Update on breeding for resistance to sunflower broomrape

Abstract

Sunflower broomrape (Orobanche cumana Wallr.) is currently regarded as one of the most important constraints in sunflower (Helianthus annuus L.) production. In the last fifteen years, efforts to introduce genetic sources of resistance to this parasite in sunflower hybrids were followed by the occurrence of new virulent races that promptly overcame all known resistance genes. Due to this situation, most of the research efforts have focused on the development and characterization of new sources of genetic resistance to the most virulent races and also on the genetic structure and variability of O. cumana populations and their evolution, in order to develop long-term strategies for sunflower broomrape management. Results of evaluation of sunflower germplasm for resistance to different broomrape races have demonstrated that wild Helianthus species constitute the major reservoir of genes conferring resistance to new virulent races. Cultivated germplasms are also valuable sources of resistance genes. Most of the resistant sources have been found to be controlled by major genes, although quantitative resistance and epistatic effects have also been reported. Different dominance reactions, depending on the race of broomrape, the source of resistance and the susceptible parental line used for crossing have been observed. Molecular studies have shown that phenotypic variance for race E resistance was mainly explained by a major QTL associated to the resistance or susceptibility character, while race F resistance was explained by several QTL with a small to moderate effect, mainly associated with the number of broomrapes per plant, suggesting the existence of a quantitative component in the resistance to race F. The relevance of these findings for sunflower breeding for resistance to broomrape Is discussed.