Cause The fungi Golovinomyces cichoracearum (formerly Erysiphe cichoracearum) and Podosphaera macularis have been reported from Oregon (found by the OSU Plant Clinic) and Washington. These fungi are favored by conditions that produce high humidity but dry leaves. They are highly specialized pathogens that form a close association with the host. Conditions that favor the host also favor the pathogen. Much of the fungus remains outside infected plant parts where it grows on the surface but sinks root-like structures called haustoria into plant cells to obtain nutrients. The white growth seen is composed of both mycelium and fungal spores. Chasmothecia are formed in the late summer and start the disease cycle again next spring.
Symptoms White, powdery spots may develop on leaves late in the growing season. Older leaves may be entirely coated with powdery mildew. Leaves become chlorotic, then necrotic and all from the plant. Severe defoliation may kill plants. Flowers as well as stems are susceptible to infection.
- Space plants for good air circulation.
- Provide proper light levels.
- Move plants from shaded to sunnier areas of the landscape.
- There are differences in cultivar reaction so select ones that are more resistant.
Chemical control Fungicides will do best when used before symptoms develop. Few materials have good eradicant activity. Use at 7- to 14-day intervals; using shorter intervals when environmental conditions favor disease development. Alternate or tank-mix products from different groups that have different modes of action. Limit the use of any one group during crop production.
- Bayleton 50 T&O at 5.5 oz/275 to 550 gal water. Landscape only, not for use on plants for sale. Group 3 fungicide. 12-hr reentry.
- Bicarbonate-based products. Might supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Thorough coverage is essential. O
- 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.
- 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. Group 7 + 11 fungicide. 12-hr reentry.
- Pipron at 4 to 8 fl oz/100 gal water. Use only in greenhouses. Group 5 fungicide. 12-hr reentry.
- Propiconazole-based products. Group 3 fungicides.
- Banner MAXX at 5 to 8 fl oz/100 gal water. 12-hr reentry.
- Infuse Systemic Disease Control at 1 Tbsp/gal water. H
- ProPensity 1.3 ME at 5 to 8 fl oz/100 gal water. 24-hr reentry.
- Safer Garden Fungicide II (Ready To Use 0.4% sulfur) thoroughly sprayed over the entire plant. Do not use when the temperature is over 85°F or within a few weeks of an oil spray. H
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H
- Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Trigo at 3 to 9 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.
- Zyban WSB at 24 oz/100 gal water. Not to be confused with the smoking cessation drug. Group 1 + M3 fungicide. 24-hr reentry.
- Bayer Advanced Natria Disease Control RTU (Bacillus subtilis strain QST 713) is registered for the home garden. Active ingredient is a small protein. H O
- 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
References Farinas, C., Jourdan, P., Paul, P.A., and Peduto Hand, F. 2019. Development and Evaluation of Laboratory Bioassays to Study Powdery Mildew Pathogens of Phlox In Vitro. Plant Disease 103:1536-1543.
Perry, L. 2005. Comparison of powdery mildew among Garden Phlox cultivars, 2004. B&C Tests Vol 20:O001