Downy Mildew on Plants: Signs, Causes & Treatment
Learn how to recognize downy mildew, understand the cool, moist conditions that encourage it, and use practical steps to manage and prevent the disease.
Infected leaves may develop pale green, yellow or brown areas on top, while fuzzy growth can appear on the underside when humidity is high. The exact symptoms depend on the host and pathogen.
What Is Downy Mildew?
Downy mildew is a group of diseases caused by oomycetes, sometimes called water molds. Despite the name, downy mildew is not the same disease as powdery mildew.
Different downy mildew pathogens are associated with different host plants. Commonly affected crops and ornamentals can include cucurbits, basil, roses, impatiens and other susceptible plants.
Signs of Downy Mildew
Look for a combination of leaf discoloration and conditions that favor moisture:
What Conditions Encourage Downy Mildew?
- High humidity
- Extended leaf wetness or free moisture
- Cool to moderate temperatures for many downy mildew diseases
- Poor air movement around dense foliage
- Crowded plantings that dry slowly
- Susceptible host plants and favorable disease conditions
The exact temperature and moisture requirements vary among downy mildew pathogens, so plant-specific guidance is important.
How to Identify Downy Mildew
Check both sides of the leaf. Upper-surface yellow or pale lesions combined with fuzzy growth on the underside during humid conditions can be a useful clue.
| What you see | What it may suggest |
|---|---|
| Yellow or pale angular patches with fuzzy growth underneath | Downy mildew is a possibility, especially in humid conditions. |
| White powdery coating mainly on leaf surfaces | More consistent with powdery mildew. |
| Orange or rust-colored pustules | More consistent with plant rust. |
| Brown circular or irregular spots without underside growth | Could be another leaf-spot disease or environmental damage. |
| Soft, wet roots with wilting | Consider a root-related problem rather than downy mildew. |
How to Manage Downy Mildew
1. Remove severely affected tissue
Where practical, remove badly infected leaves or plant parts. Avoid spreading infected material around healthy plants.
2. Improve airflow
Space plants appropriately and reduce dense growth where possible so foliage dries more quickly.
3. Reduce prolonged leaf wetness
Avoid unnecessary overhead watering and water the growing medium when appropriate. Give foliage time to dry.
4. Manage humidity
For indoor or protected growing areas, improve ventilation and avoid persistently humid conditions around leaves.
5. Use disease-specific products carefully
Some downy mildew diseases can be managed with fungicides or other labeled disease-control products, especially when used preventively. Product choice depends on the host and pathogen. Always follow the product label.
How to Prevent Downy Mildew
- Start with healthy plants and clean planting material.
- Give plants adequate spacing for air circulation.
- Water in a way that minimizes unnecessary leaf wetness.
- Inspect both upper and lower leaf surfaces regularly.
- Remove severely infected plant material where appropriate.
- Clean up infected debris according to plant-specific recommendations.
- Choose resistant varieties when reliable resistance is available.
What Plants Can Get Downy Mildew?
Downy mildew affects many plant groups, but the disease is usually host-specific. Different pathogens can affect crops and ornamentals such as cucumbers, melons, grapes, basil, roses and other susceptible plants.
Not Sure What’s Affecting Your Plant?
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Diagnose My PlantFrequently Asked Questions
What does downy mildew look like on plants?
It often causes pale green, yellow or brown areas on the upper leaf surface. Under humid conditions, fuzzy growth may appear on the underside of affected leaves.
What causes downy mildew?
Downy mildew is caused by oomycetes and is favored by moisture, high humidity and suitable temperatures. Different pathogens affect different host plants.
How do I manage downy mildew?
Remove badly affected tissue where practical, improve airflow, reduce prolonged leaf wetness and avoid unnecessary overhead watering. Use only appropriately labeled products when chemical control is suitable.
Is downy mildew the same as powdery mildew?
No. They are different diseases. Powdery mildew typically produces a powdery coating on leaf surfaces, while downy mildew commonly causes upper-surface discoloration with growth developing mainly on the underside in humid conditions.