Garden Detective

What’s that white stuff on my shrub?

Powdery mildew almost covers the leaves of this peony. It’s a common fungal disease on ornamental plants.
Powdery mildew almost covers the leaves of this peony. It’s a common fungal disease on ornamental plants. AP

Experts tackle readers’ garden questions.

Q: I was wondering what I can do for powdery mildew on my red tip photinia. I’ve tried a granular systemic without much success. Is there a spray I could use? I’d hate to lose these plants.

Mark Cabral, Granite Bay

Sacramento County Master Gardener Carmen Schindler: Powdery mildew is a common disease on many types of plants and is prevalent in California, including the Sacramento Valley. The disease is easy to spot by the white, powdery spore growth on leaf surfaces. Powdery mildew can infect old and new foliage, and can be serious on woody species, such as photinia. If left untreated, leaves will turn yellow, then brown and die.

To control powdery mildew in most situations, avoid planting the most susceptible cultivars, provide good air circulation and do not over-fertilize because new growth is more susceptible to disease. Instead, apply a slow-release fertilizer that provides controlled growth.

Moderate temperatures of 60 to 80 degrees and shady conditions are favorable for powdery mildew development, so locate new plants in sunny areas as much as possible.

Several less-toxic fungicides are available to control the disease. Fungicides can function as protectants, which prevent new infections from occurring, or eradicants, which can kill an existing infection. Apply protectant fungicides to highly susceptible plants before the disease appears. Use eradicants at the earliest signs of the disease. Once mildew growth is extensive, controlling the situation with any fungicide becomes more difficult.

In mild to moderate cases of powdery mildew, horticultural and plant-based oils, such as neem and jojoba oils, can be used as eradicants and protectants. Several other least-toxic fungicides also are available, including sulfur, potassium bicarbonate and the biological fungicide Serenade.

These products are effective only on contact, so applications must thoroughly cover all susceptible plant parts. As plants grow and produce new tissue, additional applications may be necessary at seven- to 10-day intervals. Always remember to thoroughly read and follow all label instructions.

On perennial hosts, the fungi can survive from one season to the next, and wind will carry powdery mildew spores to new hosts. As with any plant disease, the best method of control is prevention.

For more information on powdery mildew on ornamentals, visit the University of California (UC) Integrated Pest Management website at ipm.ucanr.edu.

Carmen Schindler is a UC Cooperative Extension master gardener for Sacramento County.

Garden questions?

Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to h&g@sacbee.com. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:

  • Sacramento: 916-875-6913; 9 a.m.-noon and 1-4 p.m. Monday-Thursday
  • Amador: 209-223-6838; 10 a.m.-noon Monday-Thursday; website: ceamador.ucdavis.edu
  • Butte: 530-538-7201; 8 a.m.-noon and 1-5 p.m. weekdays
  • Colusa: 530-458-0570; 9 a.m.-noon and 1-4 p.m. Tuesdays; website: cecolusa.ucanr.edu
  • El Dorado: 530-621-5512; 9 a.m.-noon Tuesday-Friday
  • Placer: 530-889-7388; 9 a.m.-noon Tuesday-Thursday or leave a message and calls will be returned; website: pcmg.ucanr.org/got_questions
  • Nevada: 530-273-0919; 9 a.m.-noon Tuesday-Thursday or leave a message
  • Shasta, Tehama, Trinity: 530-242-2219; email mastergardener@shastacollege.edu
  • Solano: 707-784-1322; leave a message and calls will be returned
  • Sutter, Yuba: 530-822-7515; 9 a.m.-noon Monday-Tuesday and 1-4 p.m. Thursdays
  • Yolo: 530-666-8737; 9-11 a.m. Tuesdays and Fridays, or leave a message and calls will be returned
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