Treatments for Mildew on Roses

Downey Mildew

• Downey mildew is the more serious of the two types of fungus. When the humidity goes over 85 percent and the temperature dips to below 75 degrees F, conditions are right for downey mildew. Dark burgundy splotches on the leaves appear first. These splotches are streaky and irregular in shape. There may also be a light fuzzy coating on the underside of the leaves. As the fungus progresses, the burgundy splotches develop brown dead tissue at the center, and the leaves will turn yellow, wilt and fall off the bush. Fresh new shoots will wither and die. Burgundy splotches on the canes indicate a severe infection. If left untreated, the entire rose bush will die.

Downey Mildew Treatment

• The first course of action in the treatment of downey mildew is the removal of the affected parts of the plant. Prune off all leaves and stems that show any sign of the fungus. Remove them completely from the area by placing them in a garbage bin to be hauled away from the property. Never place diseased leaves, branches or mulch in a compost pile. Fungal spores will survive even through winter, and can reinfect in as little as three days when the climate is right. Remove mulch and debris from around the bush. Early in the morning before the sun is fully up or while the bush is shaded, water it generously. Spray the entire bush with a hose until it's completely drenched. Thoroughly mix 2 tbsp. of fine horticultural oil, 1 tbsp. of mild dish soap and 1 heaping tbsp. of baking soda with 1 gallon of water -- and spray the entire bush. Drench the entire bush, including the undersides of the leaves and the ground around it. Leave the soil unmulched for a few days to dry, then place fresh new mulch around the bush. Spray the bush with the solution every two weeks. Always spray early in the morning so that the bush is dry before sunset. Similar premixed solutions are available commercially.

Powdery Mildew

• Powdery mildew flourishes in dry climates when the days are warm and the nights are cool and damp. White powdery film forms on the rose bush leaves, stems and flowers. Young tender growth will twist and distort. The buds and tips of new growth may be killed. Watch closely for these symptoms, especially in the spring and fall.

Powdery Mildew Treatment

• When powdery mildew is detected early on, it can be washed away using a garden hose and spray nozzle. This must be done early in the morning, however, so that the bush is dry before sunset. Hose the bush off at least twice a week and always early in the morning. If the fungal infection is severe -- covering the leaves, stems and flowers -- follow the recommended treatment for downey mildew.

Chemical Treatments

• Chemical fungicides formulated to treat fungal infections on rose bushes are available. Use extreme caution when using chemical fungicides, however. They will kill both harmful fungus and the beneficial fungus that rose bushes rely on for nutrient and water absorption. Follow the label instructions very carefully.