The first symptoms of the disease are small, disperse, white colonies first appearing on the older leaves. Often the initial colonies first appear on the underside of the leaves. As the disease progresses, younger leaves become infected, and the plant takes on a dusty white appearance. The disease can cover entire leaves, and severely infected leaves may become yellowed, then purplish-brown. Specific moisture and temperature conditions are important in the development of powdery mildew.
Free moisture inhibits spore germination and colony development.
For these reasons powdery mildew is well adapted to environmental conditions of semi-arid regions with warm, dry climates and large diurnal fluctuations of temperature and relative humidity. Because the microclimate under sprinkler irrigation will be more moist, disease development is usually slower than under surface irrigation. Because powdery mildew has the ability to increase rapidly, the concept of reaching a disease threshold before treating is of little practical value. Early detection is essential for good control, and it is far more effective to treat at first appearance of the disease, or preferably just prior to its appearance, than to wait until the disease is widespread. When the disease is treated is far more important than which fungicide is used.