Mildews are fungal diseases of plants that reduce vigor and, in some cases, kill the plant. Mildews harm plants by invading their tissue and feeding off nutrients that the plant produces for its own needs. Treating mildew diseases combines good cultural practices with the application of approved fungicides.
Treatment of plant diseases first requires accurate diagnosis of the disease. According to the University of Illinois Extension, there are two types of mildew diseases common to plants. Powdery mildew typically appears as whitish-gray spots on the tops of leaves and vines, although it can also spread to the undersides of leaves and sometimes fruit. Downy mildew begins with pale spots on the tops of leaves that spread. You can observe the fungus itself as a purple or gray fuzz on the undersides of leaves.
According to the Cornell University Plant Disease Diagnostic Clinic, powdery mildew varies in its severity depending on species. Some plants, such as lilacs, can survive infections, while other species die. Likewise, downy mildew causes different severity of symptoms depending on species. If you identify a mildew disease on your plants, it is best to pursue treatment, as the diseases can be fatal in some cases.
Good cultural practices can prevent mildew infections and avoid reinfection in future years. Gather dead leaves from houseplants or from outdoor plants in the autumn and destroy them. Moist environments favor the development of mildew diseases, so water outdoor plants in the morning to give wet leaves a chance to dry. Avoid wetting the leaves of houseplants or outdoor plants in conditions where leaves will not have the chance to dry. For some plant species, mildew-resistant varieties are available and should be planted if mildew diseases have been a problem for you in the past or are common in your area.
Fungicides can prevent or control mildew diseases. As Jay W. Pscheidt, a plant pathologist with the Oregon State University Extension, notes, it is vital to only use fungicides approved for the plant you're treating and never to exceed recommended dosages. In many cases, fungicides prevent damage to healthy tissue rather than ridding the plant of the disease, so spraying at-risk plants before the disease appears may be warranted in some cases.
Treatment should also take into account weather conditions favorable for the development of mildew diseases. Mildews tend to develop in cool, wet or humid conditions, and when this type of weather persists for an extended period of time, according to the Cornell University Plant Disease Diagnostic Clinic, it may be advisable to apply fungicides in anticipation of mildew developing.