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Epidemiology of spinach downy mildew, including insights on oospore production and global transport on seed.

Krishna Subbarao: University of California at Davis, c/o U.S. Agricultural Research Station

<div><em>Peronospora effusa </em>is an oomycete pathogen, and the cause of spinach downy mildew. The recent demonstration of viable oospores of <em>P. effusa</em> in modern seed lots has raised concerns about the global transport of the pathogen, especially to organic production areas, where control of downy mildew with fungicides is not an option and to areas newly brought under spinach cultivation. The detection of oospores in >19% of all seed lots tested, and in the infected leaves from spinach field plots, shows the widespread occurrence of oospores. Moreover, we recently observed the germination of the oospores obtained from spinach leaves, indicating that the oospores may serve as primary inoculum to introduce the pathogen in a growing season. To the best of our knowledge, this is only the second such observation of oospores germination during the last 100 years. Sexual reproduction of <em>P. effusa</em>, as in other oomycete systems, rapidly increases genetic diversity of the pathogen, leading to the emergence of new pathotypes. The appearance of new pathotypes makes the task of breeding resistance for this disease difficult. Seed treatment is the most economical option for reducing viable oospore levels in spinach seed lots, as opposed to discarding of infested seed lots or switching them from organic to conventional production. Current research is focused on use of seed treatments to reduce oospore viability, and on elucidation of the potential role of soil-and seed-borne oospores in initiating primary infections on spinach.</div>