Variability of Orobanche ramosa populations in France as revealed by cross infestations and molecular markers

The dramatic spread of root holoparasite Orobanche ramosa L. (branched broomrape) has been observed in the last few years in France in fields of Brassica napus L., Nicotiana tabacum L., and Cannabis sativa L. In the face of this occurrence and considering its tremendous capacity to adapt to its host, the question about variability of O. ramosa populations arises. The virulence of broomrape populations was investigated by cross infestations performed under greenhouse conditions using the three host species mentioned above and broomrape seeds collected from Orobanche plants parasitizing them. After 45 days of co-culture, the intensity and kinetics of infestation were determined. In all cases, parasites developed but differences were observed among associations between the host species and the broomrape population. Differences in virulence of O. ramosa populations and greater affinity between a host species and “its own” broomrape population were demonstrated by statistical analyses (F-test, CFAs). These results led to the concept of host specificity and suggestions about the existence of three different pathovars in O. ramosa. Analysis of genetic variability of the three O. ramosa populations by PCR of RAPD markers confirmed that they may be considered as three distinct pathovars.