Azalea (Rhododendron spp.)-Powdery Mildew

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Cause The fungus Erysiphe azaleae (formerly Microsphaera azaleae) is found in western Oregon and Washington and southern British Columbia on garden azalea and rhododendron species and hybrids. Erysiphe vaccinii (formerly Microsphaera vaccinii) has been found only on wild R. occidentale growing on the southern Oregon coast. The disease is most common on deciduous azaleas but not as frequent on evergreen azaleas. The cultivars Homebush and White Lights are susceptible to powdery mildew.

These fungi are obligate parasites that produce two kinds of spores. Asexual conidia are most frequent; they spread by wind and produce new colonies that produce more spores. Many infection cycles may occur in a summer. The disease develops late in summer, but overwintering colonies have been seen throughout the winter on leaves' undersides. Sexual spores are produced in small, black, spherical structures (chasmothecia). They are produced in fall in great numbers on azalea. High humidity and high nitrogen fertility favors the disease.

Symptoms White powdery spots develop on young leaves. The whitish growth can occur on both sides of the leaf and may cover the leaf by the end of summer. Powdery mildew on some azalea cultivars may look like subtle leaf spots that are not white and fuzzy. This symptom is more typical on evergreen Rhododendrons.

Cultural control

  • Reduce relative humidity by adjusting irrigation practices and spacing plants for good air circulation.
  • Avoid planting in heavily-shaded areas.
  • Use a minimal level of nitrogen fertility.
  • Plant resistant cultivars. The following were disease free in Ohio and Minnesota over 3-year period: Fragrant Star, Garden Party, Millennium, Parade, Popsicle, and June Flame.

Chemical control Begin multiple applications when you first notice the disease on current-year leaves. Early detection and scouting aid overall control. Alternate or tank-mix products from different groups that have different modes of action.

  • Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
  • Avelyo at 3 to 5 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • Bayer Advanced Disease Control at 0.75 fl oz/gal water. H
  • Bayleton 50 T&O at 5.5 oz/275 to 550 gal water. Apply at 7- to 10-day intervals. Landscape only, not for use on plants for sale. Group 3 fungicide. 12-hr reentry.
  • Bicarbonate-based products (85% potassium bicarbonate). Might be used to supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Thorough coverage is essential. O
    • MilStop at 2.5 to 5 lb/A in the field or 1.25 to 5 lb/100 gal water in the greenhouse. Oregon and Washington only. 1-hr reentry
    • Monterey Bi-Carb Old Fashioned Fungicide at 4 teaspoons/2 gal water. H
  • Broadform at 2 to 4 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
  • Compass 50 WDG at 1 to 2 oz/100 gal water. Do not use organosilicate additives. Group 11 fungicide. 12-hr reentry.
  • Concert at 22 to 35 fl oz/100 gal water. May cause injury to buds, blooms or tender new growth. Landscape use only. Group 3 + M5 fungicide. 12-hr reentry.
  • Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
  • Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
  • Insignia SC at 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
  • Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
  • Orkestra at 6 to 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
  • Pageant at 6 to 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
  • Propiconazole-based products. Group 3 fungicides.
    • Banner MAXX at 5 to 8 fl oz/100 gal water. 12-hr reentry.
    • ProCon-Z at 5 to 8 oz/100 gal water. 24-hr reentry.
  • Regime at 20.5 to 45.7 fl oz/A. Reapply if rain occurs within 12 hr of original application. Group BM01 fungicide. 4-hr reentry.
  • Safer Garden Fungicide (Ready To Use 0.4% sulfur) thoroughly sprayed over the entire plant. Do not use when temperature is over 85°F or within a few weeks of an oil spray. H
  • Seido at 4 to 5 fl oz/100 gal water plus an adjuvant. Group 50 fungicide. 4-hr reentry.
  • SuffOil-X at 1 to 2 gal/100 gal water. Do not use in conjunction with sulfur. Do not use when plants are under heat or moisture stress. 4-hr reentry.
  • Tebuzol 3.6F at 4 to 10 fl oz/100 gal water. Group 3 fungicide. No reentry interval listed.
  • Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • Torque at 4 to 10 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • Tourney 50 WDG at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • Trigo at 1.5 to 2.4 oz/100 gal water. Group 3 + 11 fungicide. 12-hr reentry.
  • Trinity at 4 to 12 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.

Biological control

  • Cease or Rhapsody (Bacillus subtilis strain QST 713) at 2 to 8 quarts/100 gal water. Active ingredient is a small protein. Efficacy in the Pacific Northwest is unknown. 4-hr reentry. O

Reference Long, M.C., Krebs, S.L., and Hokanson, S.C. 2010. Field and growth chamber evaluation of powdery mildew disease on deciduous azaleas. HortScience 45:784-789.