Powdery Mildew is a common disease that affects courgettes in Scotland. Here is what it looks like, the conditions it needs, and the controls that actually work.
What does it look like?
A white, dusty coating on leaves, stems and flowers. Also leaf distortion, discolouration, stunted growth, and occasionally scarring on succulent plants.
Powdery mildew is a DRY-conditions disease. Most gardeners assume the opposite, and act on that assumption.
Powdery mildew favours dry conditions over wet. Its spores have 'unusually high water content, enabling them to infect under drier conditions'. High humidity without free water promotes spore production, but direct water contact actually inhibits the disease. Overhead watering at midday in dry weather can even reduce infection.
This inverts standard practice. Gardeners are taught to avoid wetting foliage — correct for botrytis and rust, wrong for powdery mildew. And the usual instinct on seeing mildew is to water less; RHS's advice is the reverse: water regularly to avoid drought stress, and mulch to retain soil moisture.
Why this matters in Scotland
The one disease on the site where Scotland's wet climate is a straightforward advantage outdoors — and a warning about growing under cover.
Nearly every Scottish angle in this build is a limitation: colder, later, wetter, shorter. Powdery mildew is the inverse. It wants dry conditions and is inhibited by direct water contact, so an open Scottish garden is a hostile place for it. But the caveat is sharp: high humidity without free water promotes spore production, which describes a polytunnel exactly. Foliage stays dry under cover while humidity stays high. So Scottish courgettes grown outdoors are relatively safe from mildew, while the same crop under cover faces ideal conditions for it. Cover is again a trade, not a shield.
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.
Fungicides
RHS: 'The RHS recommends that you don't use fungicides.' They may reduce biodiversity and harm soil health.
Water regularly during dry periods to avoid drought stress; mulch to retain soil moisture
Water regularly through dry periods to avoid drought stress, and mulch to retain soil moisture. Overhead watering at midday in dry weather can also reduce infection.
Plant in full sun with good air circulation; avoid dense spacing near walls
Air circulation is the one control that works for powdery mildew, botrytis and blight alike: the common lever, despite their opposite moisture preferences.
Remove infected fallen leaves in autumn; prune infected shoots promptly
Grow mildew-resistant cultivars
Check seed catalogues for mildew-resistant cultivars of vegetables and ornamentals.
Frequently asked questions
What should I do first if I find powdery mildew in my garden?
Start with water regularly during dry periods to avoid drought stress; mulch to retain soil moisture. After that, plant in full sun with good air circulation; avoid dense spacing near walls. 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 powdery mildew. The controls that work are cultural: the measures listed above.
Sources
1 source, recorded with what each was used for
- RHS1: group of fungi with narrow host ranges, white dusty coating and other symptoms, courgette susceptibility, DRY-conditions preference and spore water content, inhibition by direct water, midday overhead watering, advice against fungicides, watering/mulching/airflow/hygiene, existence of resistant cultivars