Disease Note

An Aggressive Race of Fusarium oxysporum f. sp. niveum New to the United States. R. D. Martyn, Department of Plant Pathology & Microbiology, Texas A&M University, College Station 77843. Plant Dis. 69:1007. Accepted for publication 19 July 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-1007c.

A highly aggressive race of Fusarium oxysporum f. sp. niveum (E. F. Sm.) Synd. & Hans. (FON), causal agent of Fusarium wilt of watermelon (Citrullus lanatus (Thunb.) Matsun. & Nakai), was isolated in 1981 from wilted plants in south central Texas and in 1984 from a commercial seed lot produced in north central Texas. In greenhouse tests at 10⁶ microconidia per milliliter, the plant and seed isolates each caused 87–100% wilt in 14 watermelon cultivars, 11 of which are considered moderately or highly resistant. In contrast, race 1 (ATCC 18467) caused > 80% wilt in only one highly wilt-resistant cultivar, with the remaining cultivars showing 0–53% wilt. Two of the three described races of FON (0 and 1, sensu Cirulli) are known to be in the United States. Race 2 was described from Israel in 1976 and is highly aggressive to several American cultivars with high wilt resistance. The Texas isolates appear similar to race 2, but until comparative testing is done with the Israeli race 2, no race number designation is being assigned. The detection of this new race in growers’ fields in south Texas and commercial seed from north Texas suggests it is already widespread.