Fusarium oxysporum Schechtend.:Fr. f. sp. phaseoli J. B. Kendrick & W. C. Snyder (FOP) is the causal agent of the common bean (Phaseolus vulgaris L.) disease known as Fusarium wilt or Fusarium yellows. FOP has been reported from the Castilla y Leon region in Spain, where it is a serious problem on most commercial bean cultivars (1). Five FOP isolates from Spain (AB-6, AB-111, AB-112, AS-1, and AS-4) obtained from J. M. Díaz-Mínguez and the isolate FOP-CO1 (ATCC 90245) from Colorado were tested for pathogenicity on two American lines: Pinto U.I. 114, considered as a universal susceptible check, and Flor de Mayo, a Mexican landrace. Seedlings were root-clip inoculated and evaluated according to the CIAT 1 to 9 severity scale, in which 1 to 3 = resistant, 3.1 to 6 = intermediate, and 6.1 to 9 = susceptible. Three inoculum concentrations were tested: 104, 105, and 106 conidia/ml. AS-1 and AS-4 produced resistant reactions in both U.I. 114 (1.5 to 2.1 and 1.2 to 1.5) and Flor de Mayo (1.9 to 2.3 and 1.4), respectively, at all inoculum concentrations. Susceptible reactions to AB-6 (7.3 to 8.9), AB-111 (7.6 to 8.9), AB-112 (7.5 to 7.9), and FOP-CO1 (8.1 to 8.6) were observed in Pinto U.I. 114, regardless of inoculum concentration, although severity ratings increased as the inoculum concentration was increased. Flor de Mayo exhibited a susceptible reaction (6.7 to 7.8) to FOP-CO1 at all inoculum concentrations tested, and intermediate reactions to AB-6 (4.9), AB-111 (5.4), and AB-112 (5.0) at the lowest inoculum concentration. Susceptible reactions (7 to 8.2 for AB-6; 6.8 to 7.0 for AB-111, and 7.3 to 7.7 for AB-112) occurred with higher inoculum concentrations, and severity ratings increased as the inoculum concentration increased. Recently, FOP isolates from Greece and Italy were recognized as each belonging to different pathogenic races (2); consequently, more research on the Spanish isolates (AB-6, AB-111, and AB-112) is needed to determine if they are similar to the races reported from the Mediterranean Basin or should be classified as a new FOP race (s).
References: (1) J. M. Díaz-Mínguez et al. Plant Dis. 80:600, 1996. (2) S. L. Woo et al. Phytopathology 86:966, 1996.
