ABSTRACT
Four physiological races (0, 1, 2, and 1.2) of Fusarium oxysporum f. sp. melonis, causal agent of Fusarium wilt in melons, have been described. Whereas resistance against each of the races 0, 1, and 2 is encoded by a single dominant R-gene, resistance against the fourth race, FOM 1.2, is polygenically inherited and was recently characterized in the cultivar Isabelle. In the present study, we report on an independently derived source of resistance to race 1.2, the Israeli breeding line BIZ. The disease response of BIZ seedlings was compared with two susceptible genotypes, Line 33 and PI 414723, and the partially resistant genotype, Isabelle, at increasing concentrations of inoculum. BIZ exhibited near-complete resistance to race 1.2, even at inoculum levels of 106 spores per ml and root wounding, indicating that such resistance is stronger than that in Isabelle. Although in previous studies the F1 hybrids between BIZ and the sensitive lines displayed full resistance in the field, in the present study they were susceptible under artificial inoculation conditions that involve high inoculum concentrations of 106 spores per ml and root wounding; under intermediate inoculum levels (105 and 5 × 105 spores per ml) they were partially resistant. Segregation of the resistance response in the F2 and back-cross-1 progeny from a cross between BIZ and PI 414723 supported a model in which two complementary, recessive genes are required to obtain full resistance. While the fungus was shown to colonize root tissue of both resistant and susceptible lines, colonization was markedly lower in the resistant plants, and the fungus was nearly absent from upper stem segments.
