Powdery Mildew is a common disease that affects courgettes, swede and turnip 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.
🌱 Plants at risk
Powdery Mildew also affects these crops:
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, which is correct for botrytis and rust but wrong for powdery mildew. And the usual instinct on seeing mildew is to water less, when the right move is the reverse: water regularly to avoid drought stress, and mulchA layer of material spread over the soil around plants to suppress weeds, retain moisture, and improve soil structure as it breaks down. Full definition → to retain soil moisture.
Why this matters in Scotland
The one disease where Scotland's wet climate is a straightforward advantage outdoors, and a warning about growing under cover.
Most of what makes Scotland harder for growers 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
Not every method here works equally well: some are backed by real evidence, and one has been tested and shown not to help despite how often it gets repeated.
Fungicides
Not recommended: fungicides 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.
Powdery mildew is inhibited by direct water contact, so midday overhead watering can actually help against it. Grey mould and blight are the opposite: wet foliage is exactly what lets them take hold, so keeping leaves dry is the right advice there. Both pieces of advice are correct for their own disease. On a crop at risk from both, courgettes get powdery mildew and are also botrytis-susceptible, that's a genuine trade-off: keeping foliage dry is the safer default, since grey mould tends to do more damage, with overhead watering reserved as a targeted response once powdery mildew has actually appeared rather than a routine habit.
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 cultivarsA plant variety that arose or was bred under cultivation and is kept true by deliberate propagation, rather than a naturally occurring wild species. Full definition →
Check seed catalogues for mildew-resistant cultivars of vegetables and ornamentals.
Myths about powdery mildew
5 common claims about powdery mildew, checked: 3 busted2 disputed0 untested0 hold up
“Powdery mildew only takes hold on wet or damp leaves, the same as most fungal diseases”
Powdery mildew spores carry unusually high water content, which lets them germinateA seed sprouting: breaking dormancy and putting out its first root and shoot, given the right warmth and moisture. Full definition → and infect in drier conditions than most fungal pathogens need. Direct contact with water on the leaf surface actually inhibits the fungus rather than helping it. What the disease does want is high humidity in the air without standing water on foliage, a narrower and less intuitive requirement than the general dampness gardeners tend to blame.
Our read This disease is an exception to the usual fungal-disease logic, not a variation on it. Unlike most fungi, it doesn't need free water, because its spores carry an unusually high water content of their own.
What to do: Don't withhold water from stressed plants in the hope of drying out mildew. Water the roots as normal to avoid drought stress, and focus prevention on lowering still, humid air around the foliage rather than on keeping leaves dry.
In Scotland: This is one disease where an open, breezy Scottish garden works in the grower's favour outdoors; the risk concentrates instead in polytunnels and greenhouses, where humidity builds but leaves stay dry.
2 sources
- RHS: Powdery mildewsPowdery mildew spores have unusually high water content that lets them infect under drier conditions than most fungal pathogens, and the fungus does not like direct contact with water on the leaf.
- Clemson HGIC: Powdery MildewUnlike most fungi, powdery mildews do not require free water to germinate; only a high level of relative humidity is needed, and low relative humidity actually favours spore dispersal.
“Powdery mildew is a single fungus that spreads between any plant it touches”
"Powdery mildew" is a name for a large group of related but distinct fungal species, most with narrow host ranges limited to a few related plants. The species infecting peas is not the species infecting apples, and mildew on a rose is not a threat to the courgettes growing nearby. Seeing it on one plant doesn't mean it will spread to an unrelated crop, though it can move between closely related hosts.
What to do: Don't assume mildew on one crop puts every other plant in the garden at risk, and don't assume a resistant variety of one crop protects an unrelated species growing next to it. Treat each outbreak as specific to that plant and its close relatives.
1 source
- RHS: Powdery mildewsPowdery mildews usually have narrow host ranges comprising just a few related plants; the species affecting peas is different from the one attacking apples.
“Choosing a "powdery mildew resistant" variety means it won't get the disease”
Resistance is not immunity. Resistant varieties are bred to develop symptoms more slowly and stay healthier for longer under disease pressure, but they can still become infected if conditions favour the fungus for long enough. Severity still depends on environmental conditions and how the plant is grown, not on the variety label alone.
What to do: Treat a resistant variety as a way to buy time and reduce severity, not as a guarantee. Keep up the other cultural controls (airflow, sanitation, avoiding drought stress) even on resistant plants.
2 sources
- Clemson HGIC: Powdery MildewResistance does not mean immunity; disease severity may still vary based on environmental conditions and plant care.
- RHS: Powdery mildewsSeed producers sometimes offer powdery mildew-resistant cultivars as one option among the wider set of cultural controls.
“Spraying a baking soda (sodium bicarbonate) solution on the leaves cures powdery mildew”
The research on sodium bicarbonate is genuinely mixed rather than a clean yes or no. Some trials found it suppressed mildew on crops including cucumbers, tomatoes and peppers, but plain bicarbonate sprayed alone has also failed outright in other trials, and it has caused leaf damage on sensitive plants when overused. Where it has performed best, it was combined with a horticultural oil rather than used alone, and even then some researchers found the combination worked no better than the oil by itself.
Our read The WSU literature compilation reads as a mixed bag rather than an endorsement: real suppression in some crop trials, no effect or leaf damage in others, and stronger results generally reported for bicarbonate-plus-oil rather than bicarbonate on its own. More broadly, homemade fungicides are unregulated and usually untested.
What to do: Don't treat a baking soda spray as a reliable cure once mildew is established. If trying it at all, use it preventatively at a low concentration, watch for leaf scorch, and don't expect it to outperform simply improving airflow and removing infected growth.
2 sources
- WSU: Literature on Baking Soda (Linda Chalker-Scott)Sodium bicarbonate has suppressed powdery mildew in some crop trials, works better combined with horticultural oil than alone, and carries a risk of foliar damage including chlorosis on sensitive plants.
- RHS: Powdery mildewsHomemade fungicide products are not recommended by the RHS because they are unregulated and usually untested.
“Improving air circulation, by spacing and pruning plants, will prevent powdery mildew on its own”
Air circulation is a genuine and recommended control, not a placebo; both sources list spacing and pruning as ways to reduce the still, humid air the fungus favours. But neither treats it as sufficient by itself. It sits alongside avoiding overhead watering, sanitation of infected material, and variety choice, and even well-spaced plants can still develop mildew if conditions turn favourable.
What to do: Space and prune for airflow as one layer of prevention, not the whole plan. Pair it with removing infected leaves and shoots promptly and choosing resistant varieties where available, rather than relying on spacing alone.
2 sources
- RHS: Powdery mildewsRecommended cultural controls include avoiding sheltered, still locations, planting and pruning for an open structure to improve air circulation, and destroying infected fallen leaves and shoots.
- Clemson HGIC: Powdery MildewAdequate spacing to reduce crowding is recommended alongside avoiding overhead irrigation, plant selection and sanitation, as part of a combined management strategy rather than a standalone fix.
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. Work through the rest in order of how well-evidenced they are if the first two aren’t enough on their own.
Is there a spray that works?
No. There are no amateur chemical treatments available for powdery mildew. The controls that work are cultural: the measures listed above.
How much of this advice is actually proven?
4 of the 5 controls above are evidence-based, not just repeated folklore. Start with the evidence-based ones and treat anything lower down the list as worth trying, not worth relying on.
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