Downy Mildew is a common disease that affects spinach in Scotland. Here is what it looks like, the conditions it needs, and the controls that actually work.
What does it look like?
Discoloured areas on the upper leaf surface, with white, grey or purple mould underneath. Symptoms vary by host: lettuce downy mildew gives yellow patches with fuzzy white mould, turning brown as the tissue dies.
Why this matters in Scotland
Another wet-weather disease — so another one Scotland's climate encourages.
Downy mildew is a disease of wet weather, favoured by prolonged leaf wetness. That places it alongside leek rust and botrytis as diseases Scotland's rainfall actively supports. It also joins the polytunnel cluster, where the answer is the same in every case: avoid prolonged leaf wetness and high humidity, open doors and vents for air movement, and do not water in the evening.
Controls: what actually works
Every method below is graded by evidence, not by how often it gets repeated. Where a claim has been tested and failed, we say so plainly.
Chemical control
Control depends on cultural techniques, as no effective fungicides are available to amateur gardeners.
What actually works: With no fungicide available, the fight is about keeping leaves dry: skip overhead and evening watering, ventilate under cover, and remove affected leaves as soon as they appear.
Avoid prolonged leaf wetness — no overhead watering, no evening watering, ventilate under cover
In glasshouses, avoid prolonged leaf wetness and periods of high humidity; avoid overhead watering where possible; open doors and vents to encourage air movement; and avoid watering in the evening.
Evening watering leaves foliage wet overnight, the longest possible wet period. The advice is really about how long the leaf stays wet, not about the water itself.
Pick off and dispose of affected leaves as soon as symptoms appear
Frequently asked questions
What should I do first if I find downy mildew in my garden?
Start with avoid prolonged leaf wetness — no overhead watering, no evening watering, ventilate under cover. After that, pick off and dispose of affected leaves as soon as symptoms appear. The full list of evidenced controls is above, each graded by how much evidence supports it.
Is there a spray that works?
No. There are no amateur fungicides available for downy mildew. The controls that work are cultural: the measures listed above.