“Ask a Master Gardener” fields questions directly from Yolo County gardeners and other garden enthusiasts. Questions for this column can be sent to the authors at jmbaumbach@ucanr.edu, with “Ask MGs” in the subject line. People submitting questions will remain anonymous, if desired. When emailing a question, please include as many details as possible. The master gardener authors strongly encourage including photographs, not only to help with finding the correct answer but also potentially to include in the column. Photos can be published credited or uncredited, depending on the contributor’s wishes. We cannot guarantee that we will be able to answer all the questions submitted but will do our best to get an answer for you.
Question: I have powdery mildew on “everything.” What do I do?
Answer: Powdery mildew is common this time of year. It is a fungal disease that appears as white, powdery patches on the upper surface of leaves, stems, and buds. Poor air circulation and shade promote growth. Unlike many fungal diseases, it thrives in warm, dry conditions. Our fall temperatures of 60-80 degrees are ideal for it. While it rarely kills plants outright, it weakens them by reducing photosynthesis and vigor. It is, at a minimum, unsightly. The good news is that once temperatures drop and rain starts, active infections should disappear.
Preventing the disease is more effective than eliminating it once it has started. We’ll cover that below. For now, there are a few things you can do. Remove and dispose of the most heavily infected leaves or shoots (don’t compost them). Thin out dense growth to improve air circulation and light penetration. Powdery mildew on vegetables and annual flowers this time of year is a sign that they failing and removal is warranted. Clean up fallen leaves and debris around the base of plants to reduce reinfection sources. A fresh layer of mulch can help.
To eradicate powdery mildew infections, the most environmentally friendly alternative is to use a horticultural oil or one of the plant-based oils, such as Neem oil or Jojoba oil. You should never apply oils when temperatures are above ninety degrees or to water-stressed plants. A chemical fungicide, Myclobutanil (Immunox), is available to the home gardener and functions to reduce infestations and protect against new infections. Reapply every seven to ten days, or in accord with label directions, until new growth appears clean. Even after visible mildew fades, spores can linger, so continue sprays if weather conditions favor recurrence.
Preventing infection is the best approach. Here are a few suggestions for next year. For new plantings, powdery mildew-resistant cultivars are available in many instances and are worth searching out. Space plants so that there is good air circulation. Full sun, for those plants that can tolerate it, will reduce infestations. Avoid excess nitrogen fertilizing or use a slow-release fertilizer since new growth is most susceptible.
Oil sprays also work as a preventative. Sulfur products, used for centuries to manage fungi, are effective only when applied before the disease appears. Wettable products work best, both pre-mixed and in powder form, to mix with water. Sulfur can damage some ornamental cultivars. To avoid injuring any plant, do not apply sulfur when the temperature is near or higher than ninety degrees, and do not apply it within two weeks of an oil spray.
UC IPM's article Powdery Mildew on Ornamentals provides additional information.
Have a gardening question? Send it to jmbaumbach@ucanr.edu, with “Ask MGs” in the subject line. Include as much detail as possible and pictures if you have them.