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Disease Note:

PI 296341, a Source of Resistance in Watermelon to Race 2 of Fusarium oxysporum f. sp. niveum. D. Netzer, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. R. D. Martyn, Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77843. Plant Dis. 73:518. Accepted for publication 14 March 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0518E.

Race 2 of Fusarium oxysporum Schlecht. (emend. Snyd. & Hans.) f. sp. niveum E. F. Smith, causal agent of Fusarium wilt of watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) overcomes all current wilt-resistant cultivars as well as numerous accessions and wild species collected in Israel and subtropical Africa. Race 2 was first observed in Israel, and its occurrence also has been confirmed in the United States-in Texas, Oklahoma, and Florida (1,2; R. D. Martyn and B. D. Bruton, unpublished). Screening for race 2-resistant germ plasm began in 1978, and in 1986 the original planting of one Citrullus sp. accession, PI 296341, segregated into resistant and susceptible plants when inoculated with the Israeli isolate of race 2 of F. o. f. sp. niveum. Resistant plants were self-pollinated for two generations and retested for resistance using three isolates of race 2: IS-59 (Israel), TX-XID (Texas), and OK-270 (Oklahoma). Root-dip inoculation (1.5 X 106 conidia per milliliter) of 10- to 12-day-old seedlings resulted in inconsistent wilt (25-65% after 3 wk), whereas plants inoculated at the three-leaf stage (21-24 days old) were uniformly resistant; cv. Calhoun Gray (resistant to races 0 and I) was 100% wilted. This PI entry may serve as a valuable source of resistance to F. o. f. sp. niveum race 2.

References: (1) B. D. Bruton et al. Plant Dis. 72:734. 1988. (2) R. D. Martyn. Plant Dis. 71:233, 1987.