Fusarium oxysporum f. sp. niveum race 1 is uniformly distributed throughout watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) growing regions, but F. oxysporum f. sp. niveum race 2 has a limited known distribution in the United States (Texas, Florida, Oklahoma, Maryland, and Delaware) (3,4). Since the spring of 2001, commercial watermelon fields in Knox and Gibson counties in southwestern Indiana have been observed with symptoms of one-sided wilt and vascular discoloration typical of Fusarium wilt. Race 2 of F. oxysporum f. sp. niveum was suspected as the casual agent since the diseased watermelon cultivars are considered resistant to races 0 and 1. Two isolates of F. oxysporum obtained from wilted watermelon plants in two different commercial fields and one isolate obtained from a wilted seedling in a transplant house were compared for pathogenicity in a greenhouse assay. Known isolates of F. oxysporum f. sp. niveum races 0, 1, and 2 were obtained from Don Hopkins (University of Florida, Apopka), Kate Everts (University of Maryland/University of Delaware, Salisbury, MD), and Ray Martyn (Purdue University, West Lafayette, IN), respectively, and were used for comparison. All isolates were grown in shake cultures in a mineral salts liquid medium. (1). After 72 hr, the predominately microconidal suspensions were filtered through cheesecloth and adjusted to 1 × 105 conidia/ml with the aid of a hemacytometer. A concentration of 1 × 105 condia/ml was shown previously to cause the desired disease reaction in the standard cultivars. Seedlings of the differential cvs, Black Diamond (universal susceptible), Charleston Gray (race 0 resistant), and Calhoun Gray (race 0 and 1 resistant) were grown in a 1:1, (v:v) sand/ vermiculite mixture to the first true-leaf stage after which the plants were uprooted and the roots carefully washed prior to root dip inoculation. Subsequent to inoculation, seedlings were planted in a sand/vermiculite/ peat mixture (4:1:1, [v:v:v]) with four seedlings to a 15-cm-diameter pot. The experimental design was a randomized complete block with five replications. Two isolates from the commercial field plants caused an average of 100% wilt on cv. Black Diamond, 95% wilt on cv. Charleston Gray, and 80% wilt on cv. Calhoun Gray, resulting in a designation of race 2. The isolate from a commercial transplant house resulted in 100, 60, and 15% wilt, respectively, on the three standard cultivars resulting in a race 1 designation. The presence of F. oxysporum f. sp. niveum race 2 in Indiana is significant because Indiana currently ranks fifth in the United States in watermelon production and there are no commercially available cultivars that possess resistance to race 2. To our knowledge, this is the first report of F. oxysporum f. sp. niveum race 2 in Indiana and the first report of race 2 from the Midwest region of the United States. Race 2, first described from the United States in 1985 (2), has now been confirmed in six states.
References: (1) R. Esposito and A. Fletcher. Arch. Biochem. Biophys. 93:369, 1961. (2) R. Martyn, Plant Dis. 69:1007, 1985. (3) R. Martyn, Plant Dis. 71:233, 1987. (4)
X. Zhou and K. Everts. Plant Dis. 87:692, 2003.
