Researchers, growers target threat from spinach mildew

The designation of two new races of spinach downy mildew signifies that the state's growers and researchers continue to face a moving target as they contend with the crop's most important disease.

Resistant varieties are the most important tool for managing the disease, and new races are declared when growers and researchers find that resistance no longer holds up.

"Two new races of the downy mildew pathogen on spinach have been denominated on the basis of a worldwide evaluation of isolates from growers' fields and trap nurseries," according to the International Working Group on Peronospora in spinach in the Netherlands. "Both races pose a significant threat to the spinach industry in all parts of the world, and resistance to these new races is important."

The working group standardizes the designation of spinach downy mildew races so that disease resistance in variety breeding is uniform around the world.

The appearance of two new downy mildew races, numbers 18 and 19, means that some varieties that held up under disease pressure a few years ago may no longer offer protection.

Hydrus spinach, for example, is resistant to races 1 through 18, but is susceptible to race 19, as are the varieties Pigeon and Meerkat.

Race 19, which was first discovered in California, is common in El Centro and the Yuma Valley, according to University of Arkansas plant pathologist Jim Correll.

The Pigeon, Caladonia and Meerkat varieties are all susceptible to the newly designated race 18.

"The International Working Group is continuously monitoring the appearance of strains of the pathogen that deviate in virulence from the known races," the agency said. "In this way, the working group aims to promote a consistent and clear communication between public and private entities, such as the seed industry, growers, scientists and other interested parties about all resistance-breaking races that are persistent enough to survive over several years, occur in a wide area, and cause a significant economic impact."

The challenge of new strains constantly overcoming downy mildew variety resistance is, to an extent, a case of a crop suffering from its own success.

Spinach is generally grown on densely packed 80-inch beds that are mowed, making for a nutritious crop that is easy to grow and harvest.

This helped California spinach grow into a $400 million crop almost overnight, nearly all of it in the Salinas Valley. But the densely packed beds create a humid microclimate that is ideal for development of downy mildew, and the sheer number of spinach fields means that disease spores are carried the length of the Salinas Valley with the wind every afternoon.

There has been some research showing that drip irrigation can reduce downy mildew pressure in spinach, but work must still be done to make this system pencil out.

In the meantime, growers must rely on a continuing supply of resistant varieties to manage the ever-changing disease.

(Bob Johnson is a reporter in Sacramento. He may be contacted at bjohn11135@gmail.com.)