Transmission of spinach downy mildew via seed and infested leaf debris

Authors: Klosterman, Steven; Clark, Kelley; Anchieta, Amy; Kandel, Shyam; Mou, Beiquan; MCGRATH, MARGARET - Cornell University; CORREL, JIM - University Of Arkansas; Shishkoff, Nina

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2023
Publication Date: 10/15/2023
Citation: Klosterman, S.J., Clark, K.J., Anchieta, A.G., Kandel, S.L., Mou, B., McGrath, M.T., Correl, J., Shishkoff, N. 2023. Transmission of spinach downy mildew via seed and infested leaf debris. Plant Disease. https://doi.org/10.1094/PDIS-06-23-1225-RE.
DOI: https://doi.org/10.1094/PDIS-06-23-1225-RE

Interpretive Summary: Downy mildew disease on spinach is caused by a microscopic plant pathogen known as Peronospora effusa. The pathogen can produce wind-dispersed spores, but also produces long lived survival structures known as oospores. We previously demonstrated that the oospores are viable in commercial spinach seed lots and can germinate. In this study we provide evidence that oospores carried on spinach seeds give rise to downy mildew in chambers that do not allow contamination with the wind-borne spores. Microscopy evaluations revealed that oospore-infested seed of two different cultivars yielded seedlings with the disease and sporulation of the pathogen. Moreover, we demonstrate that the oospores supplied as inoculum in leaf debri in soil can cause disease. Survival of P. effusa on spinach seeds as oospores and the ability of oospores to initiate disease from infested leaf debri in soil impacts downy mildew management strategies. Seed treatments if necessary and brief crop rotations may help to curtail the disease.

Technical Abstract: Spinach downy mildew, caused by the oomycete pathogen Peronospora effusa, is a worldwide constraint on spinach production. The role of airborne sporangia in the disease cycle of P. effusa is well established, but the role of the sexual oospores P. effusa is less clear. Surveys of modern spinach seed lot samples since 2014 have revealed the presence of oospores in 19% of the lots. To evaluate seed transmission of spinach downy mildew via oospores in this study, we employed the use of glass isolator chambers to grow out oospore-infested spinach seed and non-infested seeds mixed with oospore-infested crop debris in two independent experiments. Downy mildew diseased spinach plants were found 37 days after planting in the first experiment, and 34 days after planting in the second experiment, in the chambers that contained one of two oospore-infested seed lots or seeds coated with oospore-infested leaves. Spinach plants in glass chambers initiated from seeds without oospores did not show downy mildew symptoms. Similar findings were obtained using the same seed lot samples in a third experiment conducted in a growth chamber. Grow out tests with oospore-infested seed of two different cultivars revealed the characteristic Peronospora sporangiophores growing from the seedlings of each cultivar. The results provide evidence that oospore-infested soil can cause downy mildew and further evidence of seed transmission of downy mildew in spinach via oospores.