Downy Mildew: How to Identify and Control It on Vegetables, Basil and Flowers

By the Garden Voila editorial team · Updated October 30, 2026 · How we verify

Downy mildew is a group of plant diseases caused by water molds (oomycetes), not true fungi, that thrive in cool, wet or humid weather. Typical symptoms are pale green to yellow patches on the upper surface of leaves, often angular and bounded by leaf veins, with gray, purple, brown or white fuzzy growth on the undersides directly below. Leaves turn brown and die, sometimes very quickly. Common garden victims include cucumbers, melons, squash, basil, lettuce, spinach, brassicas, onions, grapes, roses and impatiens. To manage downy mildew, plant resistant varieties, space plants for air flow, water at the soil in the morning, remove infected plants or leaves promptly, rotate crops and, for high-value crops, use a protective fungicide labeled for downy mildew before or at the first signs.

Downy mildew at a glance

What is downy mildew?

The Wikipedia article on downy mildew explains that these pathogens are oomycetes, more closely related to algae than to fungi, which is why some fungicides that control powdery mildew have little effect on them. They produce spores on leaf undersides that spread by wind and water, and they need free moisture on leaves to infect. Most downy mildews are highly host-specific: the one on your cucumbers will not infect your basil or roses, and vice versa. Some, such as cucurbit downy mildew, blow north each summer from warmer regions, so outbreaks can appear suddenly.

How do you identify downy mildew?

• Yellow or pale patches on upper leaf surfaces, often angular because they are limited by leaf veins (especially on cucumbers and melons).
• Fuzzy growth underneath: gray, purple-black or brown on cucurbits, basil and lettuce; white on brassicas and impatiens. Check leaf undersides in the early morning, when sporulation is heaviest.
• Browning and curling of affected leaves; rapid leaf death in humid weather.
• Basil leaves turning yellow from the center outward with dark fuzz below, then black.
• Grape leaves with yellow “oil spots” on top and white cottony growth below; infected berries shrivel.
• Impatiens with pale, curled leaves, white growth underneath, then rapid leaf and flower drop.

Downy mildew vs powdery mildew

See our powdery mildew guide for that disease and the plant problem identifier if you are unsure.

Downy mildew on cucumbers, melons and squash

Cucurbit downy mildew, caused by Pseudoperonospora cubensis, is one of the most damaging diseases of cucumbers and cantaloupes in humid regions. It does not survive cold winters, so it moves north each summer on storm systems; regional disease forecasting services track its spread. Symptoms are angular yellow patches on leaves that turn brown, with dark fuzz underneath, and plants can be defoliated in a week or two. Plant resistant cucumber varieties labeled “DM” or “downy mildew resistant,” plant early so most of the crop matures before the disease arrives, and start protective sprays when it is reported in your area. Squash and pumpkins are usually less affected than cucumbers and melons. For other cucurbit problems, see cucumber beetles.

Basil downy mildew

Basil downy mildew (Peronospora belbahrii) first appeared in the US around 2007 and is now widespread. Leaves yellow, often from the middle of the leaf outward, with gray-purple fuzz beneath, then turn black. It can ruin a basil crop in a week of humid weather. The University of Minnesota Extension guide to basil downy mildew recommends resistant varieties, good air flow and harvesting early when the disease appears. Resistant sweet basils such as Prospera, Rutgers Devotion, Rutgers Obsession and Amazel are now widely available; Thai, lemon and cinnamon basils are also less susceptible. Pinch and use basil often, keep leaves dry and remove any infected plants immediately.

Downy mildew on lettuce, spinach, brassicas and onions

Lettuce downy mildew causes pale, angular spots with white fuzz underneath on outer leaves, especially in cool, damp spring and fall weather. Spinach downy mildew (blue mold) causes yellow patches with purple-gray growth beneath. Brassica downy mildew affects seedlings and older leaves of broccoli, cabbage and kale. Onion downy mildew causes pale, elongated patches on leaves that collapse. For all of these, choose resistant varieties where available, space plants, avoid overhead watering in the evening, rotate crops and remove infected plants.

Downy mildew on grapes, roses and impatiens

Grape downy mildew can destroy leaves and fruit in wet climates; prune for air flow, remove infected leaves and follow a protective spray program for susceptible varieties. Rose downy mildew causes purplish-red to brown blotches and sudden leaf drop in cool, humid weather, especially in greenhouses and coastal gardens. Impatiens downy mildew devastated garden impatiens (Impatiens walleriana) beginning around 2011; plant resistant series such as Beacon and Imara, or substitute New Guinea impatiens, begonias or coleus.

How do you treat downy mildew?

1. Confirm the diagnosis by checking leaf undersides for fuzzy growth in the morning.
2. Remove infected leaves or plants promptly and bag them; do not compost them.
3. Improve air flow by thinning and spacing plants; use the plant spacing calculator.
4. Keep leaves dry: water at the base in the morning with drip or soaker hoses.
5. Harvest what you can from basil, lettuce and cucumbers before the disease spreads.
6. Use protective fungicides labeled for downy mildew on your crop, such as copper or specific downy mildew products, applied before or at the first symptoms and repeated as the label directs. Many powdery mildew products do not work on downy mildew. See copper fungicide.

