Using a host differential screening method in pot and in hydroponic co-cultures, four populations of the sunflower root parasite Orobanche cumana Wallr. were studied for their pathogenicity against seven sunflower genotypes that are thought to carry specific resistance genes. These broomrape populations, which belong to three supposed races (D, E, and F), were observed to differ by their virulence. The Spanish supposed-race F population was the most virulent, showing the greatest number of Orobanche fixations and biomass reduction on sunflower genotypes resistant to the other tested populations (Romanian supposed-race D population and both Spanish and Bulgarian supposed-race E populations). However, from the differential screening none of the sunflowers was absolutely immune to all populations of broomrape since at least a few broomrape attachments, which sometimes became necrosed, were observed during some weeks on the most resistant roots. However, typical hypersensitive r! esponse was not observed on the host roots, whose exudates stimulate germination of all four O. cumana populations to the same extent, but with some variations depending on the Sunflower genotypes. Before attachment, Orobanche seedlings released cell-wall-degrading enzymes such as pectin methylesterase (PME, EC 3.1.1.11) and polygalacturonase (PG, EC 3.2.1.15). These pectinolytic activities were higher in the most virulent and recently discovered "race F".