Mold on Rose Leaves

Identification

• Powdery mildew can make the leaves of plants, including roses, appear moldy or powdery. The disease also makes young plant leaves curl and sometimes turn purple. Powdery mildew retards the growth of the plant and, in some cases, kills it. Typically, powdery mildew first appears on new growth. Sometimes the front of the leaf looks healthy though the back is covered with mildew.

Prevention

• Powdery mildew commonly appears in humid weather, or when hot days are followed by cool nights. The disease is caused by a fungus called Sphaerotheca pannosa, whose spores lay dormant, waiting for the right conditions before growing on a plant. Gardeners can help prevent the right conditions for powdery mildew by not watering from overhead, watering in the morning (not in the evening), and by planting roses in full sun. Overcrowding may lead to poor air circulation and a greater tendency toward powdery mildew. Finally, plants with nutrient deficiencies are more likely to become diseased.

Traditional Treatment

• To treat powdery mildew, start by removing all affected areas of the plant and discarding the diseased pieces in the trash, since composting affected plant clippings can spread the disease. Pick up and throw away any leaves that fall on the ground. Plants may also be sprayed with a fungicide, although these are most effective as a preventive measure. The Ohio State University Extension website recommends fungicides containing triadimefon, triforine or thiophanate-methyl.

Organic Treatment

• A mixture of 1 part cow's milk and 9 parts water is also an effective treatment for powdery mildew, as scientist Wagner Bettiol reported in the September 1999 issue of "Crop Protection." Not only is this a cheap, organic treatment, but it's proven to reduce the disease by up to 90 percent.