CALIFORNIA
Vegetables A SPECIAL GROWERS’ REPORT OF AG ALERT ®
Evolving downy mildew strains test spinach growers
By Bob Johnson Most afternoons, a coastal breeze blows in from Castroville to King City, cooling the Salinas Valley and supporting a wide range of vegetable crops from artichokes to broccoli to spinach. U.S. Department of Agriculture researchers say spore traps placed throughout the region confirm that the breeze also carries spinach downy mildew spores, transporting a persistent plant-disease chal- lenge for growers. As a result, researchers and growers in the region are constantly working to develop and plant new spinach varieties that offer broad resistance to ev- er-evolving strains of spinach downy mildew. As the most widespread spinach disease in California, the pathogen can spread rapidly in cool, wet conditions, curling and blighting leaves while causing extensive crop damage. Jim Correll, a University of Arkansas plant pa- thologist specializing in spinach research, told the California Leafy Greens Research Board Conference that incidents of spinach downy mil- dew disease have remained low in recent growing seasons thanks to continued success in developing resistant spinach varieties. Yet, the disease also con- tinues to reinvent itself. In the 2022-23 season, Correll said, research- ers identified 19 distinct races of the crop dis- ease. In May, the International Working Group on Peronospora on spinach designated a 20th race of downy mildew. While spore traps can confirm the presence of the disease before Salinas Valley crops show any symptoms, researchers say they can’t immediately tell which race of the disease is in the area. “The latest monitoring tools do not tell what race is present, and we are still searching for race-specif- ic markers,” said Steve Klosterman, a USDA senior researcher based in Salinas. Each new race of spinach downy mildew is des- ignated after researchers find it shows an ability to infect previously resistant spinach varieties. Each new downy mildew discovery creates pressure to develop new spinach varieties. Under the direction of University of California, Davis, Plant Breeding Center Director Charles Brummer, researchers are working to breed spin- ach with enduring resistance and the ability to limit uptake of cadmium from the soil, an issue with some spinach varieties. Brummer’s research group has identified a few promising breeding lines for increased produc- tion by seed companies. But researchers have yet to develop a super spinach variety with universal resistance. Salinas Valley growers must constantly adapt by employing new varieties that are resistant to current strains of downy mildew. As the UC Pest Management Guidelines explain, “the use of resistant cultivars is the most effective
Jim Correll, a University of Arkansas plant pathologist, specializes in spinach disease research, including in the Salinas Valley. While incidents of spinach downy mildew remain low, he says the disease is challenging because it continues to develop new strains.
See SPINACH, Page 7
6 Ag Alert July 10, 2024
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