Downy mildew

Worst in cool, wet, humid weather through autumn, winter and spring, March to October.

Downy mildew is a fungal-like disease that strikes in cool, wet, humid weather and hits brassicas, lettuce, onions, grapes and cucurbits. Unlike powdery mildew it shows as yellow blotches on top of the leaf with a greyish mould beneath, and it can spread fast in a damp spell.

How to identify

• Pale yellow or angular blotches on the upper leaf surface, often bounded by the veins
• Greyish, purplish or downy mould on the underside beneath the blotches
• Leaves browning, curling and dying off as patches join up
• Worst in cool, wet, still weather

How to prevent

• Space plants and prune for airflow so leaves dry quickly
• Water at the base in the morning and avoid wetting foliage
• Rotate susceptible crops and clear infected debris
• Grow resistant varieties of lettuce, brassicas and cucurbits where available

How to control organically

• Remove and bin affected leaves at the first sign
• Improve airflow by thinning and weeding around plants
• Spray a copper-based fungicide as a protectant in damp spells, covering leaf undersides
• Stop overhead watering and water early so foliage dries by night
• Clear all infected debris at the end of the crop