- Author: Steven T. Koike
Jim Correll, University of Arkansas
Steven Koike, University of California Cooperative Extension
The 2015 spinach season has had a high level of downy mildew pressure (Photo 1). In the Yuma area of Arizona as well as the Imperial and Salinas valleys of California, outbreaks of severe downy mildew have occurred in hot spots in a number of fields. From these cases, downy mildew samples were collected and tested. The results indicate that a number of outbreaks may be caused by deviating isolates of the downy mildew pathogen that do not match the profiles of the fifteen described races. Two of the more commonly found isolates are 1014 and 19B, which are able to infect a number of the newer race 1-15 resistant varieties produced by several companies. Isolates matching the 1014 type have been recovered in both 2014 and 2015. Isolates matching the 19B type have been particularly prevalent in the Salinas area in recent months. A number of older spinach hybrids (a partial list of such cultivars includes Whale, Camaro, Caladonia, Scorpius, Shelby, Tasman, and Raccoon) have resistance to 19B type isolates.
Efforts are underway to fully characterize these deviating isolates. In addition, we have proposed to the International Working Group on Peronospora (IWGP) that isolates 1014 and 19B be included in a group ring-test to confirm the virulence of these isolates. The ring-test will provide supporting information that might result in the nomination of one or both deviating isolates for race designation. The IWGP coordinates activities to identify deviating races of the spinach downy mildew pathogen and to designate them as official races.
For growers and pest control advisors, management of downy mildew on spinach remains the same. The selection and planting of cultivars that are resistant to the pathogen remain the primary and preferred means of managing this problem. For conventional growers, timely and judicious use of fungicides is an important component of the IPM strategy. Organic producers still do not have fungicide products that are consistently effective.
Photo 1. Downy mildew causes distinct yellow lesions and purple sporulation to develop on spinach leaves.