Identifying, preventing and controlling downy mildew
Downy mildew is the "bad-weather fungus" – unlike its namesake, powdery mildew, it loves moisture, cool temperatures and long periods of wet leaves. It affects cucumbers, lettuce, onions, spinach, radishes and basil among others, and can spread extremely fast in damp weather.
It is important to distinguish it early from powdery mildew, because prevention and control differ significantly. The sure sign is found on the underside of the leaf.
In short
- Yellow, angular spots on the upper leaf surface, bounded by the leaf veins.
- A grey-violet fungal layer on the underside of the leaf – the sure sign.
- Occurs in wet, cool weather and prolonged leaf moisture.
- Prevent: keep dry, plant airily, water at the base, crop rotation.
How to spot downy mildew
On the upper leaf surface yellowish, often angular spots appear first, clearly bounded by the leaf veins – like little fields. Turn the leaf over and, in damp weather, a grey to violet fungal layer shows on the underside. Later the spots turn brown and the leaves dry up. So the coating sits – unlike powdery mildew – mainly underneath.
Why downy mildew occurs
The pathogen is a fungus-like organism that needs water to germinate. Ideal conditions are cool, damp weather, prolonged leaf wetness from dew, rain or overhead watering, and dense, poorly ventilated stands. The spores survive on plant debris and in the soil.
The damage it causes
Affected leaves die off from the bottom up, the plant loses leaf mass and therefore strength. In cucumbers and lettuce a severe infestation can destroy whole stands within a few days. The fruits stay small and the harvest drops noticeably.
Preventing downy mildew
- Keep leaves dry: always water at the base, in the morning rather than the evening, so the leaves dry off during the day.
- Airy position: generous spacing for good ventilation.
- Robust varieties: especially for cucumbers there are resistant breeds.
- Crop rotation: do not grow susceptible crops in the same spot year after year.
- Strengthen: horsetail brew toughens the tissue.
- Hygiene: remove affected foliage immediately and dispose of it in the household waste.
Controlling downy mildew
Once the infestation is there, consistent drying and removal help most:
- Cut off affected leaves and dispose of them to slow the spread.
- Change watering: water only at the base, thin out the stand.
- Horsetail brew: spray preventively and alongside treatment.
- Copper products only as a last resort and sparingly – pure home remedies such as milk have little effect here, as it is not a true powdery-mildew fungus.
Tip
Memory aid: Powdery mildew sits on top and is wipeable – Downy mildew forms its layer down on the underside of the leaf.
Caution
Never water affected beds from overhead and do not work among wet plants – you spread the spores particularly easily via wet leaves and your hands.
Frequently asked questions
Is downy mildew transferable to other plants?
The pathogens are usually host-specific – cucumber mildew does not infect lettuce, for example. Even so: remove affected material consistently.
Can I still eat affected lettuce?
Better remove heavily affected leaves; healthy leaves are safe after washing.
Strong, airily grown young plants with good roots are more resistant. We raise our young plants to be robust – a good start to the gardening year.