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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. [ABSTRACT FROM AUTHOR]

Kosman diversity models were applied to analyses of virulence (disease reaction patterns) variation of 115 isolates of two cucurbit powdery mildew (CPM) species, Golovinomyces orontii (Go) and Podosphaera xanthii (Px), collected in the Czech Republic from 2010 through 2012. Diversity within and distances between Go and Px populations and each other in a spatio-temporal context and with regard to original host plant species were analyzed based on virulence patterns of individual isolates on a set of 21 melon (Cucumis melo L.) race differential genotypes. Significant differentiation among the Go and Px pathogen populations was revealed, and the results clearly demonstrate and confirm that the set of differential C. melo genotypes is well composed because of high differentiation capacity. Differentiation of pathogens among years was significant for both species. No significant difference between Go isolates from different host plant species was established due to high variability among Go isolates, but there was significant host-specific differentiation among Px isolates. Differentiation of pathogens among regions was not detected. These results revealed high virulence variation in isolates of Go and Px, and their spatio-temporal fluctuations. High diversity in virulence of Go isolates supports the treatment of Go as a complex of different (sub)species with distinct virulence factors. Similar relationships of selected Go isolates in a UPGMA dendrogram in a previously reported multigene phylogenetic tree support the logic and suitability of the Kosman assignment based approach to population studies of organisms with asexual or mixed modes of reproduction. The approach applied in this study provides a complex view of virulence structures of powdery mildew populations, and when combined with race determination and denomination on melon, it may serve as a base to understandvirulence variation of these CPM species from a spatio-temporal viewpoint. [ABSTRACT FROM AUTHOR]

Cucurbit powdery mildew (CPM), a disease on field and greenhouse cucurbit crops worldwide, is caused most frequently by two obligate erysiphaceous ectoparasites ( Golovinomyces orontii s.l., Podosphaera xanthii) that are highly variable in their pathogenicity and virulence. Various independent systems of CPM race determination and denomination are used worldwide, having to date been differentiated on different cultivars or lines of melon ( Cucumis melo L.). We briefly review historical perspectives and the current state of understanding of the virulence variation of the two CPM pathogens at the pathogenic race level, their differentiation and their designation. Furthermore, we propose for use by the international CPM research, breeding, seed and production community new tools to enhance research, communication and management of CPM. These tools consist of five components: 1) a set of 21 differential genotypes of Cucumis melo L. for the identification of CPM races; 2) a triple-part, septet code for meaningful, concise designation of CPM races; 3) protocols for maintaining CPM isolates and differential genotypes and for laboratory assays to examine the virulence of CPM isolates; 4) rules and principles of practical application of this system in breeding, seed production and cucurbit growing, including a proposal of a race denomination suitable for practical application; and 5) crucial activities leading to the implementation and running of new tools for CPM research and management. The five components of this package have equal importance, forming a compact system, and none of them can be omitted. [ABSTRACT FROM AUTHOR]