Powdery mildew is a fungal disease that afflicts a wide range of crops including melons, cucumbers, squash, peas, tomatoes and even wheat. Powdery mildew appears as white spots on leaves and stems and spreads out as the disease progresses. The disease is particularly prevalent in locations where there is high humidity and moderate temperatures.
Here in the Bay Area of Northern California, we certainly have moderate temperatures, but the area is not particularly high in humidity. However, the cool nights we experience in this region creates conditions which lead to persistent powdery mildew problems. Specifically, we get night time temperatures that range between 50-60 degrees Fahrenheit. This is cool enough for moisture to condense as dew on the plant leaves. I live near a creek, so the moisture level of the air around my garden is particularly conducive to the creation of dew overnight.
Dealing with Powdery Mildew
Powdery mildew is spread when spores from the fungus are transferred from one plant to another by the wind, insects and yes, accidentally by gardeners themselves. Crop rotation and selection of powdery mildew resistant plant varieties are often recommended as the first step in fighting powdery mildew.
Plant selection is a reasonable approach to the problem, but it probably means that you will be growing hybrid varieties that have been specifically bred to be resistant to powdery mildew. If you have your heart set on growing heirlooms, then you'll need to adopt a more aggressive approach to dealing with powdery mildew. Likewise, crop rotation may work as a strategy for commercial farmers with multiple fields or large planting areas, but for home gardeners the distances between adjacent growing areas probably isn't far enough to be meaningful, so more aggressive strategies are required.
Treatments for Powdery Mildew
There are a number of treatments that can be used for powdery mildew. Almost all require re-application every 7-10 days.
- Sulfur dust - This is a powdered form of sulfur that you shake onto the leaves of infected plants. The sulfur changes the pH on the leaves, making them more acidic and this environment is inhospitable to the fungus. Sulfur is an approved for organic gardening. Sulfur is best used as a preventative treatment or at the first onset of the disease. One drawback of sulfur dust is that it can't be applied to the underside of leaves which are just as susceptible to powdery mildew as the tops. If you can find a sulfur treatment in spray form, this would be preferable because you can get at the underside of foliage.
- Copper spray - generally available in concentrated form, this is a blue-ish/green liquid that gets mixed with water and then applied with a sprayer. The copper is toxic to fungi because it interferes with proteins in plant tissues. However, this toxicity also applies to the plant you are trying to treat so there is some risk that you can also do damage to foliage and fruit. That said, copper is approved for organic use. As with sulfur, copper is best used as a preventative measure.
- Neem Oil - This is a vegetable oil extracted from a type of fast growing evergreen tree. The need is added to water and then sprayed on. Neem oil works by smothering the fungus and denying it the moisture it needs to survive. Neem oil is an organic treatment for fungus.
- Sereneade - This is a bio-fungicide made using a special strain of Bacillus subtillis. This bacteria attacks powdery mildew, puncturing the fungus cells so they dry out and making them more susceptible to other bacteria. Serenade can be used as a preventative and as a treatment. It can be purchased in a spray bottle or in concentrate. Serenade is approved for organic gardening. For more information on Serenade, check out the below video.
- Fungicides - There are many on the market but the most common ones contain a chemical called Chlorothalonil. This chemical interferes with the fungi's metabolic processes and causes it to die. Heavy duty fungicides like Chlorophalonil are very effective but are NOT approved for organic farming. Note: one drawback to chemical fungicides is that fungi can develop resistence to them over time. For this reason, I prefer to use other control mechanisms before moving to this type of product.
Recommendations:
In areas where powdery mildew is a persistent I'd recommend getting the jump on powdery mildew by using a preventative treatment before powdery mildew sets in. My plan of attack in 2012 is to use Serenade as a preventative treatment every 7 days beginning from the time that the plants are getting their first true leave. If I see any evidence of powdery mildew, I'll move to a weekly treatment regimen of chemically based sprayings, alternating between applications of copper and neem oil. If the infection persists, I'll hit the plants with Chlorothalonil as a treatment of last resorts.
If you have persistent problems with powdery mildew, let everynone know what you've found effective by posting a comment to this posting.