In February 2018 a coalition of spinach researchers, plant pathologists, seed company professionals, and other experts from the USA and Europe designated a new race of the spinach downy mildew pathogen, race 17. A coordinated announcement was released in the US and the EU on April 16. This coalition works together as the International Working Group on Peronospora (IWGP) and has been collecting and analyzing information on spinach downy mildew for a number of years. Here are some things to be aware of in light of this new race announcement.
- Some spinach cultivars that were resistant to previous downy mildew races and were disease-free last season could possibly develop disease from this new race 17.
- Some spinach cultivars appear to be resistant to race 17 as well as the other races that are currently found in California and Arizona.
- This latest development further illustrates the diversity of this pathogen and how prone it is to developing new races.
- For all the spinach downy mildew races, there is no evidence that the various races are affected differently by the various registered fungicides. For example, races 15, 16, and 17 appear to be equally well controlled by protectant fungicides.
- At this point there is no evidence that any of the races, including race 17, has developed resistance (or “insensitivity”) to any of the registered fungicides.
- Race 17 and others could be rapidly distributed because of the recent finding that the oospore structure of the downy mildew pathogen is present on spinach seed.
- The IWGP and participating parties serve as a useful means of standardizing information on spinach downy mildew races and communicating such information to the worldwide spinach industry.
- The spinach downy mildew situation in California and Arizona has been monitored through the cooperation between the University of Arkansas (Jim Correll program), University of California (Steve Koike program), and the seed companies, growers, pest control advisors, and other field professionals active in these states.
Steven Koike, TriCal Diagnostics
Jim Correll, University of Arkansas
Spinach downy mildew is readily recognized by the yellow leaf lesions and corresponding purple sporulation.
The races of the downy mildew pathogen are differentiated by their ability to grow or not grow on differential spinach lines and cultivars.