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At this time of year, you may be seeing leaf spots, powdery mildew, and other diseases on your plants.
Let's start with a little plant pathology 101. First, what causes disease?
There are three things: 1) the organism or the spore; 2) a plant that is susceptible to them; 3) the right environment for the disease to develop.
These three are called the disease triangle. If you knock out any one of those three corners you avoid the disease problem.
For example, if you select a crape myrtle that is resistant to powdery mildew, you've removed that corner of the triangle.
You can't control the weather, but the way you water affects the environment. For example, overhead watering or sprinkling that wets the foliage tends to promote disease. You can minimize these problems by going to drip irrigation instead. If you use a sprinkler, do it early in the morning so the leaves can dry quickly.
Where you grow the plant can make a difference. If the plant wants sun and you give it too much shade, it tends to have more problems. This is often the case with roses and the rock rose (Pavonia).
Our last resort is to deal with the disease agent itself. We can spray to kill the disease but that shouldn't be first choice. First: select the right plant. Second: Select the right spot for the plant and water it properly. If you must spray, examine choices for the least harmful. Be cautious about spraying any product when the temperature is over 85°.