Downy Mildew
Disease · Commonly affects: lettuce, spinach, rocket, cabbages, cauliflowers, broccoli, onions, spring onions
Downy mildew is a group of moisture-loving diseases that affect many vegetables and ornamentals. It is most likely in cool, damp UK weather, when leaves stay wet for long periods. Unlike powdery mildew, it usually begins within the leaf and can spread rapidly through closely planted crops, especially in sheltered gardens and polytunnels.
How to identify it
Look for angular yellow, pale green or brown blotches, often 3-15 mm wide, on the upper leaf surface. The patches are usually limited by leaf veins, giving a blocky pattern. In damp conditions, greyish, violet-grey or pale mould develops beneath the matching patches. Leaves may later brown, crinkle and collapse. On onions, long pale streaks can carry a greyish bloom; on lettuce, outer leaves often show the first symptoms.
Damage
Infected leaves lose their ability to support growth, resulting in stunted lettuce, weak brassicas and reduced onion bulb size. Severe outbreaks can destroy the outer leaves of leafy crops and make harvests unattractive or unusable. Young plants are especially vulnerable. Persistent infection also leaves crops less vigorous and more prone to other problems during wet weather.
Prevention
Use clean seed where possible and avoid crowding plants, allowing air to move freely between rows. Water early in the day and direct water onto the soil, not foliage. Improve drainage with organic matter or raised beds, especially on heavy ground. Rotate related crops around the plot and remove old plant remains. Ventilate greenhouses and polytunnels well. Choose resistant varieties when offered, particularly for lettuce and onions.
How to manage it
Pick off affected leaves promptly when symptoms are limited, taking care not to shake spores onto neighbouring plants. Harvest mature leafy crops before infection advances, discarding damaged outer leaves. Reduce humidity under cover by opening vents and doors whenever conditions allow. Pause overhead watering and keep paths clear of weeds that restrict airflow. Once a crop is badly affected, remove it and bin the material rather than composting it.
Downy Mildew: frequently asked questions
How do I identify downy mildew?
Look for angular yellow, pale green or brown blotches, often 3-15 mm wide, on the upper leaf surface.
How do I prevent downy mildew?
Use clean seed where possible and avoid crowding plants, allowing air to move freely between rows.
How do I get rid of downy mildew?
Pick off affected leaves promptly when symptoms are limited, taking care not to shake spores onto neighbouring plants.
General guidance for UK home gardens and allotments, focused on prevention and non-chemical control. For a confirmed diagnosis, the RHS or a local garden centre can help. Page updated 19 Sep 2026.