race 2. This is the first report of F. oxsporum f. sp. niveum race 2 occurring in Maryland and Delaware.
References: (1) R. D. Martyn and B. D. Bruton. HortScience 24:696, 1989. (2) D. Netzer. Phytoparasitica 4:131, 1976.