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PLANT GENETIC RESOURCES

Selection of Wild and Cultivated Sunflower for Resistance to a New Broomrape Race that Overcomes Resistance of the Or₅ Gene

^a Departamento de Mejora y Agronomía, Córdoba, Spain
^b Departamento de Protección de Cultivos, Instituto de Agricultura Sostenible, CSIC, Apdo. 4084, 14080, Córdoba, Spain
^c Departamento de Mejora y Agronomía, Centro de Investigación y Formación Agraria, Apdo. 3092, 14080, Córdoba, Spain

cs9femaj{at}uco.es

	ABSTRACT

TOP ABSTRACT INTRODUCTION Material and methods Results and discussion REFERENCES

 
Broomrape (Orobanche cernua Loefl., syn. O. cumana Wallr.) populations belonging to a new race F in Spain have overcome all known resistance genes Or₁ to Or₅ in cultivated sunflower (Helianthus annuus L.) and are spreading rapidly. All hybrids currently grown in Spain are susceptible to race F, and sources of resistance genes for this race are needed to develop new resistant cultivars. The objective of this study was to evaluate sunflower germplasm for resistance to race F (virulent population SE296). Using artificial inoculation with broomrape seed, 54 accessions of wild Helianthus spp. representing 27 perennial and four annual species and 55 cultivated accessions of sunflower were evaluated after incubation for 1 mo in a growth chamber. Helianthus seedlings were transplanted to the greenhouse for an additional 3 mo to evaluate the broomrape infection. Most perennial species of wild Helianthus were completely resistant to race F, but some accessions of the species H. divaricatus, H. maximiliani, and H. pauciflorus subsp. pauciflorus showed different proportions of susceptible plants, with a disease incidence varying from 10 to 80%. The annual wild species, H. anomalus and H. agrestis, were fully resistant, while segregation was observed in H. debilis subsp. cucumerifolius and H. exilis. Only 7.2% of the accessions of cultivated sunflower tested were fully resistant, with 20% of them segregating for resistance. The high frequency of broomrape resistance to race F observed in the perennial wild species, as well as the resistance found in wild annual and cultivated germplasm, indicates that development of sunflower cultivars resistant to this new race of the parasite is feasible.

Abbreviations: R, resistant • S, susceptible • SE, segregating

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Material and methods Results and discussion REFERENCES

 
SUNFLOWER is one of the most important annual oilseed crops in the world. Broomrape, a holoparasitic angiosperm that infects sunflower roots, is currently regarded as being one of the most important constraints for sunflower production in Spain and the regions around the Black Sea (Alonso et al., 1996; Bülbül et al., 1991; Domínguez et al., 1996a; Shindrova, 1994).

Genetic resistance to broomrape, originating mainly from the wild Helianthus species, has been introduced into sunflower cultivars from early sunflower breeding programs in the former USSR (Pustovoit, 1966). The widespread use of resistant cultivars usually leads to the appearance of new races of the parasite that overcome the resistance genes (Skoric, 1988). In the former USSR, race A of O. cernua was first described after the release of resistant cultivars such as Kruglik A-41. Race B overcame this resistance, but new resistant cultivars such as Jdanov 8281 and, later, other cultivars such as Peredovick, with resistance to both races A and B, were produced. Five races (A to E) have been reported in Romania (Vrânceanu et al., 1986) and other countries. These races can be determined by using five sunflower differentials, Kruglik A-41 (race A), Jdanov 8281 (race B), `Record' (race C), `S-1358' (race D), and `P-1380' (race E), which carry the major resistance genes Or₁ to Or₅, respectively. Broomrape races overcoming resistance provided by Or₁, Or₃, and Or₄ genes, but not by Or₂ and Or₅, were identified in Spain through using the Romanian differentials (Melero-Vara et al., 1989). Later studies have shown the evolution of the races of broomrape in Spain and a new race that overcomes all the known resistance genes, including Or₂ and Or₅ (Saavedra del Rio et al., 1994; Alonso et al., 1996; Domínguez et al., 1996a; Melero-Vara, 1997). Following the O. cernua races nomenclature of Vrânceanu et al. (1986), the new race is designated race F. Genetic resistance to race E, provided by gene Or₅, has been identified in Spain since 1989 (Saavedra del Rio et al., 1994) and has been incorporated into recent hybrids. These hybrids were used successfully in Spain until the appearance of race F. Populations of this race are present in both central and southern Spain. In random amplified polymorphic DNA analysis (Gagne et al., 1998), one population of race F showed the lowest intrapopulation genetic diversity, which was to be expected for a new race of recent origin.

