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. Powdery mildew can be caused by many different species of fungi, although the most common is Podosphaera xanthii. Powdery mildew is very easy to identify because its symptoms are white powdery spots on the leaves and stems of plants. Usually, you will 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 this is high humidity combined with moderate temperatures. It is not much of a problem in hot, dry climates, but in temperate regions of the Midwest, Northwest and Northeast, it is so common during the humid midsummer months that it is regarded almost as an inevitable stage in the gardening season. Many gardeners, in fact, don't both to treat powdery mildew, since it does not really harm plants all that much unless the infestation becomes very severe.
While there are chemical remedies for powdery mildew, few home gardeners need to use them--these controls are reserved for commercial farming operations. A better solution for home gardeners is simply to follow good cultural practices--or to use a homemade spray, as described below.
A Homemade Spray for Powdery Mildew
Here is a simple spray for controlling the spread of the fungus. It won't get rid of the fungus on leaves that already have it, but it will prevent it from spreading to the rest of the plant.
- 1 gallon of water
- 1 tablespoon of baking soda
- 1 tablespoon of vegetable oil
- 1 tablespoon of dishwashing liquid
Mix the ingredients together and add them to a spray bottle or pressure sprayer. Spray your plants weekly, preferably on overcast days to prevent it from burning the foliage.
Tips for Preventing Powdery Mildew
Good cultural practices go a long way toward preventing powdery mildew in the first place.
Follow these tips to minimize your problems with this fungus.
- Choose plants that are less susceptible to powdery mildew. When buying seeds, you may see packaging mention whether or not the plants are resistance to powdery mildew. Hollyhocks and phlox, for example, are two ornamental plants that may be available in cultivars that are resistant to powdery mildew.
- Space plants that are known to be susceptible to powdery mildew with good air space between them. Powdery mildew thrives in humid conditions, and the more space you have between plants, the better the air circulation and the less humidity can be trapped on 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 garden or 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. Also do not allow dead plant material to overwinter in the garden, because it will immediately reinfect young plants when they grow in the spring.