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Ministry of Agriculture, Food and Rural Affairs

Downy Mildew

Downy Mildew on cabbage seedlings Downy Mildew on bok choy (top of leaf) Downy Mildew on bok choy (underside of leaf)
Click to enlarge.

Beginner

Scientific Name
Hyalopernospora parasitica

Identification

  • Yellow spots on the upper leaf surface with corresponding greyish-white fungal growths on the underside
  • Infected cauliflower curds develop dark-grey spots; when the curds are cut open, a grey streaking is evident beneath the florets
  • In broccoli, grey streaking occurs beneath the beads, leads back to the main stem
  • Cabbage heads develop numerous black spots

Often Confused With
Alternaria
White leaf spot

Period of Activity
Infection can occur at any growth stage. Cool temperatures of 10˚C– 15˚C (50˚F– 59˚F) and prolonged periods of leaf wetness, dew or fog are required.

Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field. Observe both the older and newer leaf growth. Record the percentage of leaves infected and the average number of lesions per leaf. Also note any black spotting on florets or heads when inspecting mature plants. Downy mildew must be managed preventatively.

Thresholds
None established.

Advanced

Scientific Name
Hyalopernospora parasitica

Nearly all cultivated and weed plants of the mustard family can be affected by the downy mildew fungus. It can affect plants at the seedling stage and later toward the end of the season, when it can become a problem in storage.

Early season infections can cause severe losses to seedlings while late season infections can cause losses due to internal discolouration of the heads. It also leaves the heads more susceptible to soft rot bacteria and other storage rots, especially in cabbage.

Identification
In seedlings, the first sign of the disease is a slight yellowing of the upper surface of the first true leaves and cotyledons. Soon, if conditions are moist and cool, a white cottony growth is visible on the underside of these leaves. Once the leaves turn yellow they usually drop off.

Late season infections are primarily on the older leaves. The disease parts of the leaf turn dry and papery but the leaves rarely drop. The downy white growth of the fungus on the lower side of the leaves is visible in cool, damp weather. If the infection moves into the stem it can travel up to the head.

In cauliflower, dark grey spots may appear on the curd. When such curds are cut open, grey streaking is noticeable on the branches beneath the florets. In broccoli, there may be no spotting on the outside of the head but grey streaking occurs beneath the beads all the way back to the main stem. In cabbage, black spots will be evident on the head. These spots range from 1- 2 cm (2/5- 4/5 in.) in diameter.

Often Confused With
Alternaria
Cercospora leaf spot

Biology
The fungus survives between seasons in roots and crop refuse. Plants grown in infested soil will become infected before they emerge. After emergence, the fungus develops on the leaves and spores are produced. These spores are readily carried by the wind and will float in cool moist air. Under favourable weather conditions, spores will infect new leaf tissue in 3 hours and symptoms can become evident in as few as 4 days.

Period of Activity
Temperature and moisture are very important to the development of this disease. Fog, drizzling rain and dew all contribute to growth.  Cool temperatures between 10 and 15ºC (50- 59°F) are ideal. In Ontario, the disease shows up most frequently after Labour Day when the weather turns cool and wet in the fall.

Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field. Observe both the older and newer leaf growth. Record the percentage of leaves infected and the average number of lesions per leaf. Also note any black spotting on florets or heads when inspecting mature plants. Downy mildew must be managed preventatively.

Thresholds
None established.

Management Notes

  • A rotation out of crucifers for 2 years will reduce chances of disease.
  • Some resistant varieties of broccoli are available.
  • Unless all diseased crop refuse is ploughed under promptly after harvest, downy mildew (and other diseases) will continue to spread into nearby crucifer plantings by wind-blown spores.
  • Volunteer plants such as rutabagas should be destroyed, since they may harbour downy mildew as well as other diseases from year to year.
  • Plant into a well-drained soil.
  • Avoid excessive use of overhead irrigation.