Powdery Mildew

How to Prevent & Control Powdery Mildew

indoor & greenhouse gardening

Practically any indoor garden or greenhouse can fall victim to powdery mildew—a fungal disease that appears as white, powdery spots on leaves and stems. It’s caused by spores from various ascomycete fungi, some of which develop more quickly than others. Early detection is crucial; and treatment should begin at the first sign of infection.

While common, powdery mildew is far from harmless. Left untreated, it can severely damage or kill plants. In commercial cultivation, any detectable traces during purity testing can cause crops to fail. The fungus spreads quickly, so identifying and addressing it early is essential to avoid a full-blown infestation.

Fortunately, there are effective ways to control powdery mildew and prevent outbreaks. Key factors to manage include plant health, humidity levels and airflow. A range of natural and safe products are available to combat the fungus without harming crops or consumers.

Understanding Powdery Mildew

Powdery mildew is identified by white, flour-like patches on leaves, usually forming small, round clusters. As the infection spreads, leaves may yellow and wilt. Contaminated crops should not be consumed or sold, as the fungus can be harmful if inhaled or ingested.

This fungus thrives in warm, poorly ventilated environments with fluctuating humidity. Unlike bud rot caused by Botrytis cinerea, powdery mildew does not need standing water to germinate. Even brief, sharp changes in humidity can activate spores.

Three conditions must occur for powdery mildew to develop:

  • Erysiphales spores must attach to a plant’s surface
  • Temperatures between 70–80°F (22–27°C)
  • Humidity swings: High relative humidity (over 80%) during the day and low (under 40%) at night

Tips to Prevent Powdery Mildew

Maintain Cleanliness
Spores can hitch a ride on people, tools and plants. Keep your grow space as sterile as possible:

  • Do not bring outside plants, tools, or equipment into your facility
  • Require staff to change clothes, remove shoes, and wash hands before entering
  • Regularly clean surfaces, tools and floors

Improve Air Circulation
Stagnant air encourages mildew development:

  • Avoid overcrowding plants
  • Install fans for consistent airflow
  • Use intake filters to catch spores and airborne contaminants

Control Humidity
Managing humidity is critical, but it can be challenging:

  • Keep daytime humidity below 80% using dehumidifiers
  • At night, maintain humidity above 40% using humidifiers
  • Use environmental controllers with humidity sensors for automatic adjustments

Strengthen Plants
Healthy plants are naturally more resistant to fungi:

  • Add silica (Si) to your nutrient regimen to fortify cell walls
  • Strong plants are better equipped to withstand pests and disease

Use UV Light
UVB light can enhance plant growth, yield, and resistance to mildew and other pathogens. However, it must be used safely to avoid damaging crops or exposing workers to health risks.

Implement Integrated Pest Management (IPM)
A solid IPM strategy includes preventive practices that reduce the risk of powdery mildew and other threats. Consistent monitoring, cultural controls and environmental adjustments form the backbone of any effective IPM program.

Responding to an Outbreak

If powdery mildew appears:

  • Act immediately; it spreads rapidly and can linger
  • Isolate infected plants
  • Remove and destroy contaminated foliage
  • Sanitize pruning tools after each cut
  • Understand that the spores can remain viable without a host for up to 30 days

To fully eliminate the fungus:

  • Remove all plants from the facility
  • Clean walls, trays, tools and surfaces using hydrogen peroxide or other disinfectants
  • Sanitize air ducts, vents and HVAC systems with chlorine dioxide

Quick Tips to Prevent & Eliminate Powdery Mildew

  • Act Swiftly: Isolate infected plants and start treatment immediately.
  • Increase Ventilation: Use fans and ensure good airflow throughout the space.
  • Prune: Remove infected leaves and sanitize tools between cuts.
  • Natural Treatments: Neem oil, baking soda, and diluted milk sprays can slow fungal growth.
  • Commercial Fungicides: Use as directed; follow all label instructions.
  • Maintain Ideal Conditions: Keep temperature and humidity within safe ranges.
  • Stay Clean: Sanitize yourself, your tools, and your grow space regularly.
  • Choose Strong Plants: Use silica and nutrients to encourage strong cell development.

Products to Prevent, Control & Eliminate Powdery Mildew

Commercial cultivators now have access to a wide range of products formulated to prevent, control, and eliminate powdery mildew. This is not a complete list, but includes popular and proven options:

Prevent:

Control:

Neem Oil Products:

Silica Products:

Cleaners:

Final Thoughts

Preventing powdery mildew in a greenhouse or indoor grow requires vigilance, cleanliness, and environmental control. Even with the best preventive measures, outbreaks can still occur—so regular inspections and rapid responses are essential.

By maintaining a clean and controlled environment, implementing a robust IPM strategy, cultivating strong plants, and acting quickly when signs of powdery mildew appear, you can significantly reduce the risk of infection. Whether you choose natural or commercial remedies, staying proactive is your best defense against this persistent and costly fungus.

Back to Learning Center