Powdery mildew is a fungal disease that causes a white or grayish film to form on the surface of rose leaves. The disease is also common on berries, vegetables, and vines. It has also been reported on lawns. Powdery mildew thrives in moist, shady areas, but it can become especially active when it’s hot and the humidity is high at night. Ideal temperatures for powdery mildew are 68 to 77 degrees with 40% to 100% humidity. Often the leaves also curl up and turn purple.
Powdery mildew is common in cold climates, as well as among store-bought potted plants, so check yours carefully before you buy. The disease can spread in 72 hours and can complete a full development cycle (from exposure to new spore production) in 7 to 10 days. Any rose can get powdery mildew, but those that are particularly susceptible include Chinese (although rare in colder areas), deep pink and red hybrid tea roses, polyanthas, and wichuriana rose.
You can prevent powdery mildew by ensuring adequate spacing between your shrubs and keeping them evenly moist. Mulch the base of the roses to keep the soil moist and cool. Make sure your roses get at least four hours a day of full sun. If you’ve been using inorganic fertilizers and find you’re having problems with powdery mildew, try switching to organic fertilizers – the high nitrogen content of inorganic fertilizers can induce downy mildew.
If you catch it just as the disease is starting, simply remove infected shoots and leaves; powdery mildew tends to establish itself first on new growth. If you have a more advanced case, you can control it by making a solution of three tablespoons of baking soda to one gallon of water and misting the bushes well. Garlic is also an old cure: take 3 or 4 cloves and crush them, then mix them in about 16 ounces of water and spray your roses.
Alternatively, you can try mixing two tablespoons of Lysol bleach per gallon of water; if you use this mixture, spray only in the middle of the day. If neither of these treatments is effective, you can up the ante by adding 2 tablespoons of wettable sulfur or horticultural oil per gallon. You will need to use the sulfur treatment for 14 days to ensure that you have killed all stages of mold growth. Follow the directions on the bottle of any treatment you use carefully. Never use sulfur-based products when the outside temperature is above 85 degrees Fahrenheit. If this still doesn’t solve the problem, consider removing the plants that have been most affected, possibly even removing some of the topsoil around them. Powdery mildew can survive through the winter.
Of course, wash any tools you use around powdery mildewed plants to ensure you don’t infect the next plant you use the tool on. The classic remedy is to make a small amount of 1 part bleach to 9 parts water, soak a rag in the solution, and then wipe down any tools you used near infected plants.