🐛✗ Tested and false“Downy mildew and powdery mildew are basically the same disease, so the same treatment works for both”
The two are caused by unrelated groups of organisms and want close to opposite conditions. Downy mildew needs extended leaf wetness to infect at all, while powdery mildew is favoured by high humidity but is actively hindered by free water sitting on the leaf. Treating them as interchangeable leads straight to the wrong fix: watering foliage to discourage powdery mildew would feed a downy mildew infection instead.
What to do: Diagnose from the symptom pattern first: dusty white coating on the leaf surface points to powdery mildew, while patchy discolouration on top with fuzzy grey, white or purple growth underneath points to downy mildew. Only then pick a response, since the two call for different watering habits.
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- RHS, Downy mildewsDespite the similar name and some shared symptoms, downy mildews are unrelated organisms to powdery mildews and need extended periods of leaf wetness to infect.
- RHS, Powdery mildewsHigh humidity without free water on the leaf favours powdery mildew spore production and infection, the opposite of downy mildew's requirement for leaf wetness.
“A fungicide spray from the garden centre will clear up a downy mildew outbreak”
There is no fungicide sold for amateur use that treats downy mildew, so this claim doesn't have a product to point to in the first place. Whatever a label or a fellow gardener suggests, control has to come entirely from cultural measures: keeping foliage dry, spacing and ventilation, and removing infected leaves promptly, none of which is a spray-and-forget fix.
What to do: Don't delay leaf removal or spacing changes while waiting to find a fungicide that will work, since none is available; treat prevention as the only real lever and act on it as soon as symptoms appear.
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- RHS, Downy mildewsThere are no fungicides available to amateur gardeners for use against downy mildews.
“A downy mildew resistant lettuce variety won't get downy mildew”
Resistant varieties genuinely cut risk and are worth choosing, but resistance is tied to specific genetic races of the pathogen, and lettuce downy mildew (Bremia lactucae) keeps evolving new races that overcome those genes. Breeders and disease-tracking boards have already had to respond to new races overcoming widely used resistance genes, most recently strains identified in both the US and Europe. A resistant label is a reduced-odds bet, not a guarantee, and it can stop working partway through a season if a new race turns up locally.
Our read The lettuce-specific literature on race turnover is US and European commercial-growing research rather than home-garden Scottish data, so treat the general principle (resistance can and does break down) as solid but don't expect a Scotland-specific race map.
What to do: Grow resistant varieties as one layer of defence, not the whole plan: pair them with the same leaf-dryness and spacing habits used against non-resistant plants, and don't assume an outbreak on a 'resistant' variety is a fluke or a mislabelled seed packet.
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- RHS, Downy mildewsDisease resistance can break down if the downy mildew mutates to produce a genetically different race, and the situation with lettuce downy mildew is particularly complex because many races exist and new ones keep emerging.
“Downy mildew only takes hold when leaves are wet, humidity alone isn't enough”
Extended leaf wetness, not just damp air, is what the pathogen needs to infect. Spores are often produced overnight, so conditions that keep foliage wet after dark, mist, dew, rain, or an evening watering session, create the longest infection window. This is also the clearest way it differs from powdery mildew, which can infect leaves that are humid but dry to the touch.
What to do: Focus prevention on how long leaves stay wet rather than on the air's overall humidity: water at the base of the plant where possible, and give foliage the whole day to dry rather than the whole night.
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- RHS, Downy mildewsExtended periods of leaf wetness are required for infection, with spores often produced at night and severe outbreaks occurring during wet summers outdoors.
“Watering in the morning instead of the evening makes a real difference to downy mildew risk”
This one holds up because it follows directly from what the disease actually needs: prolonged leaf wetness. Morning watering gives foliage a full day of light, warmth and air movement to dry off, while evening watering leaves it wet through the coolest, stillest hours of the night, which is also when the pathogen tends to produce spores. It's not that evening water is somehow worse in itself, it's that it maximises how long the leaf stays wet.
What to do: Water early in the day, ideally at the base of the plant rather than over the foliage, and skip evening watering sessions on crops that have had downy mildew before.
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- RHS, Downy mildewsGardeners should avoid watering plants in the evening, as this leads to high humidity or leaf wetness persisting overnight, and should instead water early in the morning so leaf surfaces dry out rapidly.
“Wider spacing and better airflow around plants actually reduces downy mildew, it's not just generic gardening advice”
This is specific advice rather than a catch-all, because airflow directly affects how quickly foliage dries and how much still, humid air collects around leaves, both of which the disease depends on. Dense planting and weed growth trap moisture close to the leaves, while open spacing and weed control let air move through the crop, and under cover the same logic applies to opening doors and vents.
What to do: Space susceptible crops like lettuce, brassicas and spinach at the wider end of the recommended range in wet seasons, keep the base of the plants weed-free, and prop open greenhouse or polytunnel vents whenever conditions allow rather than only when it feels hot.
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- RHS, Downy mildewsAvoid dense planting and control weeds so there is good air circulation around plants, and open greenhouse doors and vents when conditions allow to encourage air movement.
