Home & Garden Garden How to Identify and Prevent Powdery Mildew Plant Disease By Ramon Gonzalez Writer Columbia College Chicago Roman Gonzalez is the creator of the urban gardening blog MrBrownThumb, founder of the Chicago Seed Library, and a co-founder of One Seed Chicago. our editorial process Ramon Gonzalez Updated October 11, 2018 kazakovmaksim / Getty Images Share Twitter Pinterest Email Garden Indoor Gardening Planting Guides Urban Farms Insects Powdery mildew is a widespread and easy to recognize plant disease. There are many types of powdery mildew fungi, but they all produce similar symptoms in plants. You know you have a powdery mildew problem when you observe white spots that make your plants look like they’re covered in flour. Just how bad a case of powdery mildew you have depends on several factors: weather conditions, variety of the plant affected, age and general health of the plant. The young, fresh growth of a plant is usually more susceptible than older plant tissue. This is why you notice it on buds and young leaves soon after they unfurl. Powdery mildew is more than just aesthetically unpleasant. It can lead to a loss of your fruit or vegetable crops and even kill entire plants. Conditions Favorable for Powdery Mildew Crowded plants in dry, warm climates with poor air circulation and damp, shaded areas are most often affected. When the relative humidity rises to 90 percent the conditions become ideal for spore germination. Controlling Powdery Mildew Avoid watering overhead to reduce the relative humidity around your plants. If the signs of a powdery infection are present: remove and destroy the infected leaves and plant parts. It is important that you do not compost the infected plant material. Selective pruning of overcrowded and overgrown plant material may help lower relative humidity and increase air circulation. Homemade Powdery Mildew Spray There are several recipes for a homemade powdery mildew spray floating around the Internet. The one I’ve used in the past calls for 1 tablespoon of baking soda, 1 tablespoon of liquid soap, and 1 tablespoon of vegetable oil mixed in 1 gallon of water. After mixing the concoction, apply a spot test to one leaf or stem to test the plant’s response. If you don’t notice any adverse affects to your plant you can apply the spray to the entire plant. It may be a good idea to apply the homemade powdery mildew spray after you have watered, and early in the morning before the sun comes up. Ideally, you should apply the spray on the infected plant on an overcast day to avoid burning your plant. Apply this spray once a week to prevent powdery mildew spreading further.