If you are seeing powdery-looking patches on the foliage of your plants, you most likely have a case of the very common powdery mildew fungal disease. While there are chemical remedies for powdery mildew, few home gardeners need to use them; these controls are usually reserved for commercial farming operations. A better solution for home gardeners is simply to follow good cultural practices and, if desired, to apply a simple homemade powdery mildew spray.
What Is Powdery Mildew?
Powdery mildew can be caused by many different species of fungi, although the most common in cucurbits is Podosphaera xanthii. Powdery mildew is very easy to identify because its symptoms are white or gray powdery spots on the leaves, stems, flowers, and even fruit of plants. Typically you first see the powdery residue on the lower leaves and stems, but as the disease progresses, the powdery coating may blanket the entire plant.
Powdery mildew grows best in conditions where you'd least expect it--instead of a cool, moist environment, powdery mildew prefers warm and dry conditions where there is high humidity combined with moderate temperatures. It often appears in late spring and early summer. While a mild case of powdery mildew typically isn't fatal to the plant, a severe infection can leech nutrients from the plant, causing leaves to wither and yellow. Loss of leaves in plants like tomatoes and peppers, for instance, can lead to sunscald of the fruit.
Spores of powdery mildew spread by wind and can survive the winter in plant debris or compost piles. Make sure to dispose of infected plants and leaves to help prevent future outbreaks.
Homemade Spray for Powdery Mildew
You can apply a homemade spray to control the spread of the powdery mildew fungus. It won't get rid of the fungus on leaves that already have it, but it can help prevent it from spreading to the rest of the plant.
Mix the ingredients thoroughly in a gallon-size container or an empty milk jug, then pour some of the mixture into a spray bottle. You can also mix and spray directly from a pressure sprayer. Spray your plants weekly, preferably on overcast days to prevent it from burning the foliage. Coat all sides of the leaves and stems, and let them dry. Save leftover spray mixture in your container, giving it a quick shake before using it again. Reapply after rain.
Tips for Preventing Powdery Mildew
Good practices go a long way toward preventing or minimizing powdery mildew formation in the first place.
- Choose plants that are less susceptible to powdery mildew. Look for seeds or plants that are advertised as being powdery mildew resistant. Hollyhocks and phlox, for example, are two ornamental plants that may be available in cultivars that are resistant to powdery mildew.
- Plant varieties that are known to be susceptible to powdery mildew with ample space between them for good airflow. The more space you have between plants, the better the air circulation and the less likely powdery mildew will spread to other plants. Similarly, dense bushy plants such as lilac (known to be susceptible to powdery mildew) can be thinned out to improve air circulation and minimize powdery mildew problems.
- Dispose of diseased leaves and plant stems by putting them in the trash or leaving them for the municipal compost pickup. Do not compost this material in your own compost pile, because the fungi spores can live on for quite a long time. Do not allow dead plant material to overwinter in the garden, because it will immediately reinfect young plants when they grow in the spring.