Powdery mildews are a group of fungi familiar to vegetable gardeners and ornamental plant lovers alike. Though there are several fungi in this group, all have a similar appearance. The powdery growth starts as a small infection on the top of a leaf and can grow to cover the entire leaf. The disease can involve the stem, flowers and fruit. While the fungus is rarely fatal to the plant, severe infections can cause yellowing and drying of leaves, significantly reduced production of fruit or vegetables and distorted flower buds that don't open.
Gardeners often associate this disease with areas of the country that receive more rainfall, but it also flourishes here in our dry steppe climate. Rather than rainfall totals, the determining factors are relative humidity and light. A high relative humidity encourages spore germination. Low light conditions increase the likelihood of infection. Poor air circulation and high nitrogen levels seal the deal.
The disease affects nearly every kind of plant in a landscape -- vegetables, ornamentals, grasses, even fruit and shade trees. You've probably seen it on roses, zinnias and squash, but crabapples, garden phlox, pears, grapes, lilacs and even turfgrasses also are susceptible.
The first line of defense is cultural controls.
Choose disease-resistant varieties. Tall garden phlox that bloom about this time of year are a great example. With age, the perennial grows into a large, dense clump, and in the past a gardener could pretty well count on getting dusty, gray leaves before the season was over. Hybridizers tackled this problem by incorporating disease resistance into the various varieties. Probably the best known of these is a white phlox named 'David.' This disease resistance has become a selling point and is often noted on plant labels.
Increase air circulation. Selective pruning of woody plants or adequate spacing for perennials and annuals will increase air flow, and in turn reduce humidity that fosters the spread of the fungus.
Sanitation. Throw away infected leaves, stems and flowers. Composting typically does not generate high enough temperatures to kill the fungus.
Fungicides are a chemical option if cultural controls don't manage the fungus.
Information on chemical controls for powdery mildews is available online at ext.colostate.edu. As with any chemical, carefully read and follow the directions.
Please note: Some fungicides, especially those containing sulfur, can cause plant damage when applied at high temperatures.
Colorado State University Extension in the City and County of Broomfield provides unbiased, research-based information about 4-H youth development, family and consumer issues, gardening, horticulture and natural resources. As part of a nationwide system, Extension brings the research and resources of the university to the community. The Broomfield County Extension office is at 1 DesCombes Drive, Broomfield, 80020. For information, call 720-887-2286.