Downy mildew on spinach and related plants

By Mike Matheron, UA Extension Plant Pathologist

Downy mildew has been observed recently on nettleleaf goosefoot and swiss chard in Yuma County, Ariz. and Imperial County, Calif. respectively.

These plants are members of the plant family called the Chenopodiaceae which also contains the important vegetable crop spinach.

Since spinach can also be infected with downy mildew, the question arises - can the pathogen from one member of this plant family infect one or more of the other family members?

University of California farm advisor Steve Koike, Monterey County, and colleagues conducted research to answer this question. The downy mildew pathogen from spinach was used to inoculate nettleleaf goosefoot and lambsquarters. The result was no disease development.

In another trial, the isolates of the pathogen from nettleleaf goosefoot, lambsquarters, beet and swiss chard were each used to inoculate spinach. The result again was no disease development.

The bottom line is that spinach growers do not need to worry about related Chenopodiaceae including nettleleaf goosefoot, lambsquarters, beets or swiss chard as potential sources of inoculum for the spinach crop.

Even though the bluish-purple evidence of the pathogen on the underside of leaves is similar in appearance on all of these plants, the particular isolates of the downy mildew pathogen have very specific and limited host ranges.

Contact Matheron: (928) 726-6856 or matheron@ag.arizona.edu.