Sporangiospore viability and oospore production in the spinach downy mildew pathogen, Peronospora effusa

Authors: DHILLON, BRAHAM - University Of Arkansas; VILLARROEL-ZEBALLOS, MARIA - University Of Arkansas; CASTROAGUDIN, VANINA - University Of Arkansas; BHATTARAI, GEHENDRA - University Of Arkansas; Klosterman, Steven; CORRELL, JAMES - University Of Arkansas

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2020
Publication Date: 4/14/2020
Citation: Dhillon, B., Villarroel-Zeballos, M.I., Castroagudin, V.L., Bhattarai, G., Klosterman, S.J., Correll, J.C. 2020. Sporangiospore viability and oospore production in the spinach downy mildew pathogen, Peronospora effusa. Plant Disease. https://doi.org/10.1094/PDIS-02-20-0334-RE.
DOI: https://doi.org/10.1094/PDIS-02-20-0334-RE

Interpretive Summary: Downy mildew is a very serious disease on spinach and other crops. This disease is especially concerning for organic spinach production where there are no effective controls for the pathogen once the plants have been infected. The organism that causes spinach downy mildew produces two different types of spores in the spinach leaves, asexually produced sporangiospores, and sexually produced oospores. Currently, we are lacking knowledge on sporangiospore survival and their infection efficiency, and the occurrence and production of oospores in fields. The aim of this current research was to evaluate the impact of desiccation on sporangiospore survival and infection efficiency and evaluate the occurrence and production of oospores. The results indicate that desiccation significantly reduces sporangiospore viability, and infection potential, and likely limits long distance dispersal. Oospores were observed in leaves of numerous commercial spinach cultivars in California and Arizona in 2018 and 2019. Knowledge on the survival of sporangiospores and oospores, and where and how often they occur could provide insights for new disease control measures. Additionally, development of the procedure for crossing strains of the pathogen to generate oospores will aid in its study.

Technical Abstract: Downy mildew of spinach, caused by the obligate pathogen Peronospora effusa, remains the most important constraint in the major spinach production areas in the U.S. The spinach downy mildew pathogen 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 infect seedlings following germination remain unclear. The primary objectives of this research were to evaluate the impact of desiccation on sporangiospore survival and infection efficiency and evaluate the occurrence and production oospores. The initial results indicate that desiccation significantly reduces sporangiospore viability, and infection potential, and likely limits long distance dispersal. Oospores were observed on numerous spinach commercial cultivars in California and Arizona in 2018 and 2019, although the frequency of occurrence varied between the two states. Oospores were produced in controlled crosses on detached leaves, as well as intact plants, confirming that P. effusa is heterothallic as previously described. Pairing of isolates also demonstrated the presence of opposing mating types in spinach production areas in the U.S. Understanding the variables that affect sporangiospore viability and oospore production will help in improving our understanding of the epidemiology of this important pathogen and in the management of spinach downy mildew.