Host-parasite interactions between Cucurbitaceae and P. cubensis exhibit significant variation. This contribution reviews the current state of knowledge regarding characterization of P. cubensis virulence variation on the level of pathotypes (variation in host genera and species host-range) and races (variation in intraspecific level). However, our knowledge of the interactions between P. cubensis isolates and the most important genera and species of cultivated cucurbits is limited. The concept of pathotypes identification was introduced in 1980s with some modifications later. The former concept was missing some crucial requirements to be applied and comparable internationally. It was the reason why an improved differential set of six cucurbit genera and 12 genotypes (Benincasa, Citrullus, Cucumis, Cucurbita, Lagenaria, and Luffa) was developed to characterize pathotypes among P. cubensis isolates, and is broadly used. This concept allows application of various mathematical approaches for virulence comparison. Long-lasting research of interactions between cucurbits (Cucumis spp., Cucurbita spp.) and Pseudoperonospora cubensis demonstrated frequent expression of race-specific reaction patterns. The differential set of 21 genotypes of Cucumis melo was developed for determination of cucurbit powdery mildew races. Most recent research of virulence variation of P. cubensis population in the Czech Republic showed very broad spectrum of virulence patterns on Cucumis melo demonstrating existence of huge number of races by this pathogen. The approach of how to determine races of P. cubensis is discussed and demonstrated. Utilization and combination of both approaches (establishment of pathotypes and races) are important for science as well as for practical application in cucurbit resistance breeding.