Fitness and Virulence ofPhytophthora infestansIsolates from Sexual and Asexual Populations

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Abstract
Isolates of A1 mating type of Phytophthora infestans from Mexico (sexual population) were compared in growth chamber experiments with A1 isolates from the United States and Wales (asexual population). Fitness components assessed included the proportion of inoculated leaflets on which lesions developed, lesion areas per infected leaflet, and sporulation capacity. In addition, a composite fitness index was calculated as the product of the three fitness components. Virulence was assessed as the ability to produce sporulating lesions on detached leaflets of nine differential (R-gene) potato genotypes. The mean number of virulence factors per isolate was seven for isolates from the sexual population and three for isolates from the asexual population. Although isolates from the sexual population infected significantly fewer leaflets than did those from the asexual population, the resulting lesions were larger for isolates from the sexual than the asexual population. No significant population differences in sporulation capacity were observed, nor were isolates from one population more fit than those from the other on the basis of the composite fitness index. The number of virulence per isolate was not significantly correlated with the fitness of the isolates.