AbstractA system for scoring the virulence of isolates of Phytophthora fragariae based on a scale of root rot from 0 (no symptoms) to 5 (76-100% roots rotted) on a series of strawberry cultivars is described. Thirty-two single-zoospore isolates from one field site were compared by subjecting their root rot scores to cluster analysis and this grouped them into two major clusters equivalent to physiologic races B66-3 and B66-11. Different sub-clusters of isolates of race B66-11 produced different degrees of rotting on the same hosts. Apart from differences in virulence between the sub-clusters there was some evidence for differences in aggressiveness between isolates within sub-clusters. Increasing inoculum concentration by over 300-fold increased rotting by c. 25% but did alter the rankings of different isolate/host combinations. Repeated passage of isolates through cultivars of differing susceptibilities did not affect their pathogenicity.