Powdery mildews are one of the most widespread and easily recognized plant diseases. They affect virtually all kinds of plants. In Colorado, powdery mildews are common on lilac, grape, roses, turfgrass, vegetables (such as cucumbers, squashes and peas), crabapple and Virginia creeper, among others.
Powdery mildews are characterized by spots or patches of white to grayish, talcum-powderlike growth. Tiny, pinhead-sized, spherical fruiting structures that are first white, later yellow-brown and finally black, may be present singly or in a group.
Powdery mildews can be severe in warm, dry climates. This is because the fungus does not need the presence of water on the leaf surface for infection to occur. However, the relative humidity of the air does need to be high for spore germination. Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Incidence of infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces are wet (e.g., in a rain shower). Young, succulent growth usually is more susceptible than older plant tissues.
Several cultural control practices will reduce or prevent powdery mildews. Many plants have been developed to be resistant or tolerant to powdery mildew. If resistant varieties are unavailable, do not plant in low, shady locations.
- Avoid late-summer applications of high nitrogen fertilizer to limit the production of succulent tissue, which is more susceptible to infection.
- Avoid overhead watering to help reduce the relative humidity and water early in the day
- Remove and destroy infected plant parts if possible. Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus.
- Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection
If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of a fungicide may be necessary. Most chemical controls are applied as a preventative measure. The chemical spray provides a protective coating on the plant which can prevent spore germination. These include:
- Bonide Copper Fungicide – environmentally soft control
- Fertilome F-Stop – effective Big Gun control!
Chemicals applications are most effective when combined with cultural controls. Apply fungicides at seven to 14-day intervals to provide continuous protection throughout the growing season. Follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications, and waiting periods before harvest.