Spore stalks of powdery mildew, Uncinula necator.
Several least-toxic fungicides are available, including horticultural oils, neem oil, jojoba oil, sulfur, potassium bicarbonate, and the biological fungicide Serenade. With the exception of the oils, these materials are primarily preventive, although potassium bicarbonate has some eradicant activity. Oils work best as eradicants but also have some protectant activity.
To eradicate mild to heavy powdery mildew infections, use a horticultural oil such as JMS Stylet Oil, Saf-T-Side Spray Oil, Sunspray Ultra-Fine Spray Oil, or one of the plant-based oils such as neem oil (e.g., Powdery Mildew Killer) or jojoba oil (e.g., E-rase). Be careful, however, never to apply an oil spray within 2 weeks of a sulfur spray, or it may injure plants. Also, you never should apply oils when temperatures are above 90°F or to water-stressed plants. Some plants may be more sensitive than others, and the interval required between sulfur and oil sprays may need to be even longer. Always consult the fungicide label for any special precautions. Of the horticultural oils, JMS Stylet Oil is the most highly refined and therefore the least likely to damage plants, but it may be more difficult to obtain than the others.
Sulfur products have been used to manage powdery mildew for centuries but are effective only when applied before the disease appears. The best sulfur products to use for powdery mildew control in gardens are wettable sulfurs that are specially formulated with surfactants similar to those in dishwashing detergent (e.g., Safer Garden Fungicide). However, you shouldn’t use dishwashing detergent with sulfur. Additionally, sulfur can damage some ornamental cultivars. To avoid injuring any plant, do not apply sulfur when the temperature is near or higher than 90°F, and do not apply it within 2 weeks of an oil spray. Other sulfur products, such as liquid lime sulfur or sulfur dust, are much more difficult to use, irritate skin and eyes, and are limited in the types plants you safely can use them on.
Also available to licensed applicators only is a fungicide containing potassium bicarbonate (e.g., Kaligreen). Sprays of potassium bicarbonate can injure the plant, so use these products with caution.
Biological fungicides (e.g., Serenade) are commercially available beneficial microorganisms formulated into a product that, when sprayed on the plant, destroys fungal pathogens. The active ingredient in Serenade is a bacterium, Bacillus subtilis, that helps prevent the powdery mildew from infecting the plant. These products have some effect in killing the powdery mildew organism but are not as effective as the oils or sulfur in controlling it.
Myclobutanil (Immunox) also is available to the home gardener and functions as an eradicant and protectant against both powdery mildew and rust.
Apply protectant fungicides to susceptible plants before or in the earliest stages of disease development. Once mildew growth is mild to moderate, it generally is too late for effective control with protectant fungicides. These are effective only on contact, so applications must thoroughly cover all susceptible plant parts. As plants grow and produce new tissue, additional applications may be necessary at 7- to 10-day intervals as long as conditions favor disease growth.
If mild to moderate powdery mildew is present, you can use horticultural and plant-based oils such as neem or jojoba oil.