Downy mildew of spinach, caused by the obligate pathogen Peronospora effusa, remains the most important constraint in the major spinach production areas in the United States. This disease can potentially be initiated by asexual sporangiospores via "green bridges", sexually derived oospores from seed or soil, or dormant mycelium. However, the relative importance of the various types of primary inoculum is not well known. The ability of P. effusa sporangiospores to withstand abiotic stress, such as desiccation, and remain viable during short- and long-distance dispersal and the ability of oospores to germinate and infect seedlings remain unclear. Thus, the primary objectives of this research were to evaluate the impact of desiccation on sporangiospore survival and infection efficiency and examine occurrence, production, and germination of oospores. Results indicate that desiccation significantly reduces sporangiospore viability as well as infection potential. Leaf wetness duration of 4 h was needed for disease establishment by spinach downy mildew sporangiospores. Oospores were observed in leaves of numerous commercial spinach cultivars grown in California in 2018 and Arizona in 2019. Frequency of occurrence varied between the two states-years. The presence of opposite mating types in spinach production areas in the United States was demonstrated by pairing isolates in controlled crosses and producing oospores on detached leaves as well as intact plants. Information from the study of variables that affect sporangiospore viability and oospore production will help in improving our understanding of the epidemiology of this important pathogen, which has implications for management of spinach downy mildew.
Keywords: Peronospora; controlled mating; desiccation; downy mildew; etiology; heterothallic; inoculum; oospore; spinach; sporangiospore viability; spore.