2019 ASHS Annual Conference

Downy mildew, caused by the obligate oomycete Peronospora effusa (= P. farinose f. sp. spinaciae [Pfs]), is the most economically important disease of spinach. The emergence of new races of the pathogen continue to overcome the new genetic resistances used in newly released resistant cultivars. Genetic resistance to Pfs in spinach governed by a number of major dominant genes (RPFs) are widely used in all major hybrid spinach cultivars. Characterizing races of the downy mildew pathogen, breeding and selection for resistance in spinach rely on a labor intensive screen on whole plants in a large tray format in temperature controlled growth and dew chambers. Resource efficient disease screening techniques minimizing space, greenhouse resources, seed, and labor could greatly improve both race typing and efforts to screen for disease resistance. The objectives of this work were to evaluate, validate, and standardize a detached leaf inoculation method to quantify downy mildew disease response and to differentiate resistant and susceptible spinach genotypes. Detached spinach leaves in petri dishes were inoculated by spraying a spore suspension followed by standard incubation protocols. Disease incidence and severity on detached leaves were compared to the corresponding variety in the standard whole plant assay. The downy mildew pathogen was found to infect, propagate, and maintain pathogenicity on the detached spinach leaves. A high positive correlation (|r| > 0.90) between disease incidence and disease severity was found between detached leaves and whole plant inoculations. The detached leaf assay was able to discriminate between the expected susceptible and resistant reactions and could greatly reduce the resources needed for whole plant evaluations.