The Q&A Archives: Powdery Mildew On Tomato Leaves

Question: I grow tomatoes in containers. All the plants are growing fine and have green tomatoes, some of which are starting to ripen. Last week I had to go out of town for a few days, so I watered them the night before I left. When I came back three days later, some of my tomato plants had white powdery mildew on the leaves. The night temperature in San Diego is probably in the mid 60s. I get about six hours of sun everyday and since I am close to the ocean I get cool winds during the day. Why am I getting this mildew? How can I correct the problem?

Answer: Powdery mildew can be a terrible problem once it gets a foot-hold in the garden! And because the disease overwinters on living plants, it is virtually impossible to eradicate. There are some things you can do to minimize its
effects. Powdery mildew is unique among plant diseases in that it doesn't require a wet leaf surface to spread. It can thus thrive during hot, dry weather. The first line of defense is to grow resistant varieties. Next, remember that while the disease is unsightly, it doesn't cause any real harm to many of its victims (though it will damage some plants.)

Here are some general rules for helping control fungal diseases. Start by making sure that your plants are getting
enough direct sunlight. (Eight to ten hours a day is generally the minimum for plants that flower or bear fruit.)
You'll also want to make sure that there's enough room between plants for air to circulate freely. Overcrowding not
only makes plants more susceptible to diseases, if leaves touch other plants, those diseases can easily be spread. The general advice to inhibit the spread of fungal diseases is to avoid wetting leaf surfaces. In the case of powdery mildew, you can actually inhibit infection with periodic strong sprays of water (not so strong as to damage the plant.) After the new growth of affected plants reaches about six inches long and UNTIL THE TEMPERATURE
REACHES 90 degrees F, you may apply dusting sulfur to foliage every 14 days. If necessary apply fungicide. Fungicide works much better as a preventative measure, before the powdery mildew becomes a problem.

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