A high level of resistance to race E of O. cernua was found in collections of both wild and cultivated sunflower (Domínguez et al., 1996b; Ruso et al., 1996). Since all broomrape-resistant sunflower hybrids currently used in Spain, as well as hybrids planted in other European countries, are based on the Or₅ gene, they are highly susceptible to race F of broomrape. The search for plant material resistant to this new race F is urgently needed. The objective of our research was to identify resistance to the new broomrape race F in wild and cultivated sunflower.

	Material and methods

TOP ABSTRACT INTRODUCTION Material and methods Results and discussion REFERENCES

 
The plant material evaluated comprised 54 accessions (Table 1) representing 27 perennial Helianthus species and subspecies, four annual species that showed resistance to race E (Ruso et al., 1996), and 55 cultivated accessions (Table 2) with different origins, showing some level of resistance to Orobanche in field tests in Thrace, Turkey (Gulya et al., 1994). All were obtained from the USDA-ARS, Regional Plant Introduction Station, Ames, IA. Three cultivated inbred lines — HA89 and P21, susceptible to race E, and the Romanian differential P-1380, with resistance to race E provided by the Or₅ gene (Vrânceanu et al., 1986) — were used as checks in the evaluation of wild material. The checks used for the evaluation of cultivated material were two commercial hybrids (`Turbo' and `Isla') resistant to race E.

Plants showing emerged or underground broomrape stalks were considered susceptible, otherwise they were regarded as resistant. Accessions were considered resistant (R) when no susceptible plants were found within the complete entry, (0% incidence), susceptible (S) when no resistant plants were present (100% incidence), and segregating (SE) when at least one plant within the entry was found to be susceptible (0% < incidence < 100%). For the susceptible plants the number of broomrape shoots per sunflower plant was also recorded and disease severity was calculated for each entry as the average of emerged plants of O. cernua per infected sunflower plant (Sukno et al., 1998). For this index, one-way classification analysis of variance was performed using the infected sunflower plants as replications. The GLM procedure of SAS (SAS Institute, 1990), which allows analysis of data with unequal sample size, was used. Mean comparisons of disease severity among entries were made by the least significant difference test.

	Results and discussion

TOP ABSTRACT INTRODUCTION Material and methods Results and discussion REFERENCES

 
All the cultivated checks, including the differential line P-1380 and hybrids Turbo and Isla carrying the gene Or₅, were fully susceptible (100% incidence) to race F (Tables 1 and 2), indicating the high virulence of the race. All accessions of 24 wild perennial species and subspecies showed complete resistance, whereas some accessions of the species H. divaricatus, H. maximiliani, and H. pauciflorus subsp. pauciflorus showed segregation for resistance. In the case of H. maximiliani, most of the accessions had a varying number of susceptible plants, with disease incidence ranging from 10 to 80% and disease severity values not significantly different from those of the P-1380 check (Table 1). For the annual species, only H. agrestis and H. anomalus showed complete resistance, while H. debilis and H. exilis were segregating (Table 1). Disease severity of susceptible entries ranged from 1.0 to 3.2 for the wild, 3.6 to 14.2 for the cultivated accessions, and 5.6 to 18.2 for the susceptible checks. The values for susceptible plants of wild segregating accessions were lower than those of cultivated accessions and susceptible checks (Tables 1 and 2).

The high frequency of resistant entries of perennial species to race F coincides with a previous evaluation by Ruso et al. (1996) in which other less virulent populations of O. cernua, unable to overcome the Or₅ gene, were used. However, the species H. gracilentus and H. nuttalli subsp. nuttalli used in our study were fully resistant to race F, whereas other accessions of these species used in a previous study were fully susceptible or segregating when inoculated with two broomrape races less virulent than race F (Ruso et al., 1996). Accessions of two wild annual species, H. debilis subsp. cucumerifolius (unpublished) and H. exilis (Ruso et al., 1996), were resistant to broomrape populations of previous races, but segregated when inoculated with race F (Table 1). Other accessions of H. debilis subsp. cucumerifolius were found to be susceptible to the former races of O. cernua (Ruso et al., 1996).

Most of the perennial species tested were highly resistant to less virulent races and to the new race F of broomrape, although intraspecific variability was evident. Three species, H. divaricatus, H. maximiliani, and H. pauciflorus subsp. pauciflorus, were segregating for resistance to race F in some accessions, while previously they showed full resistance to older races, thus confirming the greater virulence of race F (Ruso et al., 1996). The wild annual species, H. agrestis and H. anomalus, were resistant to the less virulent races and to race F of broomrape, thereby providing resistance to all known races of Orobanche found in Spain. In contrast, the resistance previously found in H. debilis subsp. cucumerifolius and H. exilis has broken down for the new race of O. cernua. Therefore, intraspecific variability should be considered in future screenings.

