Hardly anything in the bed causes as much confusion as mildew. Because behind the one name sit two fundamentally different diseases that behave almost as opposites. Mix them up and you fight the wrong one and wonder why nothing helps.
Yet they can be told apart at a single glance, and that distinction decides the right prevention. Powdery mildew is a fine-weather fungus, downy mildew a foul-weather one. Grasp this contrast and you protect courgette, cucumber and gooseberry precisely, instead of spraying blindly.
Two diseases, one confusing name
Powdery mildew is caused by true fungi from the group of sac fungi. It lives on the leaf surface and forms its white web there, which you can wipe off like flour. Because it only colonises the surface, it is easier to catch.
Downy mildew, by contrast, is not a true fungus at all but a water mould, related to the pathogen of late blight. It penetrates the leaf tissue and pushes its down out only on the underside. That is why it cannot be wiped off and is harder to fight from the outside.
How to tell at a glance which one you have
The surest distinction is the leaf underside. Simply turn an affected leaf over.
The quick comparison
- Powdery mildew: white and on top
A white, floury coating on the upper leaf side that wipes off. Appears in warm, dry weather. The underside usually stays clear.
- Downy mildew: spots on top, down beneath
Angular yellow to brown spots on top, bounded by the leaf veins, with a grey-violet fungal down beneath. Comes in damp, cool weather and does not wipe off.
- The weather gives it away
A long fine-weather spell with warm days and damp nights points to powdery mildew. Wet, cold, rainy weather points to downy mildew.
- The plant gives clues
Roses, apples, gooseberries and cereals almost always get the powdery kind. Lettuce, onion, basil and grape often suffer the downy one. Cucumber and squash can get either.
Powdery mildew: the fine-weather fungus
Powdery mildew covers leaves, stalks and sometimes fruit with a white, dusted-looking coating. Contrary to what you might think, it needs no leaf wetness. On the contrary, warm days and damp, dewy nights are enough for it, while rain tends to slow it. That is why it often appears in high and late summer after longer fine-weather spells.
Act early and you keep it small. The first tiny white spots are the best moment to remove affected leaves and give the plant more air.
Downy mildew: the foul-weather fungus
Downy mildew is the opposite: it needs wetness and mild to cool weather. After rainy spells it breaks out and can spread at breakneck speed, especially on cucumber, lettuce and grape. Because it sits inside the leaf, prevention matters even more here than with the powdery kind.
How to prevent both
Plant robust varieties
The most effective measure. There are mildew-tolerant cucumbers and courgettes and resistant gooseberries such as Gooseberry 'Invicta'. They fall ill noticeably later and more mildly.
Plant airy and with spacing
Give the plants room so the foliage dries quickly after dew and rain. Dense, damp stands are a paradise for both kinds of mildew.
Against the downy kind: keep foliage dry
Water from below at the root, never over the leaves, and best in the morning. Dry leaves take the base away from downy mildew.
Feed in balance
Avoid a nitrogen surplus. Lush, soft foliage is especially susceptible. Moderate feeding makes the plant more resistant.
Remove affected leaves early
Cut out the first affected leaves at once and put them in the residual waste, not on the compost. That slows the spread before it gets going.
Keep the rotation
Do not grow susceptible crops in the same spot year after year. Downy mildew can survive on plant debris and in the soil.
How to lower the risk for good
- Watch the variety label
Look for notes like mildew-tolerant or resistant when you buy. With cucumber, courgette, gooseberry and grape, the choice of variety makes the biggest difference.
- Water in the morning, not the evening
Watering in the morning gives the foliage all day to dry. Evening watering keeps the leaves damp overnight, a starting gun for downy mildew.
- Do not think in monoculture
Mix crops and keep your distance from already affected beds. Every pathogen is specialised on certain hosts, so diversity slows it down.
- Strengthen the plant
Strong, well-supplied plants defend themselves better. Even watering and a good spot are half the prevention.
White and wipeable on top is the powdery kind, spots on top and down beneath the downy one. First identify, then act.
The core rule on mildew
Frequently asked questions
How do I reliably tell powdery from downy mildew?
Fastest via the leaf underside. Powdery mildew forms a white, wipeable coating on top, the underside stays clean. Downy mildew makes angular spots on top and a grey fungal down beneath that does not wipe off.
Can I still eat vegetables with mildew?
Yes. The mildew sits on the foliage, the fruit stays edible. Just wash cucumbers, courgettes and the like. Only heavily affected, wilting plants yield little more, and then a fresh start beats the rescue.
Why does powdery mildew often come only in late summer?
Because warm days and damp, dew-rich nights suit it best. That combination is typical of high and late summer. Courgette and squash get it almost every year once the nights turn cooler and damper.
Does the milk spray really help against mildew?
Only to a limited degree and only against powdery mildew, as a preventive on still-healthy leaves. As the sole rescue of a heavily affected stand it is no good. Robust varieties and an airy stand work far more reliably.
Do I have to cut off affected leaves?
At the start of an attack, yes, that slows the spread noticeably. Remove the first affected leaves and dispose of them in the residual waste. With very heavy attack, cutting helps little, and then it is only about prevention for next year.
Are there really mildew-proof varieties?
Yes, and they are the best protection. With cucumber and courgette, many modern varieties are mildew-tolerant; with the gooseberry there are resistant varieties such as Gooseberry 'Invicta'. They are not spared entirely, but they fall ill much later and more mildly.
