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Ministry of Agriculture, Food and Rural Affairs

Powdery Mildew

Powdery mildew in pumpkin Powdery mildew (lower leaf surface) Fresh powdery mildew infection (upper leaf surface)
Click to enlarge.

Beginner

Scientific Names
Sphaerotheca fuliginea and Erysiphe cichoracearum

Identification

  • Initial symptoms develop on the undersides of older, shaded leaves
  • The white, powdery hyphae will spread to the upper leaf surface and down the petiole
  • Infected leaves and stems wither and die prematurely

Often Confused With
Downy mildew

Period of Activity
Spores usually arrive in mid-summer. Peak infection periods occur when temperatures are in the range of 20˚C– 26˚C (68˚F– 80˚F). Disease development slows when temperatures climb above 26˚C (80˚F).

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

Thresholds
Begin applying protectant fungicides at the first sign of disease (2% of leaves infected with one lesion).  Late July through August is usually the most critical powdery mildew control period.

Advanced

Scientific Names
Sphaerotheca fuliginea and Erysiphe cichoracearum

Powdery Mildew is a significant disease of all cucurbits; however, pumpkins and squash are the most susceptible.  Powdery mildew reduces both the yield and quality of the crop.  Plants infected during early fruit development have a lower fruit count and tend to produce smaller fruit.  Premature defoliation may also result in sunscald.

Identification
Initial symptoms usually appear on the older, shaded leaves.  A dense, white fungal (powdery) growth develops on the lower leaf surface.  A pale green-to-yellow discolouration may also appear on the corresponding upper leaf surface. 

The white, powdery growth will spread to the upper leaf surface and down the petiole, until both leaf surfaces are entirely covered by the mildew.  Older infections of S. fuliginea may turn dirty-white with age, making them easily confused with downy mildew. 

Severely infected leaves turn brown, wither and die prematurely.  The fruit petioles of infected plants rapidly deteriorate, causing them break apart and crumble when picked.

In melons and squash, powdery mildew may result in poor internal colour development, lower sugar content and reduced storability.  In pumpkins and squash powdery mildew infections cause the fruit petioles (handles) to break down, rendering the fruit unmarketable. 

Often Confused With
Downy mildew

Biology
Powdery mildew does not need free moisture for infection.  Fungal conidia are able to germinate and spread at a relative humidity of 20% (or lower).  Infection rates do increase at higher humidity levels although humid weather conditions and heavy dews lead to more rapid disease development. Under these conditions, visual symptoms may appear 3- 7 days after the initial infection.  Infections develop most rapidly when there are significant fluctuations in day and night-time temperatures.

Leaves are most susceptible to infection 16- 23 days after unfolding.  Shaded canopies caused by excess nitrogen and/or dense weed pressure are more prone to powdery mildew infections.

Period of Activity
Spores usually arrive in mid-summer. Peak infection periods occur when temperatures are in the range of 20- 26°C (68°F- 80°F). Disease development slows when temperatures climb above 26°C (80°F).

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

Thresholds
Begin applying protectant fungicides at the first sign of disease (2% of leaves with one lesion). Late July through August is usually the most critical powdery mildew control period.

Management Notes

  • 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). 
  • Always rotate between fungicides from different chemical families.
  • Maintain good weed control, and avoid unnecessarily dense canopies due to over-fertilization.
  • There is a wide selection of powdery mildew–resistant varieties available for all types of cucurbits.