|There seems to be a white powder on my zinnias. They are planted in full sun but it appears that I have a mold or fungus growing on my plants. Now it is spreading to my cosmos.
|It sounds like powdery mildew. 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.
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 (but not so strong as to damage the
plant.) After the new growth of affected plants reaches about six inches long you may apply dusting sulfur to foliage every 14 days. If the temperatures reach 90F, cease using sulfur, because it can kill plant tissue at high temperatures. Fungicides work much better as a preventative measure, before the powdery mildew becomes a problem.