Broomrape (Orobanche cumana Wallr.) is a root parasite of sunflower that is regarded as one of the most important constraints of sunflower production in the Mediterranean region. Breeding for resistance is the most effective method of control. P-96 is a sunflower line which shows dominant resistance to broomrape race E and recessive resistance to the very new race F. The objective of this study was to map and characterize quantitative trait loci (QTL) for resistance to race E and to race F of broomrape in P-96. A population from a cross between P-96 and the susceptible line P-21 was phenotyped for broomrape resistance in four experiments, two for race E and two for race F, by measuring different resistance parameters (resistance or susceptibility, number of broomrape per plant, and proportion of resistant plants per F(3) family). This population was also genotyped with microsatellite and RFLP markers. A linkage map comprising 103 marker loci distributed on 17 linkage groups was developed, and composite interval mapping analyses were performed. In total, five QTL ( or1.1, or3.1, or7.1 or13.1 and or13.2) for resistance to race E and six QTL ( or1.1, or4.1, or5.1, or13.1, or13.2 and or16.1) for resistance to race F of broomrape were detected on 7 of the 17 linkage groups. Phenotypic variance for race E resistance was mainly explained by the major QTL or3.1 associated to the resistance or susceptibility character ( R(2)=59%), while race F resistance was explained by QTL with a small to moderate effect ( R(2) from 15.0% to 38.7%), mainly associated with the number of broomrape per plant. Or3.1 was race E-specific, while or1.1, or13.1 and or13.2 of were non-race specific. Or13.1, and or13.2 were stable across the four experiments. Or3.1, and or7.1 were stable over the two race E experiments and or1.1 and or5.1 over the two race F experiments. The results from this study suggest that resistance to broomrape in sunflower is controlled by a combination of qualitative, race-specific resistance affecting the presence or absence of broomrape and a quantitative non-race specific resistance affecting their number.

A new race F of broomrape overcomes all known resistance genes in cultivated sunflower, but recently, sources of resistance against race F have been developed. The objective of the present research was to study the inheritance of resistance to race F in crosses between 12 resistant sunflower breeding lines, derived from three different sources of resistance, and the susceptible male-sterile line P-21. Parental lines and F 1 , F 2 , F 3 and BC 1 generations were evaluated for broomrape resistance. Segregations in the F 2 and BC 1 to resistant parent approached resistant to susceptible ratios of 1 : 15 and 1 : 3, respectively, in most of the crosses, suggesting a double dominant epistasis. However, segregations of 3 : 13 and 1 : 1 for F 2 and BC 1 , respectively, indicating a dominant-recessive epistasis, were also found. The F 3 data confirmed these results. Owing to the recessive nature of this resistance, it must be incorporated into both parental lines for developing resistant hybrid cultivars.

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Broomrape (Orobanche cumana Wallr.) populations belonging to the new race F in Spain have overcome all known resistance genes, Or 1 to Or 5 , in cultivated sunflower (Helianthus annuus L.) and are spreading rapidly. Resistance to race F of this parasitic weed has been found in wild perennial species of Helianthus and has been introgressed into cultivated sunflower. The objective of this study was to characterize the inheritance of resistance genes in cultivated sunflower derived from wild perennial species H. divaricatus and H. grosseserratus, respectively. Crosses between resistant cultivated lines and the susceptible line P21 were made, and the F 1 's were resistant when evaluated for broomrape resistance using a highly virulent population of race F, indicating dominance of resistance genes. Comparison of resistance of the segregating populations, F 2 and BC 1 F 1 , to both parents confirmed the dominance observed in the F 1 and indicated that resistance is under the control of a single dominant gene. This dominance of resistance genes will greatly simplify the breeding for resistance.

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