How to Prevent and Manage Downy Mildew on Brassicas, Onions, and Lettuce

Damp, cool weather may feel great for lush garden growth, but it’s also prime time for downy mildew, a stealthy disease that loves to attack crops like brassicas, onions, and lettuce. It creeps in quietly, yellowing leaves and leaving greyish, fuzzy spores in its wake. But with a little planning and quick action, we can prevent and manage downy mildew before it takes hold of our harvest.

What Is Downy Mildew?

Downy mildew is caused by water moulds that thrive in humid, wet conditions. It’s not the same as powdery mildew, it affects the undersides of leaves and tends to strike during prolonged damp weather, especially when nights are cool and mornings are dewy.

Here’s what it looks like:

• Yellow or pale patches on upper leaf surfaces
  
• Grey, white, or purple fuzz underneath
  
• Leaf distortion or collapse as the infection progresses

Maximise Airflow and Spacing

Since downy mildew thrives in still, damp air, give your crops plenty of breathing room. That means:

• Generous spacing between rows and plants
  
• Removing lower leaves on brassicas as they mature
  
• Trellising or growing vertically when possible

Also try to avoid overcrowding in the greenhouse, tight spacing is a fast track to fungal trouble.

Water Wisely

Wet leaves are a big part of the problem. Water early in the day and aim directly at the soil, not the foliage. This gives plants time to dry before evening.

Use Crop Rotation

Downy mildew spores can linger in soil and on plant debris. Rotate crops on at least a 3-4 year cycle, especially for brassicas and alliums, to break the disease cycle.

Try not to plant lettuce after lettuce or onions after onions in the same spot each year if mildew has been an issue.

Keep Leaves Dry and Clean

Regularly inspect plants, especially during damp spells. At the first sign of infection, you can:

• Remove affected leaves and bin them (never compost)
  
• Improve ventilation by pruning excess growth
  
• Avoid overhead watering

Sanitation matters, fungal spores can cling to tools, gloves, or boots, so we clean up between beds when disease is present.

Organic Sprays for Prevention

While downy mildew can be tough to stop once it takes hold, early application of organic treatments can help. You can try:

• Potassium bicarbonate sprays to alter leaf surface pH
  
• Copper-based fungicides (where permitted) as a last resort
  
• Seaweed foliar feeds to boost plant health and resilience

Apply during dry weather and repeat every 7–10 days when conditions are damp.

Mulch and Soil Health

A layer of well-rotted compost or straw mulch keeps rain from splashing fungal spores up from the soil onto lower leaves. Mulching also helps regulate moisture levels and encourages stronger root systems.

Healthy soil equals healthy plants and healthy plants are far more capable of resisting infections like downy mildew.

Downy mildew on brassicas, onions, and lettuce doesn’t have to spell disaster. With a mix of smart prevention, good hygiene, and early intervention, you can keep your garden beds healthy and productive.