Powdery Mildew is one of the most common garden diseases. The fungus usually appears as white, powdery spots on leaves or yellow irregular blotches on tomato plants. It spreads easily through the air. Leaves covered in mildew die back resulting in lower vegetable yields.
Prevention is the key to controlling this disease. Plant disease resistant varieties. Make sure your plants have adequate sunlight and airflow. Don’t water leaves late in the day.
Fungicides can be effective treatment for mildew but most need to be applied before infection occurs. The safest ones use natural types of bacteria such as bacillus amyloliquefacius or bacillus subtillus. Wettable sulfur is effective at preventing mildew but is incompatible with horticultural oils like neem oil. Once a plant is infected, horticultural oils are the most effective remedy. Do not water leaves once a fungicide is applied as this will reduce its effectiveness.
Another treatment option is to wash off infected leaves early in the morning. This will slow down the spread of the fungus.
Source: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7406.html .
