Pseudoperonospora cubensis, causal agent of cucurbit downy mildew, is a common and highly variable pathogen of cucurbit vegetables. Pathogen virulence can be considered on two different levels, that is, pathotypes and races. Pathotypes characterize the interactions between pathogen isolates and the set of genotypes represented by different species and/or genera of the host plant family. Races are characterized by the interactions between pathogen isolates and the set of genotypes (e.g., cultivars, lines, accessions) of one host species (or very closely related species). Differential sets for pathotype and/or race determination are composed of distinct (different) genotypes. Until recently, only differentiation of pathotypes was used to characterize P. cubensis. Most recent research of virulence variation in P. cubensis populations in the Czech Republic working with the Cucumis melo differential set demonstrated the existence of a large number of races of this pathogen. The differential capacity of this set was primarily created for the determination and denomination of virulence phenotypes/races of cucurbit powdery mildew (Golovinomyces orontii, Podosphaera xanthii). However, this set is also efficient for differentiating virulence phenotypes in P. cubensis. This means that one set of C. melo differential genotypes can be used for two independent plant–pathogen systems: cucurbit powdery mildew and cucurbit downy mildew. Results showed that isolates of P. cubensis developing identical virulence reaction patterns at the level of pathotype, develop different reaction patterns at the level of races and vice versa. It is demonstrated and explained that pathotypes and races are two different categories for the assessment of virulence variation, which should be clearly defined, recognized and used within international scientific and breeding communities.