Fifteen of the 55 cultivated accessions tested showed resistant plants, but only four of them did not segregate for resistance (Table 2). In a previous study, most of these cultivated entries were tested against race E (Domínguez, 1997, unpublished data). The results of these tests showed a higher percentage of entries resistant or segregating to race E compared with when tested against race F. This was also observed by Sukno et al. (1999), who found that only two out of six lines showing resistance to previous races were also resistant to race F. This would suggest that in the sunflower germplasm collection, the frequency of the Or₅ resistance gene is higher than the gene(s) conferring resistance to race F. Sukno et al. (1999) suggested that resistance to race F could be conferred by additional alleles at the Or locus or by a cluster of very tightly linked nonallelic genes. In any case, since those entries showing resistance to the new race F are also resistant to previous races, genes or alleles conferring resistance to race F are also functional against other less virulent races. This fact had been reported previously by Vrânceanu et al. (1986), who found five dominant genes Or₁ to Or₅ each conferring resistance to the last race discovered and to the previous races.

The main goal of this research was to identify sources of resistance to race F of O. cernua present in Spain, which are not currently available. The results look very promising since both cultivated and wild germplasm were identified that can be used in breeding for resistance to race F. Although a higher frequency of resistant entries was found in wild perennial accessions, cultivated accessions with resistant or a reduced number of susceptible plants are preferred for short-term breeding programs and are recommended for advancement in cultivar development programs, especially some parental lines (RHA 276, RHA 278, and RHA 801) released by the USDA (Fick et al., 1979; Roath et al., 1981). These three lines as well as other resistant or segregating accessions had shown some degree of resistance to a virulent race in Turkey in previous studies (Gulya et al., 1994). Therefore, the derived plant breeding material could also be of interest for Turkey and other Eastern European countries where Orobanche races have also rapidly evolved. The segregation for broomrape resistance of the parental lines, also observed by Gulya et al. (1994), is not completely unexpected since they were not selected for this trait during the process of selection and they were released in the relatively early F₅ generation (Fick et al., 1979; Roath et al., 1981). Sunflower lines released in early generations have been reported to segregate for other traits such as resistance to downy mildew [Plasmopara halstedii (Farl.) Berl. and de Toni] and rust (Puccinia helianthi Schwein.) and seed coat color (Fick and Zimmer, 1974).

The resistance to broomrape in wild Helianthus species has been known since early sunflower breeding research in the former USSR (Pustovoit, 1966) to recent reports (Korrell et al., 1996; Ruso et al., 1996). We examined a greater number of perennial species showing complete resistance and low intraspecific variability. Because of the high number of resistant perennial Helianthus spp. and their great genetic diversity, the potential of these species for providing resistance to future races of broomrape seems to be greater than that of cultivated germplasm. Genes for resistance to other sunflower pathogens, such as rust, appear to be present in wild species with a high frequency, but completely resistant or completely susceptible populations are rarely found (Quresh et al., 1993). Sunflower rust is present in North America, where wild species are native and host–pathogen coevolution probably enhanced the diversity for resistance in host populations. In the case of O. cernua, which is not present in North America, the high levels of resistance present in wild Helianthus species may have occurred fortuitously. The lower frequency of resistance in cultivated accessions, many of them originating in the former USSR and tracing back to perennial species (Pustovoit, 1966), suggest that natural and conscious selection pressure in Europe shifted this germplasm to susceptibility. This resistance is suitable for breeding purposes, since the transfer of resistance to O. cernua from resistant perennial Helianthus species with different ploidy levels to cultivated sunflower has been achieved (Sukno et al., 1998). However, the process is much more laborious and time consuming than when resistant cultivated accessions are used. Wild annual species, except H. agrestis, show higher cross compatibility with cultivated sunflower than the perennials, thus facilitating the introgression of resistance into commercial breeding lines.

The identification of genes or alleles controlling broomrape resistance, as well as the type of gene action involved, is urgently needed. Studies to transfer resistance from the wild species and studies of the genetic and allelic relationship of the resistance to the new race F of O. cernua are underway. In addition, an accurate molecular characterization of O. cernua races, as an alternative to the conventional characterization by the reactions on sunflower differentials, is crucial in the search for stable sources of resistance.

Received for publication January 7, 1998.

	REFERENCES

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