Powdery mildew is one of the most commonly occurring plant problems. It is a fungal disease that affects plant leaves and stems, coating them in what looks like a white or gray powder-like substance. In severe cases, powdery mildew can even spread to the buds, flowers, and fruits of plants. Although any plant can get powdery mildew, some are very susceptible—such as crab apples, cucumbers and all types of squash, lilacs, phlox, and roses.
The white coating greatly diminishes the appearance of the plant, but it is not fatal unless left uncontrolled. However, as it spreads, it stresses and weakens the plant and makes it hard for photosynthesis to occur. Left untreated, powdery mildew can leech nutrients from the plant, cause leaves to yellow and wither, exposing fruit to sunburn. It can even affect the flavor of fruit and reduce blooms on plants. Most importantly, powdery mildew on one plant can quickly spread to other plants, so it's important to prevent its spread.
Controlling Powdery Mildew With Baking Soda
Baking soda alone isn't effective in controlling powdery mildew, but when combined with liquid, non-detergent soap and water, it works well as a preventative. If you know which plants are susceptible, spraying them weekly with the baking soda/soap recipe (and reapplying after rain) can greatly reduce the incidence of powdery mildew in your garden. Powdery mildew typically occurs in late spring/early summer. New foliage is especially susceptible to the fungus, which, unlike other mildews, occurs in moderate temperatures with warm days/cool evenings, low light, high humidity—but dry foliage. Spacing plants for good air circulation also helps prevent the spread of powdery mildew.
To control powdery mildew on plants with a baking soda spray, mix together the following:
- 1 tablespoon of baking soda
- 1/2 teaspoon of liquid, non-detergent soap
- 1 gallon of water
Pour the mix into a sprayer, and evenly coat all areas of the plant, including the underside of leaves and stems.
Do not store the unused mixture. While this recipe has been known to be effective, it can burn the leaves of some plants. It is recommended that you water your infected plants well a couple of days before applying this mixture and don’t apply it in full sun. Try on a small area first to test the plant’s response before spraying the entire plant.
The soap helps the mix spread and cling to the leaf surface.
Control Versus Cure
Unfortunately, this baking soda mixture works best as a preventative, applied before powdery mildew has a chance to spread on your plant. It is less effective as a cure once the fungus has taken hold. If you know a plant is affected by powdery mildew year after year, as is the case with many monarda, phlox, and lilacs, then spraying early in the season may prevent any occurrence that year. In the first signs of infection on a plant, remove the leaves with powdery mildew, if there aren't too many, and spray the rest of the plant. Spray any susceptible plants located nearby, too.