Genetic diversity and race composition of sunflower broomrape populations from Tunisia

Sunflower broomrape ( Orobanche cumana Wallr.) was detected in 2010 for the first time in sunflower fields of Tunisia, in the Beja region. No information is available on the race composition and genetic diversity of the broomrape populations parasitizing sunflower in that area. Plant tissue and seeds were collected from nine populations in Beja Sud (ORD, ORE), Amdoun (ORF, ORG), and Beja Nord (ORH to ORL) areas of the Beja region. Virulence studies on populations ORD, ORH and ORK revealed that the ORD population was essentially race E, whereas race G individuals were present in ORH and ORK poulations, with greater frequency in the ORK population. Cluster analysis of inter-population relatedness indicated that Tunisian populations were more related to Eastern European than to Spanish populations, with the Beja Nord populations clustering separately from the rest of Tunisian populations. Analysis at the intra-population level of the ORD, ORG, ORH and ORK populations revealed the existence of two gene pools (GP1 and GP2), that were present in all populations at different frequencies. GP2 was at a lower frequency (1/14) in ORD and ORG but at higher frequencies in ORH (5/15) and ORK (8/14). This indicates that GP1 was probably an initial introduction of a race E population, while GP2 is most likely a later introduction of a race G population, introduced initially in Beja Nord area but now spreading to other areas in the region. The risk of a generalized expansion of race G of broomrape in this sunflower cultivation area is discussed.