Downy mildew is the disease that gardeners most often confuse with powdery mildew, and the confusion matters, because the two need different conditions and different responses. Downy mildew shows as yellow spots on the upper surface of the leaf, with a grey or purple-grey fuzz on the underside, and it needs wet leaves to spread. It attacks cucumbers, lettuce, grapes, onions, roses and many other plants, and in a wet season it can defoliate a crop in a matter of weeks. It is a more serious disease than powdery mildew, and it is harder to control once it is established — which is why prevention, and early removal, matter so much.
This guide explains the symptoms, the conditions that favour downy mildew, cultural prevention, what to remove, what not to compost, and the control options. If you are not sure the spots on your leaves are downy mildew, the plant disease checker can help you compare, and our powdery mildew treatment guide covers the disease it is most often confused with.
Symptoms
Downy mildew starts as pale yellow or light green spots on the upper surface of the leaves, often bounded by the leaf veins so the spots look angular. On the underside of the leaf, directly beneath the yellow spots, there is a grey, purple-grey or white downy growth — the fuzz that gives the disease its name. The spots spread and merge, the leaves turn brown and dry out, and heavily infected leaves drop. On cucumbers and lettuce, the whole plant can be defoliated in a wet season. On grapes, the leaves yellow and drop, and the fruit may be affected. On onions, the leaves show pale spots and a grey-violet fuzz, and the bulbs are small.
The key difference from powdery mildew is the location and the colour: powdery mildew is a white, powdery coating on the upper leaf surface, while downy mildew is yellow spots above and grey fuzz below. If you are unsure, look at the underside of the leaf — the fuzz is the giveaway.
Conditions that favour it
Downy mildew needs wet leaves to spread. The spores germinate in water on the leaf surface, so rain, dew, overhead watering and leaves that stay wet overnight all favour it. It thrives in cool to warm, humid conditions, and it spreads rapidly in wet weather, carried by wind and by splashing water. Crowded plants, shaded beds and poor airflow make it worse, because the leaves stay wet longer. It overwinters on plant debris and, in some species, in the soil, and it can also be carried on seed.
Different plants have different downy mildew fungi, and most are specific to one plant family — the mildew on your cucumbers will not infect your lettuce. That matters for rotation, but it does not make the disease any less serious for the crop it does attack.
Cultural prevention
Cultural prevention is the most effective control, and it starts before the disease appears. Space plants generously so air moves freely between them and the leaves dry quickly after rain. Plant in a sunny, open position, not in a shaded corner, and prune for airflow. Water at soil level, in the morning, so the leaves stay dry and the soil surface dries before nightfall. Avoid overhead watering entirely in a wet season, and never water in the evening, when the leaves will stay wet all night. Mulch to prevent soil splashing onto the leaves, since the spores can be carried in splashing water.
Avoid heavy nitrogen feeding, which produces soft, lush growth that is more vulnerable, and feed steadily with a balanced fertiliser instead. Choose resistant varieties where they are available — many modern cucumber, lettuce and grape varieties are bred for downy mildew resistance, and your local extension service or seed supplier can advise on what suits your area. Rotate cucumbers, lettuce and other host crops to fresh ground each year, and do not plant them where an infected crop grew the previous season.
What to remove, and what not to compost
Remove infected leaves as soon as you see them, and put them in the bin or a sealed bag — do not compost them, because the fungus can survive in a cool home compost heap and reinfect the next crop. At the end of the season, clear away all infected debris, including fallen leaves and dead vines, and do not leave them in the bed over winter. Do not compost any of it unless your heap reliably stays hot enough to kill fungal spores — and if you are unsure, the bin is the safe choice. Check your local extension service for composting guidance in your area. Avoid handling infected plants when they are wet, since the spores spread easily on wet hands and tools — wait until the leaves are dry, and wash your hands and tools after.
Treatment, in order
1. Cultural control first. Improve airflow by thinning and pruning, water at soil level in the morning, and remove infected leaves promptly. This alone can slow a mild attack, and it is the foundation of any treatment plan. In a wet season, cultural control is often the only control that works reliably.
2. Remove and isolate. If a plant is heavily infected, remove the whole plant rather than trying to save it — it is a source of spores for the rest of the bed. Do not leave it on the ground or in the compost. Check the neighbouring plants daily and remove any leaves that show the first yellow spots.
3. Copper and other least-toxic options. Copper-based fungicides are the traditional treatment for downy mildew, and they can protect healthy leaves if applied early and thoroughly. They are protectants, not cures — they do not cure leaves that are already infected, and they need reapplying after rain. Copper accumulates in the soil, so use it sparingly and only when needed, and always read the label. Neem oil and bicarbonate sprays have limited effect on downy mildew, which is harder to control than powdery mildew. Always check your local extension service for products approved in your area, and avoid broad-spectrum fungicides as a routine.
4. Know when to stop. If the disease is widespread and the weather stays wet, the honest answer is often to remove the crop, clear the bed, and plant something else, rather than to keep spraying. Downy mildew in a wet season can outrun any treatment, and the spores will be waiting for the next planting if the debris is left behind.
Frequently asked questions
Is downy mildew the same as powdery mildew? No, though they are often confused. Powdery mildew is a white, powdery coating on the upper leaf surface that thrives in warm, dry conditions. Downy mildew is yellow spots above and grey fuzz below, and it needs wet leaves. See our powdery mildew treatment guide for the comparison.
Will downy mildew kill my plants? It can defoliate a crop in a wet season, and a badly defoliated plant will produce little or nothing. Young plants are the most vulnerable. The main loss is the harvest, and the risk is highest in wet, humid weather.
Can I eat cucumbers from a plant with downy mildew? Yes, if the fruit itself is healthy. The disease affects the leaves, not the fruit, though a badly defoliated plant may produce small or misshapen fruit. Wash the fruit well, as always.
Should I compost leaves with downy mildew? No, unless your compost heap reliably stays hot. The fungus can survive in a cool home compost heap and reinfect the next crop. Put infected leaves in the bin.
How do I prevent downy mildew next year? Space plants for airflow, water at soil level in the morning, choose resistant varieties, rotate the crop, and clear away all infected debris at the end of the season. Check your local extension service for resistant varieties suited to your area, and check the planting calendar for zone 9 in March for sowing times that may let you grow a crop before the wettest weather.
Downy mildew is a disease where the weather matters more than the cure. Give the plants sun, space and air, keep the leaves dry, choose resistant varieties, and remove the infected leaves promptly — and the yellow spots will usually stay at a level the garden can live with. For the related disease, see our powdery mildew treatment guide, and for general help use the plant disease checker.