Characterization of Fusarium race 1.2 resistance in melon and mapping of a major QTL for this trait near a fruit netting locus

An Israeli breeding line, BIZ, resistant to all four races of Fusarium oxysporum f.sp. melonis (FOM), has been characterized regarding its response to FOM race 1.2. The infection process of a FOM 1.2 strain that expresses the GFP reporter protein was monitored, suggesting the timing and sites in which fungal progression differs between a resistant and a susceptible genotype. A mapping population derived from the cross PI 414723 and BIZ was used to construct a linkage map with ~90 traits and markers, and QTL analysis of FOM1.2 resistance was initiated. Other traits such as flesh color and rind netting were mapped as well. A major recessive QTL for FOM1.2 resistance was located in linkage group 2, near a major locus controlling net development. INTRODUCTION Fusarium wilt of melon is caused by the soil fungus Fusarium oxysporum Schlechtend f.sp. melonis Snyd. & Hans. (FOM). The pathogen penetrates through the root system, invades the vascular elements and causes wilting and plant death (Gordon and Martyn 1997; Sherf and Macnab 1986). FOM isolates have been classified into four pathogenic races, according to their ability to infect a known set of differential melon genotypes (Risser et al 1976). Resistance to races 0, 1 and 2 is monogenic and dominant. Resistance to the fourth race, 1.2, is complex, and appears to be controlled by multiple recessive genes. Such resistance was described in the Far-Eastern line Ogon 9 and introgressed into Isabelle, a partially FOM 1.2-resistant cultivar (Perchepied and Pitrat 2004; Perchepied et al 2005), and QTL analysis revealed nine loci affecting the trait. In Israel, unpublished work by J. Katan and breeders from Zeraim Gedera Ltd. resulted in the development of a FOM 1.2 resistant line, designated BIZ, by selection of breeding material in the laboratory and in the field. An F1 hybrid, 'Adir', derived by crossing BIZ with a non-resistant counterpart, displayed good field resistance to all four races of Fusarium. 'Adir' was also used as a rootstock for the susceptible melon 1 Cucurbitaceae 2008, Proceedings of the IX EUCARPIA meeting on genetics and breeding of Cucurbitaceae (Pitrat M, ed), INRA, Avignon (France), May 21-24, 2008