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Powdery Mildew

Practical biological control, IPM and environmental pest-management knowledge.

Powdery Mildew

Powdery mildew refers to a group of fungal diseases that produce white or grey powdery growth on leaves, stems, flowers or fruit. Different crops have different powdery mildew species, but the environmental pattern is often similar.

Unlike many fungal diseases, powdery mildew does not always need free water on the leaf surface. It often thrives in humid, sheltered crop canopies where leaves remain dry but the air around them is moist.

Description

Powdery mildew usually appears as white powdery patches on the upper leaf surface, although it can also occur on stems, flowers and fruit depending on the crop.

Typical symptoms include:

  • White or grey powdery patches
  • Distorted young leaves
  • Reduced leaf function
  • Yellowing or browning of infected tissue
  • Poor crop appearance in ornamentals
  • Reduced fruit or flower quality

In protected crops and dense outdoor canopies, mildew can build gradually before becoming obvious.

Environmental drivers

Powdery mildew risk is usually highest where conditions are mild, humid and poorly ventilated.

Key drivers include:

  • Mild temperatures, often around 15–25°C
  • High humidity around the leaf surface
  • Low air movement
  • Dense crop canopies
  • Shaded or sheltered areas
  • Large day/night humidity swings
  • Soft, lush growth
  • Plant stress from irregular watering or high VPD periods

Free water is not always required. In some crops, heavy rain may physically reduce spores, while humid dry-canopy conditions can still favour mildew development.

Weather risk signals

Powdery mildew pressure is more likely when:

  • Temperatures are mild
  • RH is moderate to high
  • VPD is low to moderate
  • Air movement is poor
  • Canopies stay humid overnight
  • Growth is soft and dense

Management principles

Powdery mildew management is about reducing sheltered humid microclimates and avoiding soft, susceptible growth.

Useful cultural actions include:

  • Improve airflow around plants
  • Avoid overly dense canopies
  • Manage nitrogen to avoid excessive soft growth
  • Reduce plant stress from irregular watering
  • Remove heavily infected leaves where practical
  • Monitor sheltered or shaded areas first
  • Avoid large humidity swings where possible in protected crops

Biopredator weather link

The Biopredator weather tool flags mildew risk when mild temperatures combine with high humidity, low VPD and slow-drying canopy conditions.