How do you prevent downy mildew?

• Grow resistant varieties of cucumbers, melons, basil, lettuce, spinach and impatiens.
• Space and prune for good air circulation.
• Avoid evening watering and overhead sprinklers; water in the morning so foliage dries quickly. Use the garden watering calculator to water efficiently.
• Rotate crops and clean up debris at the end of the season.
• Buy healthy transplants, especially basil and impatiens, and inspect leaf undersides before buying.
• Time plantings so crops mature before peak downy mildew season in your area.
• Follow disease alerts from your Extension service for cucurbit and basil downy mildew.

Still not sure what you are looking at? Compare symptoms in the plant pests and diseases library.

Downy mildew FAQ

What weather triggers downy mildew?

Most downy mildews need leaves to stay wet for several hours to infect, and they produce spores best when nights are humid and temperatures are cool to moderate, roughly 50 to 75°F depending on the species. Rainy spells, foggy mornings, heavy dew and overhead watering late in the day all create those conditions. Cucurbit downy mildew is an exception in that it also thrives in warm, humid summers once it arrives. Long dry periods and hot sun slow or stop most downy mildews. Watch the forecast: after several days of rain or fog, check susceptible plants closely, especially the undersides of leaves.

How does basil downy mildew spread so fast?

A single infected basil leaf can produce thousands of spores overnight in humid conditions, and these blow to nearby plants or are splashed by water. The pathogen can also arrive on infected seed and transplants, which is why it appeared across North America within a few years of its first detection. The UMass Extension fact sheet on basil downy mildew notes that the disease can spread through a planting very quickly and recommends resistant varieties and close monitoring. Growing basil in containers in a sunny, breezy spot, spacing plants widely and harvesting frequently all reduce risk.

Can you grow basil indoors to avoid downy mildew?

Indoor basil on a sunny windowsill or under grow lights is less exposed to airborne spores and rain, but it can still get downy mildew if you bring in infected plants or keep the air humid and still. Start with resistant varieties or seed from reputable suppliers, give plants strong light and air movement, water at the soil, and avoid misting the leaves.

Should you remove the whole plant?

For basil, impatiens and lettuce, yes: once downy mildew is established, the whole plant usually declines, and it becomes a source of spores for others. Pull and bag infected plants promptly. For cucumbers, melons, grapes and roses, removing infected leaves and protecting new growth can keep plants productive for a while, especially if the weather turns dry. Do not compost infected material.

Which crops are most at risk from downy mildew?

Knowing which crops in your garden are most vulnerable lets you focus prevention where it pays off most.

Can downy mildew be prevented with organic methods?

Yes, largely. Resistant varieties are the most powerful organic tool, followed by spacing, morning watering at the base, crop rotation, prompt removal of infected plants and timing plantings to avoid the wettest periods. Copper products approved for organic production can protect susceptible crops such as grapes and cucumbers when used preventively, though they must be applied before infection and repeated after rain. Biological fungicides based on Bacillus bacteria offer some protection on a few crops. Combining several of these measures works far better than relying on any single one.

Above all, watch the weather and the undersides of your leaves: catching downy mildew in its first day or two makes all the difference.

Keep a simple log of which plants showed downy mildew and when; over a few seasons it will show you the riskiest weeks in your garden, so you can plan resistant varieties and protective steps just before the disease usually appears.

Sources: University of Minnesota Extension, Basil Downy Mildew; UMass Extension, Basil Downy Mildew; Wikipedia, Downy mildew.

Downy mildew facts that change how you manage it

University of Minnesota Extension’s page on downy mildew of cucumber, melon and squash shows why this disease behaves differently from the powdery kind. Spots are yellow to pale green on the upper leaf surface, turning brown, and are bounded by the veins, giving an angular look that is often mistaken for angular leaf spot. In humid weather a dark purplish-grey fuzz appears on the underside. The pathogen infects at 41 to 86°F (5 to 30°C) with the most severe damage at 59 to 68°F (15 to 20°C), and it needs moisture on the leaf surface to germinate.

• Spread: spores arrive on air currents from the south, and in many years never reach cold northern states. The pathogen survives only on living tissue, so it cannot overwinter in Minnesota; early outbreaks may come from greenhouse-grown cucurbits. In eastern states it typically arrives late, often in August.
• Two races: one infects cucumbers and melons, the other pumpkins and squash, so a healthy pumpkin patch can sit beside a badly infected cucumber bed.
• Home garden management: drip irrigation, wide row spacing and trellising to dry the leaves, removal of symptomatic plants, and resistant varieties where the disease has occurred before. The university states that no effective pesticides are available to home gardeners.
• Resistance: cucumber varieties with moderate to high resistance exist; melon options are moderate to low; some pumpkin and squash resistance is specific to certain pathogen variants and may not work everywhere.

Use our plant spacing calculator to give cucurbits the airflow that wet-leaf diseases hate.

Reviewed October 2026. Sources: University of Minnesota Extension, Downy mildew of cucumber, melon and squash