Downy mildew disease, caused by Peronospora farinosa f. sp. spinaciae, is the most important disease problem facing the extensive spinach industry in California.

In recent years, several new downy mildew races have appeared in the state in rapid succession, raising great concerns about the ability to manage this threat and causing the industry to consider research strategies to address the problem.

In October 2009, a downy mildew “summit meeting” was held in Salinas to discuss this concern. Sponsored by UC Cooperative Extension, the California Leafy Greens Research Board, and seed industry leaders, the intent of the meeting was to mobilize all components of the spinach industry and to generate ideas for research and collaborations.

The major concern is the recent proliferation of races. While downy mildew has been around California spinach fields for decades, the last few years have seen the development of four or five new races.

Each new race potentially overcomes the resistance factors in the cultivars being planted at that time, leaving the crop susceptible to severe damage. This Salinas spinach meeting helped bring out several points:

1. Basic information on the pathogen biology is missing. Information is needed regarding exact conditions for infection, mechanisms of genetic change and the rise of new races, how the pathogen survives the winter, and other aspects. Very pressing is the question of the importance of seedborne inoculum.

Researchers have found downy mildew survival structures on spinach seed. However, it is unknown whether this seedborne inoculum is important, how commonly it occurs, or whether the pathogen on seed is alive and able to cause downy mildew on the germinating seedling.

2. Role of organic spinach in epidemics remains unknown. Some suggest that organic spinach plantings, by virtue of using untreated seed (presuming seedborne inoculum, discussed above) or lacking effective fungicides, might be a source of new races. These hypotheses are derived from field observations related to some of these recent outbreaks, plus the 2003 stoppage of synthetic fungicide treatments used on seed for organic plantings. However, from a research-based perspective such assertions are yet unproven and require investigation. A connection here may be possible, but requires research to substantiate such a link.

3. Integrated management steps must be used. There is consensus that standard IPM disease strategies must be employed and improved upon. Resistant cultivars will remain a foundational piece of such a program. Judicious use of effective fungicides will remain important. Timely disking of harvested, old, or severely diseased fields is warranted. Failure to destroy these fields results in “green bridges” in which pathogen inoculum from the old fields can “bridge” over and infect newly planted fields. For organic production, effective fungicides need to be developed.

4. Research is needed. Certainly more research is needed to fill in the knowledge gaps regarding basic biology of the mildew, disease development, inoculum sources, role of seedborne downy mildew, sources of genetic resistance of spinach, and improved fungicides.

5. A model for collaboration and progress. Growers of conventional and organic spinach, seed industry personnel, plant breeders, pest control advisors, allied industry members, and researchers must team together to work on solutions. It is hoped that this October meeting in Salinas will be the first of many steps taken to build such cooperation and develop responses to the spinach downy mildew challenge in California.