Phenotypic and Genotypic Characterization of Novel Strains of Spinach Downy Mildew Pathogen

Downy mildew, caused by the obligate pathogen <i>Peronospora effusa</i>, can have a devastating economic impact on spinach production. Growing resistant cultivars is the most economical way to manage this disease and the only viable management practice for organic spinach production. However, rapidly emerging races or novel strains may result in a breakdown of the resistance deployed. It is therefore critical to monitor the population dynamics of <i>P. effusa</i> and to determine the disease reactions of newly released cultivars to the new races and novel strains. In this study, 74 isolates of <i>P. effusa</i> were examined for their pathogenicity on differential host cultivars, resulting in the identification of three new races and 18 novel strains with unique virulence pathotypes. Of those identified, race 19 of <i>P. effusa</i> could infect many widely grown cultivars with resistance to <i>P. effusa</i> races 1 to 17. Targeted sequencing of DNA isolated from lesions of <i>P. effusa</i> race 19 isolates revealed genetic variations among isolates and within isolates of <i>P. effusa</i> race 19. The isolates showed either no genetic variation (only one genotype was found from multiple lesions of an isolate tested), limited, or abundant genetic variation (multiple genotypes were found within an isolate tested). Additionally, 70 commercial spinach cultivars were tested with two <i>P. effusa</i> races, 18 and 19, and four novel strains. The results of this study are helpful for growers in selecting suitable cultivars for production and for breeders in developing downy mildew-resistant cultivars.