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Ministry of Agriculture, Food and Rural Affairs

Downy Mildew

Downy mildew in cucumber Downy mildew on pumpkin leaf Downy mildew on pumpkin leaf Downy mildew (lower leaf surface)
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Beginner

Scientific Name
Pseudoperonospora cubensis

Identification

  • Infected leaves develop pale green areas on the upper leaf surface 
  • The corresponding lower leaf surface may be covered with a downy, grey-to-purple mould with visible black spores (sporangia) 
  • As the infection progresses the lesions become yellow, then brown and necrotic
  • In cucumbers the lesions are often confined by the leaf veins and take on an angular appearance 
  • During favourable environmental conditions downy mildew can completely kill a crop within a week of the initial infection
  • Fruit from infected plants are usually undersized, deformed and unmarketable

Often Confused With
Powdery mildew
Angular leaf spot

Period of Activity
Downy mildew is favoured by cool, wet or humid weather conditions.  Spore production is highest at temperatures of 15- 20°C (59- 68°F).  Under favourable weather conditions the pathogen will produce new inoculum in as little as 4 days.

Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field.  Observe both the upper and lower leaf surface of the old and new leaves.  Divide the number of infected leaves by 200 to determine the percent infection.  Record the average number of lesions per leaf.

Thresholds
Downy mildew must be managed preventatively.  There are no thresholds.

Advanced

Scientific Name
Pseudoperonospora cubensis

Downy mildew is a serious disease of cucurbit crops grown in Ontario.  Once established in a region, it can spread rapidly causing significant loss of fruit quality and yield.  Downy mildew infects gourds, squash, pumpkins, melons and cucumber.  However, cucumbers are the most susceptible.

Downy mildew primarily infects the leaves, resulting in decreased photosynthesis.  Occasionally the fruit of melons can become infected as well.  During favourable environmental conditions downy mildew can defoliate plants and destroy entire fields within a week.  Fruit of infected plants are usually undersized and misshapen.  They are prone to sun scald which further reduces their quality

Identification
Downy mildew symptoms develop 4- 12 days after the initial infection.  They first appear as water-soaked lesions or small greenish-yellow spots on the upper surface of older leaves.  In cucumbers the lesions are often confined by the leaf veins and take on an angular appearance. 

As the disease progresses, the centre of the lesion will eventually turn tan-to-dark-brown and die.  Downy mildew lesions often develop a greyish-purple downy growth on the lower leaf surface.  This is particularly noticeable in the mornings after a period of wet weather or heavy dew.  Dark purple sporangia may also be present.

As the lesions expand and multiply, the entire field takes on a brown and “crispy” appearance.  During favourable environmental conditions the disease may move onto the new growth.  Once established in a region it spreads rapidly killing the crop prematurely and causing significant yield losses.

Often Confused With
Powdery mildew
Angular leaf spot

Biology
Cucurbit downy mildew is specific to cucurbits.  It will not cause downy mildew infections in onions, spinach or other vegetable crops.  Species of downy mildew infecting other vegetables will not infect cucurbits.  Cucurbit downy mildew has several different strains (pathotypes).  Some only infect cucumbers while others can infect melons, cucumbers, pumpkins and squash.

This disease is primarily spread via air-borne sporangia.  These spores can travel on wind currents for up to 4 days.  During prolonged cool, wet periods, the sporangia can also burst open and release many zoospores. These zoospores swim through the film of water along the leaf surface towards the stomates.  These natural pores are a primary point of entry for the pathogen, resulting in multiple infections on the leaf.

Cucurbit downy mildew is an obligate parasite.  It requires living green plant tissue on which to survive.  Killing frosts and cold winters effectively prevent spores from overwintering in the field in Ontario.  There is the potential for downy mildew to overwinter on living cucurbit plant material in the greenhouse.

Downy mildew primarily overwinters in the Southern United States, Cuba and Mexico.  Sporangia from infected fields are carried long distances by storms. The infection moves northwards as spores are deposited on cucurbit crops in the temperate growing areas.  Downy mildew typically arrives in Ontario on summer storms in early to late summer.

Period of Activity
Downy mildew is favoured by cool, wet or humid weather conditions.  Spore production is highest at temperatures of 15- 20°C (59- 68°F).  Under favourable weather conditions, the pathogen will produce new inoculum in as little as 4 days.

Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field.  Observe both the upper and lower leaf surface of the old and new leaves.  Divide the number of infected leaves by 200 to determine the percent infection.  Record the average number of lesions per leaf.

Thresholds
Downy mildew must be managed preventatively.  There are no thresholds.

Management Notes

  • Follow a basic 7- 14 day preventative program using recommended broad spectrum fungicides.  Initiate the program no later than runnering.  If suitable environmental conditions exist during the early stages of crop development, use a banded application to protect the young plants.
  • Switch to a targeted downy mildew fungicide if downy mildew is identified in the Great Lakes region or during prolonged periods of cool, humid weather.  If downy mildew has been identified in an area, maintain a strict 5- 7 day fungicide schedule.
  • There are several different fungicides recommended for both conventional and organic growers.  Always rotate between fungicides from different chemical families.
  • Ensure good spray coverage.  Select nozzles and set the air pressure to deliver a medium-to-fine droplet size.  All fungicides should be applied in a minimum 30 gal water/ac (or higher.)
  • Growers who plant cucurbit transplants should ensure that the transplants are free from disease and grown in a greenhouse dedicated solely for transplant production.  For cucumbers grown under plastic tunnels, apply a preventative fungicide immediately after transplanting and a second application as soon as the row covers are removed.