Powdery Mildew is a common disease that affects courgettes, swede and turnip.
What does it look like?
White, powdery, spreading patches of fungus on the upper or lower leaf surfaces, stems, flowers and fruit. Heavily infected leaves may distort and growth may be stunted. Each powdery mildew has a narrow host range, so the mildew on peas is not the one on apples.
Is this disease active now?
From spring onwards. It follows dry spells and drought stress more than a date, so a dry month with humid nights can bring it on at any point in the growing season.
🌱 Plants at risk
Powdery mildew doesn't need wet leaves, and water on the leaf works against it. Most gardeners assume the opposite.
The spores have an unusually high water content, enabling them to infect under drier conditions than most other fungal pathogens. Humid air, but not free water, favours spore production and infection, and the fungus does not like direct contact with water. Plants under intermittent drought stress are more susceptible, and overhead watering in mid-morning in dry weather can reduce it.
This inverts standard practice. Gardeners are taught to avoid wetting foliage, which is right for botrytis and rust but not for powdery mildew. The instinct on seeing mildew is to water less, when the better move is the reverse: water regularly through dry spells 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
No Scottish figures on powdery mildew pressure were found, but the biology points to where the risk sits: outdoors, rain and open air work against it, while still, humid air under cover favours it.
The fungus does not like direct contact with water, and humid air without free water favours its spores, which describes a polytunnel or greenhouse with the leaves kept dry. Plants in still, sheltered spots, such as climbers against walls and fences, are often heavily infected. A courgette grown outdoors in rain and moving air is therefore likely to be less exposed than the same crop under cover, though this is reasoning from the biology, not a Scottish measurement.
Controls: what 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
The RHS recommends not using fungicides, including organic types, because they may reduce biodiversity, affect soil health and have wider adverse environmental effects. US extension advice (UC IPM) takes a different line and lists least-toxic sprays for very susceptible crops such as cucurbits, applied no later than the first sign of disease.
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 in mid-morning in dry weather can also reduce infection; avoid watering overhead early in the morning or in the evening, and avoid leaves staying wet for long, as that may favour other diseases. US extension advice (UC IPM) is more cautious about overhead sprinklers on vegetables for the same reason.
Powdery mildew does not like direct contact with water, so mid-morning overhead watering in dry weather can help against it. Grey mould and blight are the opposite: wet foliage 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, such as courgettes, keeping foliage dry is the safer default, since grey mould tends to do more damage, with overhead watering reserved as a response once powdery mildew has appeared.
Plant in full sun with good air circulation, avoid dense spacing near walls and go easy on nitrogen
Give plants space and avoid sheltered places, such as against walls or fences, where the air is likely to be still. Don't over-fertilise, especially with nitrogen, because succulent growth is particularly susceptible.
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
Destroying fallen infected leaves in autumn reduces the spores available next spring, and promptly pruning out infected shoots reduces later infection.
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 →
Some seed producers offer powdery mildew-resistant cultivars, so check catalogues. Resistance is not always to every race of the fungus, so it reduces the risk rather than removing it.
Myths about powdery mildew
“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 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 keep the air moving around the foliage.
In Scotland: Outdoors, rain and moving air work against it, and the risk is likelier to concentrate in polytunnels and greenhouses, where humidity builds but leaves stay dry. That is our reasoning from the biology, not a Scottish measurement.
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 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.
“Improving air circulation, by spacing and pruning plants, will prevent powdery mildew on its own”
Air circulation is a 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.
“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 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.
Frequently asked questions
What should I do first if I find powdery mildew in my garden?
The first thing to try: Water regularly during dry periods to avoid drought stress; mulch to retain soil moisture. Then: Plant in full sun with good air circulation, avoid dense spacing near walls and go easy on nitrogen.
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.
When should I watch for powdery mildew?
From spring onwards. It follows dry spells and drought stress more than a date, so a dry month with humid nights can bring it on at any point in the growing season.
Sources
Show the 2 sources
- RHS1: group of fungi with narrow host ranges, white powdery patches, courgettes, marrows, cucumbers, peas, apples and gooseberries among plants affected, spores' high water content, dislike of direct contact with water, drought stress, mid-morning overhead watering in dry weather, sheltered still air, nitrogen, mulch, fallen leaves, pruning out shoots, resistant cultivars from seed producers, advice against fungicides with its reasons
- UC IPM: infection without free water, moisture inhibiting spores, mid-morning sprinkling with a caution for vegetables, spacing, nitrogen, resistant cultivars of cole crops, cucurbits and peas that may not resist all races, least-toxic fungicides for very susceptible crops