Powdery mildew first appears on new foliage and flower buds as a white dust similar to talcum powder. Affected parts quickly yellow and become distorted. 

An infected rose leaf shows typical signs of the disease: a white powder and distortion.

Cause & Prevention: When selecting plants, look for varieties that are resistant to powdery mildew. Provide susceptible plants with morning sun so dew quickly evaporates. Rain and cool nights followed by humid days, overhead watering, fog and dry soil also encourages this prevalent disease. When watering, avoid getting foliage wet and don't water late in the day. Wet foliage and dry soil encourages this fungus so water before soil it becomes too dry. Don’t crowd plants as good air circulation deters the fungal spores from settling. 

Powdery mildew affects all plant parts including stems, flowers and leaves as noted on this speedwell.

The beginnings of powdery mildew on a catalpa leaf.

Control: Since the disease overwinters, remove infected plant parts including infected fallen leaves. Apply organic mulch and water often to reduce water stress. Infected tissues will not recover but applying a fungicide will reduce the spread of the disease 

​Apply sulfur or copper according to the manufacturers instructions, every 7 to 10 days. Effective non chemical controls include baking soda or milk. Whatever you use, apply to all areas of the plant thoroughly including the ground, but don’t apply in full sun to prevent tissue damage.

​Homemade Solutions
Baking Soda
To prevent and to control powdery mildew, spray susceptible plants weekly.
1 tablespoon baking soda
¼ teaspoon of liquid dishwashing soap
½ teaspoon of vegetable oil
1 gallon of water
Milk Spray
1 part milk to 2 to 3 parts of water
apply every 10 to 14 days

Members of the cucumber family are susceptible to powdery mildew as this zucchini plants shows.

Garden phlox are very prone to powdery mildew.

This tomato has succumbed to powdery mildew late in the season due to foggy mornings.