|Everything in my garden has powdery mildew on it. I've tried using a fungicide, but nothing seems to help. What can I do?
|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 aday 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 advise 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.) Some people have had good luck with pesticides containing neem, a plant extract. You can also try using a homemade baking soda spray--mix 1 teaspoon baking soda per quart of water; add 1/4 tsp mild soap to help it stick to the leaves. Apply this once a week, and hose down plants between sprayings. Be sure than any fungicide you use is labeled for powdery mildew, since some are ineffective against this disease.
Since it's late in the growing season, you should remove all of the diseased plants - don't compost; instead, bury or throw everything in the garbage. Next year, where possible grow resistant varieties, and choose plants well adapted to your climate. Following the ideas above, your mildew problem should be better under control.