Sporangiospore Viability and Oospore Production in the Spinach Downy Mildew Pathogen, Peronospora effusa

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 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.

Sexual reproduction contributes to the evolution of resistance‐breaking isolates of the spinach pathogen Peronospora effusa

“…Isolates with similar virulence spectrum are often simultaneously found at different geographical locations (Irish et al ., 2003 ; Satou et al ., 2006 ), which is likely due to the globalized spinach market where new spinach varieties are distributed and sold worldwide. Possibly, the sexually produced oospores of P. effusa that are abundant in spinach fields drive the repeated emergence of new races (Dhillon et al ., 2020 ), followed by a rapid spread over a large area. Therefore, it is difficult to determine where a novel resistance‐breaking isolate first occurred, and in our case, to know the exact geographical origin of the denominated P. effusa races.…”

Genetic diversity of the spinach downy mildew pathogen based on hierarchical sampling

“…Oospores of P. effusa have been observed from spinach seed [12][13][14] and spinach plants in one downy mildew sample of 2008 [5], two samples in 2016, and 28 samples in 2018 collected from CA (Correll et al, unpublished). Difference was found in the mating proteins in the genome sequences of isolates of P. effusa [42], oospores can be produced by co-inoculation of two isolates selected accordingly to different mating features [47], indicating heterothallic sexual reproduction occurs in P. effusa, which was also found in several other oomycetes, such as Phytophthora, Bremia, and Plasmopara [21]. Among those isolates that rejected sexual reproduction hypotheses, nine isolates contained multiple lesions, but only one genotype was observed, suggesting clonal production among these isolates of P. effusa.…”

“…The fast emergence of new races may be driven by multiple molecular mechanisms. The ability to do controlled crosses and generate sexual progeny of P. effusa represents a significant advance in our understanding of genetic diversity of this important pathogen [47]. The relative importance of asexual variation versus sexual variation in virulence in the P. effusa population will help determine the rate at which novel strains overcoming the deployed resistance can develop.…”

Mapping and selection of downy mildew resistance in spinach cv. whale by low coverage whole genome sequencing

“…Significant increase in the production area in recent decades and planting in a higher density, year-round production, and use of resistant cultivars with narrow genetic backgrounds increase selection pressure, and increased organic production provides favors P. effusa growth and multiplication, and all these phenomena promote emergengence of new pathogen races. Recent studies have reported asexual genetic variation, the presence of opposite mating types among California isolates ( Dhillon et al., 2020 ), and sexual recombination within the P. effusa population ( Lyon et al., 2016 ; Kandel et al., 2019 ), all of which might contribute to the emergence of new races.…”