Identifying the Symptoms
Penn State Extension describes powdery mildew as a white, powdery fungus growing on leaf surfaces, often on the upper surface of the lower leaves first. Affected leaves can twist, distort, wilt and eventually die.
It starts as small patches that can merge into a continuous coating. Unlike dust, it smears rather than wiping away cleanly, and it tends to return. Check both sides of the leaves, and the stems and buds too.
What Favors Powdery Mildew
Penn State Extension lists the conditions that favor powdery mildew as high relative humidity at night, lower humidity during the day, and moderate temperatures of about 70 to 80°F (22 to 27°C). That combination is common in spring and fall. Poor air movement and crowded plants add to the problem by trapping humid air around the leaves.
Stagnant Air
Still air gives spores a calm place to settle. Improving circulation around plants, for example with a small fan on a low setting, helps reduce humidity at leaf level.
Crowded Plant Spacing
Leaves touching leaves hold humid air and let the disease pass from plant to plant. Giving plants room helps.
Powdery mildew does not need wet leaves, so avoiding overhead watering will not prevent it by itself. Look at humidity swings and airflow instead.
Treatment: Step by Step
Act as soon as you confirm the problem, because mildew can spread to neighboring leaves and plants.
- Isolate the plant. Move it away from other plants to limit the spread of spores.
- Remove badly affected leaves. Cut them off with clean shears and discard them in a sealed bag. Do not compost them.
- Improve airflow and spacing. Give the plant room and keep air moving gently across the leaves, without blasting them directly.
- Consider a fungicide only if needed. Penn State Extension recommends a fungicide, biological control or anti-transpirant to protect plants, and stresses checking that the product is labeled for your plant. Read and follow the label for rates and frequency.
- Keep checking. Mildew can return from surviving spores, so check the plant regularly for several weeks and repeat what works.
Preventing Recurrence
Environmental Adjustments
Avoid long periods of very humid, still air. In an enclosed space such as a grow tent or a bathroom shelf, add ventilation or move the plant somewhere with better airflow. Space plants so leaves are not pressed together.
Inspection Routine
Check susceptible plants every week, including the lower leaves. Removing a single affected leaf early is far easier than treating the whole plant. Wipe down shelves and trays now and then, since spores can linger on surfaces.
When to Remove the Plant Entirely
If a plant is badly affected and keeps getting worse despite several rounds of treatment and better airflow, consider replacing it. Removing it also protects the plants around it from ongoing exposure.
Read more on diagnosing and treating fungal leaf problems.
