Spinach downy mildew: Advances in our understanding of the disease cycle and prospects for disease management

Authors: Kandel, Shyam; Mou, Beiquan; Shishkoff, Nina; SHI, AINONG - University Of Arkansas; SUBBARAO, KRISHNA - University Of California; Klosterman, Steven

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2018
Publication Date: 4/2/2019
Citation: Kandel, S.L., Mou, B., Shishkoff, N., Shi, A., Subbarao, K.V., Klosterman, S.J. 2019. Spinach downy mildew: Advances in our understanding of the disease cycle and prospects for disease management. Plant Disease. 103:791-803. https://doi.org/10.1094/PDIS-10-18-1720-FE.
DOI: https://doi.org/10.1094/PDIS-10-18-1720-FE

Interpretive Summary: Downy mildew disease on spinach is very destructive, especially for organic production in the U.S. and wherever spinach is grown organically since the standard synthetic fungicides cannot be applied for disease control. In some instances entire fields were lost to the disease because the typical symptoms of leaf yellowing rendered the leaves unmarketable. Peronospora effusa is the oomycete organism that causes spinach downy mildew, and like other oomycetes, produces long-lived sexual spores known as oospores. This article describes and illustrates germination of oospores of P. effusa, an observation that has not been reported in the literature for 100 years. The article also discusses recent finds of oospores on modern commercial seed lots and the control measures that are being deployed to stop the pathogen, including host resistance.

Technical Abstract: Downy mildew on spinach is caused by Peronospora effusa, an oomycete pathogen that poses a challenge to spinach production worldwide, especially in organic production. Following infection, P. effusa produces abundant amounts of asexual sporangia. Sporangia become windborne and initiate new infections locally or distantly, leading to widespread epidemics. Oospores produced from the union of opposite mating types have been observed within infected leaves and seeds, and may remain viable for many years. Sexual reproduction increases the genetic diversity of P. effusa through sexual recombination, and thus, the movement of oospores on seed has likely fueled the rapid explosion of new pathotypes in different regions of the world over the past 20 years. This review summarizes recent advances in spinach downy mildew research, especially in light of the findings of oospores in contemporary commercial spinach seed lots as well as their germination. Knowledge of the role of the oospores and other aspects of the disease cycle can directly translate into new and effective disease management strategies.