vick, with resistance to both races A and B, were produced. Five races (A to E) have been reported in Roma-Broomrape (Orobanche cernua Loefl., syn. O. cumana Wallr.) nia (Vrâ nceanu et al., 1986) and other countries. These populations belonging to a new race F in Spain have overcome all races can be determined by using five sunflower differknown resistance genes Or 1 to Or 5 in cultivated sunflower (Helianthus annuus L.) and are spreading rapidly. All hybrids currently grown in entials, Kruglik A-41 (race A), Jdanov 8281 (race B), Spain are susceptible to race F, and sources of resistance genes for 'Record' (race C), 'S-1358' (race D), and 'P-1380' (race this race are needed to develop new resistant cultivars. The objective E), which carry the major resistance genes Or 1 to Or 5 , of this study was to evaluate sunflower germplasm for resistance to respectively. Broomrape races overcoming resistance race F (virulent population SE296). Using artificial inoculation with provided by Or 1 , Or 3 , and Or 4 genes, but not by Or 2 broomrape seed, 54 accessions of wild Helianthus spp. representing and Or 5 , were identified in Spain through using the 27 perennial and four annual species and 55 cultivated accessions of Romanian differentials (Melero-Vara et al., 1989). Later sunflower were evaluated after incubation for ≈ 1 mo in a growth studies have shown the evolution of the races of chamber. Helianthus seedlings were transplanted to the greenhouse broomrape in Spain and a new race that overcomes for an additional ≈ 3 mo to evaluate the broomrape infection. Most all the known resistance genes, including Or 2 and Or 5 perennial species of wild Helianthus were completely resistant to race F, but some accessions of the species H. divaricatus, H. maximiliani, (Saavedra del Rio et al., 1994;Alonso et al., 1996; Dom-and H. pauciflorus subsp. pauciflorus showed different proportions ínguez et al., 1996a; Melero-Vara, 1997). Following the of susceptible plants, with a disease incidence varying from 10 to 80%.O. cernua races nomenclature of Vrâ nceanu et al. The annual wild species, H. anomalus and H. agrestis, were fully (1986), the new race is designated race F. Genetic resisresistant, while segregation was observed in H. debilis subsp. cucumertance to race E, provided by gene Or 5 , has been identiifolius and H. exilis. Only 7.2% of the accessions of cultivated sunfied in Spain since 1989 (Saavedra del Rio et al., 1994) flower tested were fully resistant, with 20% of them segregating for and has been incorporated into recent hybrids. These resistance. The high frequency of broomrape resistance to race F hybrids were used successfully in Spain until the appearobserved in the perennial wild species, as well as the resistance found ance of race F. Populations of this race are present in in wild annual and cultivated germplasm, indicates that development both central and southern Spain. In random amplified of sunflower cultivars resistant to this new race of the parasite is feasible.

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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