Downy Mildew

Table of contents

    Downy Mildew

    Figure 37

    Photo by Phil Thomas

    Downy mildew is caused by the fungus Peronspora parasitica, a close relative of white rust. Downy mildew occurs in almost constant association with staghead as part of the white rust/downy mildew disease complex. Downy mildew also appears alone on infected seedlings and leaves of the rosette stage of B. rapa during periods of cool, wet weather in the spring. The fungus is soil-borne and seedborne and may persist in the soil for five to 10 years. The fungus appears as a white mealy growth on the lower surface of leaves (Figure 37) and on green stagheads caused by the white rust fungus. The upper leaf surface over the infected area usually turns yellow. The disease cycle is similar to that of staghead. The control recommendations are the same as for staghead. Use a crop rotation with noncruciferous crops making certain to control volunteer canola, stinkweed and wild mustard.