Downy Mildew is a common disease that affects spinach, beetroot and chard.
What does it look like?
Look for blotches on the upper leaf surface, pale green, yellow, purple or brown, with a mould-like growth underneath that may be white, grey or purple depending on the species. Downy mildew is not as easy to recognise as powdery mildew, and the mould underneath is the tell. Each downy mildew usually infects only a few related plants, so the one on spinach is not the one on your lettuce or onions.
What causes it
It is a disease of wet weather: infection is favoured by prolonged leaf wetness, and extended high humidity or overnight leaf wetness can cause severe outbreaks. Wet summers make outbreaks likely, and cool, damp conditions suit it on spinach.
Is this disease active now?
Mainly summer and autumn, particularly in wet conditions. It is driven by leaf wetness rather than a date, so a wet spell in any month can bring it on.
🌱 Plants at risk
Life cycle
Most downy mildews produce resting spores (oospores) inside infected tissue. They are released into the soil as the diseased material rots and may survive for up to five years. The disease spreads from plant to plant by airborne spores, which are splashed by rain and sometimes carried long distances on the wind.
Despite the name, downy mildew and powdery mildew are unrelated organisms, and they differ on water.
Despite a similar name and certain similarities in symptoms, downy mildews are unrelated to powdery mildews.
Seeing 'mildew' and reaching for the wrong advice can make things worse: overhead watering to suppress powdery mildew would favour downy mildew.
Why this matters in Scotland
An agronomist at East of Scotland Growers in Fife described downy mildew as a growing concern in brassicasThe cabbage family: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip and more, sharing the same pests, diseases and rotation rules. Full definition →, particularly in Scotland (AHDB, 2020), and the grower trial there tested fungicides against it in broccoli and cauliflower.
On brassicas it shows as blemishes on the underside of the leaves, and once it reaches the head it can cause grey streaking in the stems. Under cover the practical answer is to avoid prolonged leaf wetness and high humidity, open doors and vents for air movement, and not water in the evening.
Controls: what works
Chemical control
There are no fungicides available to amateur gardeners for use against downy mildews, so control depends on cultural measures.
What to do instead: Keep leaves dry and the air moving: skip overhead and evening watering, ventilate under cover, space plants, 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, and open doors and vents to encourage air movement. Outdoors, water in the morning so the leaves dry quickly; evening watering can leave them wet all night.
The advice is about how long the leaf stays wet, not about the water itself.
Pick off and dispose of affected leaves as soon as symptoms appear
Destroy severely affected plants, and dispose of affected leaves by deep burial, burning or council green waste rather than composting them at home, since the resting spores survive in rotting tissue.
Space plants and control weeds so air circulates
Avoid dense planting and keep weeds down so there is good air circulation around the plants, which helps the leaves dry.
Rotate: don't replant the same host in the same ground for at least a year
Practise crop rotationMoving each crop family to different ground each year, on a multi-year cycle, so pests and diseases that build up in soil don't get a repeat host. Full definition → for vegetables, and avoid replanting the same host for at least a year. Resting spores can survive in soil for up to five years, so rotation reduces rather than removes the risk.
Choose 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 → where they exist
Cultivars with resistance to downy mildew are available for some vegetable crops, such as lettuce and onions, and for spinach. Resistance can break down as the disease changes, so it is a help rather than a guarantee.
Myths about downy mildew
“Downy mildew and powdery mildew are basically the same disease, so the same treatment works for both”
The two are caused by unrelated organisms and differ on water. Downy mildew is a disease of wet weather, favoured by prolonged leaf wetness, while powdery mildew is favoured by humid air but does not like direct contact with water. Treating them as interchangeable leads to the wrong fix: watering overhead to discourage powdery mildew would favour downy mildew.
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.
2 sources
- 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 are no fungicides available to amateur gardeners for use against downy mildews, so there is no product to point to. Control comes from cultural measures: keeping foliage dry, spacing and ventilation, removing infected leaves promptly, rotating crops and choosing resistant cultivars where they exist.
What to do: Don't delay leaf removal or spacing changes while looking for a fungicide, since none is available to amateur gardeners; act on the cultural measures as soon as symptoms appear.
1 source
- 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 cut risk and are worth choosing, but resistance is tied to specific genetic races of the pathogen, and many races of lettuce downy mildew exist, with new ones emerging. A resistant label is a reduced-odds bet, not a guarantee.
Our read No Scotland-specific race information was found, so treat the general principle, that resistance can break down, as the useful point.
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.
1 source
- 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”
Prolonged leaf wetness is what favours infection, more than damp air alone. Conditions that keep foliage wet after dark, such as dew, rain or an evening watering session, create the longest infection window. This is also the clearest way it differs from powdery mildew, which does not like direct contact with water.
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.
1 source
- RHS, Downy mildewsExtended periods of leaf wetness are required for infection, with overnight leaf wetness and wet summers causing severe outbreaks.
“Watering in the morning instead of the evening makes a real difference to downy mildew risk”
This holds up because it follows from what the disease needs: prolonged leaf wetness. Morning watering gives foliage a full day to dry off, while evening watering can leave it wet all night. It's not that evening water is worse in itself; 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.
1 source
- 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 reduce downy mildew”
Airflow affects how quickly foliage dries and how much still, humid air collects around the leaves. Dense planting and weeds 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 open greenhouse or polytunnel vents whenever conditions allow.
1 source
- 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.
Frequently asked questions
What should I do first if I find downy mildew in my garden?
The first thing to try: Avoid prolonged leaf wetness. Then: Pick off and dispose of affected leaves as soon as symptoms appear.
Is there a spray that works?
No. There are no amateur chemical treatments available for downy mildew. The controls that work are cultural: the measures listed above.
When should I watch for downy mildew?
Mainly summer and autumn, particularly in wet conditions. It is driven by leaf wetness rather than a date, so a wet spell in any month can bring it on.
Sources
Show the 4 sources
- RHS1: unrelated to powdery mildew, upper-surface blotches with mould below, each species infecting a few related plants, wet weather and prolonged leaf wetness, resting spores surviving up to five years, spread by airborne spores, no fungicides for amateur gardeners, removal and disposal of affected leaves, spacing, rotation, resistant cultivars, glasshouse ventilation and evening watering
- UC IPM: cool, damp conditions favouring spinach downy mildew, air circulation, drying between irrigations, a year out of spinach, destroying infected debris, resistant cultivars available
- archive.ahdb.org.uk: downy mildew described by a Scottish agronomist as a growing concern in brassicas, particularly in Scotland; blemishes on the underside of broccoli and cauliflower leaves and grey streaking in the stems
- RHS1: the contrast between the two diseases