Downy mildew of spinach can be an important disease of spinach. The lesions are often a bright yellow with irregular margins (Figure 1). With time, these lesions can become a light brown. The underside of the lesion is dark black or purple under moist conditions. Under very conducive conditions, sporulation can occur on the top of the leaf.
Figure 1. Downy mildew of spinach can cause bright yellow lesions with irregular margins.
This disease may spread rapidly under cool, wet conditions: the optimal temperatures for lesion development is 59-77 °F. Wind can spread the spores and leaf wetness is required for infection. Any factor which increases leaf wetness, as, for example, closely spaced spinach plants, can increase disease severity.
The best method of managing downy mildew of spinach is to use varieties with host resistance. However, new races of the pathogen may develop in which case the most resistant varieties may change. Several fungicides are listed in the Midwest Vegetable Production Guide (ID-56) that may be used to protect emerged crops.