Indigenous highly virulent accessions of the sunflower root parasitic weed Orobanche cumana

10 pages; 5 tables; 2 figuresOrobanche cumana (broomrape) is a severe constraint to sunflower production in southern and eastern Europe and the Middle East. Races A to E of this parasitic weed controlled by genes Or1 to Or5 have been described. A study of 38 seed accessions of O. cumana collected from different locations in Spain between 1983 and 2003 investigated the effect of long-term storage in the laboratory on germination and infectivity and assessed their virulence on a number of sunflower cultivars. Only 18 infected the susceptible cultivar B117. Infectivity was maintained for up to 17 years of storage, but with a greatly decreased vigour as compared with that of recently collected seed. The 12 oldest viable accessions overcame the resistance of the gene Or5 (in resistant line NR5). Seven out of them, in particular those collected in 1988 and 1989, were identified as race F. Three accessions were identified as race E allegedly holding components of higher virulence. Our results show evidence of the occurrence of race F prior to the use of sunflower hybrids resistant to race E, suggesting the former as indigenous to the country. This finding suggests the necessity of a continuous breeding of sunflower for resistance to O. cumana. The effectiveness and sustainability of genetic resistance must rely on the knowledge of the diverse virulence characteristics of O. cumana accessions.Ramón Areces Foundation (Madrid, Spain). Pioneer Hi-Bred SL (Sevilla, Spain), PR64A71Peer reviewe