As I write this article, it is 87 degrees outside. With these warm temperatures, I’m a little surprised to see powdery mildew (PM) occurring on my rambling roses. This fungal disease grows best at temperatures of 60-80 degrees. It rarely develops at temperatures close to or above 90 degrees. PM is a common fungal disease on many types of plants. In addition to roses, zinnias and other flowers, many varieties of vegetables such as beans, beets, cucumbers, eggplants, peas, peppers, melons, squash pumpkins and tomatoes also are affected.
PM does not need moist conditions or free water to establish. It first appears as white powdery spots on both surfaces of the leaves, on shoots, and sometimes on flowers and fruit. These spots spread and may cover large areas. The PM that infects tomatoes and peppers causes yellow patches on leaves, but little powdery growth. PM spreads by spores carried on the wind.
Prevention is the best control. This starts with choosing PM resistant varieties, and planting in full sun. Fungicides can be used as protectants or eradicants. Using fungicides successfully requires timing their application either before or at the first sign of disease. Once mildew growth has developed, control with a fungicide is difficult. Reapplication every seven to 10 days may be necessary.
Fungicides available include horticulture oils, neem oil, sulfur or biological fungicides. Oils work primarily as eradicants, with some protection potential. Sulfur is only effective prior to the appearance of symptoms. It must be used carefully or it can burn plants. Sulfur should not be used if the air temperature is around 90 degrees or higher and should not be applied within two weeks of an oil spray. Biological fungicides are made from beneficial bacteria. When sprayed on a plant, they destroy PM fungi. Always read and follow pesticide labels carefully.
I prevent PM on my roses simply by hosing them off with water as new growth develops, before I ever see the powder. Overhead irrigation can reduce PM development too. I hadn’t seen any PM this year, so I got out of the habit of hosing off my roses and now I’m paying the price!
The next class in the Grow Your Own series, “Managing Pests in your Landscape,” is Sept. 23, followed by “Caring for your Landscape Plants,” Sept. 25. Both run 6 to 8 p.m. at 2621 Northgate Lane, Suite 12, Carson City. To reserve a seat for these classes, contact Teri at 775-887-2252 or