Downy mildew is a damaging plant disease that strikes in cool, wet weather — and despite the similar name it is very different from powdery mildew. It spreads fast in the right conditions and can ruin cucurbits, grapes, brassicas, basil and many ornamentals. This is a general guide; downy mildews are a group of host-specific pathogens, so susceptibility and the right response vary by plant — treat this as the framework.
What downy mildew is
Downy mildew is caused by water-mould organisms (oomycetes), not true fungi. Crucially, it needs moisture — wet leaves, high humidity and cool temperatures — to infect and spread, which is the opposite of powdery mildew. It’s largely host-specific: the downy mildew on one plant type generally won’t jump to unrelated plants, but it moves fast among susceptible ones.
Downy mildew vs powdery mildew
- Downy mildew — needs wet/cool/humid conditions; growth appears mostly on the underside of leaves; top surface shows yellow, often angular patches bounded by the veins.
- Powdery mildew — thrives in warm, humid but dry conditions; white powder on the upper surface. (See powdery mildew.)
Getting the ID right matters because the conditions and prevention differ.
Common symptoms
- Pale green to yellow patches on the upper leaf surface, often angular (bounded by leaf veins).
- A greyish, purplish or downy fuzzy growth on the underside, usually beneath those patches (most visible in humid mornings).
- Patches browning and dying; leaves curling, withering and dropping.
- Stunted growth, and on some crops, damaged flowers, heads or fruit.
Conditions that favour it
- Cool, wet, humid weather; prolonged leaf wetness (dew, rain, overhead watering).
- Crowded plantings and poor air circulation.
- Still, damp microclimates — shaded, sheltered, or under cover without ventilation.
Commonly affected plants
Cucurbits (cucumber, melon, squash), grapes, brassicas, lettuce, spinach, basil, onions, and ornamentals such as impatiens, roses and some bedding plants each have their own downy mildew. Check susceptibility for your specific plant.
Airflow & spacing
Because the disease depends on moisture and still air, improving airflow is one of the strongest preventives: space plants adequately, thin congested growth, weed, and ventilate greenhouses and covers so foliage dries quickly.
Watering practices
Keep leaves as dry as possible — water at the base, not over the foliage, and water early in the day so any splashed leaves dry before evening. Avoid overhead watering and working among wet plants (which spreads spores).
Resistant varieties & sanitation
Where downy mildew is a recurring problem (e.g. basil, cucurbits, impatiens), choosing resistant varieties is often the most effective single step. Remove and dispose of infected leaves and end-of-season debris (don’t compost heavily infected material), since some downy mildews overwinter on plant remains.
Treatment principles
- Act early and fix conditions — improve airflow, reduce leaf wetness, remove affected leaves. Downy mildew is hard to cure once established, so prevention and early action matter most.
- Remove and dispose of infected material promptly to slow spread.
- If using a product, some protectant options exist (for example copper-based and other labelled products, and biologicals). No single product suits every plant or every downy mildew, and most work best preventively — choose one labelled for your plant and this disease, follow the label exactly, and don’t exceed labelled rates.
Flowering & fruiting impact
On some crops downy mildew hits more than leaves — it can damage flower heads, reduce yield, and spot or distort developing produce (for example on grapes and cucurbits). Protecting foliage through the season also protects the crop.
Common mistakes
- Confusing it with powdery mildew and treating for the wrong conditions.
- Overhead watering and wetting foliage late in the day.
- Crowding plants / ignoring airflow.
- Waiting too long — it’s far easier to prevent than to cure.
- Composting heavily infected debris near susceptible plants.
Quick questions
How do I tell downy from powdery mildew? Downy mildew makes yellow angular patches on top with fuzzy growth underneath and needs wet, cool conditions; powdery mildew is white powder on the upper surface in warm, dry-but-humid weather.
Can I cure downy mildew? It’s hard to reverse once established — focus on airflow, dry foliage, removing infected leaves and resistant varieties; products work mainly preventively.
Will it spread to my other plants? Mostly to related, susceptible plants — downy mildews are largely host-specific, but they move fast among plants of the same type in wet weather.
Related guides
- Powdery mildew · botrytis (grey mould) · all plant pests & protection
- Choosing a plant sprayer
- Rust – another common fungal leaf disease.
- anthracnose
- leaf spot
