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Brassica Diseases | Downy Mildew

Downy Mildew


Plants can be infected at any stage of development. In seedbeds, cotyledons and primary leaves are invaded first, resulting in fungal growth visible on the underside of the leaf, while chlorosis develops on the upper side of the leaf. The young leaves or cotyledons, when severely chlorotic, may drop off prematurely.  Older leaves usually persist and infected areas gradually enlarge, turn bright yellow, and then become tan and papery. In rare cases, the infected leaves may develop hundreds of minute darkened specks. Under cool, moist conditions, a white mildew forms on the undersides of infected leaf lesions.

Downy mildew on mustard green.

Signs of the pathogen and disease symptoms may appear on other plant parts as well. Pathogen growth can be seen on fleshy roots of turnips, and radishes may develop an internal, irregularly shaped discoloration extending from the crown downward. The flesh may be brown or black or show a form of net necrosis. In advanced stages, the skin becomes roughened by minute cracks and the root may split open. Dark grey-black, sunken spots develop on cauliflower curds and cabbage heads, often followed by secondary invasion by soft-rotting bacteria and fungi.  The disease may progress systemically through the plant, resulting in dark brown to black streaks in the veins.

Downy mildew on radish foliage (left; center) and cauliflower (right).

Pathogen Biology

The fungus causing downy mildew in crucifers, Peronospora parasitica, overwinters in roots or in decaying portions of diseased plants. Thick-walled resting spores may form in stems, cotyledons, and other fleshy parts of infected host plants and weeds. On growing plants, the fungus produces large numbers of spores that are blown about by wind and splashed by rain. Moisture and temperature are important in the spread and reproduction of this fungus. High relative humidity during cool or warm, but not hot, periods promotes its growth and sporulation. Presence of a water film on the foliage from fog, drizzling rain, or dew allows spores to germinate, infect, and produce more spores on a susceptible host in as few as 4 days.

Favorable Environmental Conditions

Cool, wet conditions are conducive for the development of downy mildew.  Abundant sporulation and rapid disease development occur at greater than 98% relative humidity, when leaves are wet and temperatures are between 46-60°F. Downy mildew becomes severe in several days under these conditions, especially when plants remain wet for a prolonged period of time.

Often Confused With

  • Rhizoctonia on turnip and radish – Rhizoctonia causes internal decay and necrotic lesions on turnip and radish similar to that caused by downy mildew; however, on turnip the decay is much more watery than the necrosis caused by downy mildew. On radish, Rhizoctonia symptoms are more superficial than those caused by downy mildew, although surface cracking is common for both diseases.
  • White rust – White rust is caused by Albugo candida and causes similar symptoms to those caused by downy mildew. There is a white grown on the undersides of the leaves that can be confused with downy mildew; however, the upper sides of the leaves have different symptoms.  White rust causes pustules while downy mildew causes chlorotic lesions.

Scouting Notes

After long periods of cool, wet weather, check the undersides of leaves for any white mycelial growth.  Leaves that become severely chlorotic after periods of favorable weather could also be another indicator of downy mildew infection.


Currently there is no threshold information for downy mildew.

Management Notes

  • Use sound cultural practices – Use a crop rotation plan that excludes production of any type of cruciferous crop for at least 2 out of every 3 years. Practice sanitary measures such as keeping seedbeds away from other crucifer production, destroying any cruciferous weeds or using a planting site and plant spacing pattern that exposes plants to full sun throughout the day.
  • Use of fungicides – Under environmental conditions that favor disease development, apply a registered fungicide weekly beginning soon after emergence, or as soon as symptoms are observed. Consult the Midwest Vegetable Production Guide for Commercial Growers for current fungicide recommendations.
  • Start with disease-free or disease-resistant varieties – Always start with certified disease free seed to avoid introduction of downy mildew into your plant system. Disease resistant cultivars are not available for most cruciferous crops; however, some hybrid cultivars of broccoli are resistant or tolerant to downy mildew.