Powdery mildew is a fungal disease that causes a white or grayish film to form on the surface of rosebush leaves. The disease is also common on berries, vegetables and grape vines. It has also been reported on lawns. Powdery mildew thrives in damp, shady areas, but can become especially active when the weather is hot and evening humidity is high. Ideal temperatures for powdery mildew are 68 to 77 degrees with 40% to 100% humidity. Often 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 purchasing. The disease can spread within 72 hours, and it can complete a full development cycle (from exposure to producing new spores) in 7 to 10 days. Any rose can contract powdery mildew, but those that are particularly susceptible include Chinas (although these are rare in colder areas), deep pink and red hybrid tea roses, polyanthas, and rosa wichuriana.
You can prevent powdery mildew by ensuring adequate space between your bushes and keeping them evenly moist. Mulch around the base of roses to keep the soil damp and cool. Make sure your roses get at least four hours a day of full sun. If you have been using inorganic fertilizers and find you have problems with powdery mildew, try switching to organic fertilizer — the high nitrogen content of inorganic fertilizers can actually induce mildew.
If you catch it just as the disease is starting, simply remove the infected shoots and leaves — powdery mildew tends to establish itself on new growth first. 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 spraying your bushes thoroughly. Garlic is also an old cure — take 3-4 cloves and crush them, then mix them into about 16 ounces of water and spray on your roses.
Alternately, you may try mixing two tablespoons of Lysol bleach per gallon of water — if using this mixture, spray only in the middle of the day. If none of these treatments is effective, you can up the ante by adding 2 tablespoons of wettable sulphur or horticultural oil per gallon. You will have to use the sulphur treatment for 14 days to be sure you have knocked out the mildew in all its growth stages. Follow the directions on the bottle of whatever treatment you use carefully. Never use sulphur-based products when the outdoor temperature is above 85 degrees Fahrenheit. If this still does not get rid of the problem, consider removing the plants that have been affected the worst, and possibly even remove some of the top soil around them. Powdery mildew can survive through the winter.
Of course, wash any tools you use around plants with powdery mildew to make sure you do not infect the next plant you use the tool on. The classic remedy is making a small amount of 1 part bleach to 9 parts water, soaking a rag in the solution, and then wiping down any tools you’ve used near the infected plants.