Races of Isolates of Plasmopara halstedii from Spain and Studies on Their Virulence

Abstract

Plants infected with downy mildew were collected from 1994 to 2000 in sunflower fields in Spain. The race of 102 bulk isolates of Plasmopara halstedii obtained from them was determined by inoculation of sunflower lines traditionally used as differentials for characterization of the pathogen. Nine different races of the fungus were determined. Although race 1 was most common and was the only one found in central Spain, races 4, 6, and 7 were widespread in southern Spain. The results allowed the identification of a new race of the pathogen, race 10, and of a race not previously reported in Europe, race 8. According to the proposal of a new system for characterization of the isolates of P. halstedii in the late 1990s, Coded Virulence Formulae (CVF) were assigned to bulk isolates and single-sporangium (ss) isolates obtained from them. The CVF of the bulk isolates (CVFi) did not always fit with the previous designation. Similarly, ss isolates from the same bulk isolate exhibited different CVF, not only among themselves, but also compared with the CVF of the source isolate. Although a revision of the differential lines used to perform the racial characterization of fungal isolates seems to be needed, the occurrence of a diversity of genotypes in field populations of P. halstedii and a high frequency of recombination and/or mutation of the fungal genome is also suggested.