Cause The most frequently reported genera causing powdery mildew on Phlox are Golovinomyces magnicellulatus and Podosphaera sp. 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. Annual phlox are more resistant than perennial phlox.
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 fall from the plant. In some cases, leaf infection may show the green island effect where areas around colonies remain green while the rest of the plant tissue appears senescent (yellow). Severe defoliation may kill plants. Flowers as well as stems are susceptible to infection.
Cultural control
- 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 in your area.
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.
- Avelyo at 3 to 5 fl oz/100 gal water. 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.
- Mural at 4 to 7 oz/100 gal water. Group 7 + 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.
- Ortho Rose & Flower Disease Control at 2 fl oz/gal water. Group 3 fungicide. H
- Pageant at 6 to 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. 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.
- Postiva at 10 to 28 fl oz/100 gal water. Group 3 + 7 fungicide. 12-hr reentry.
- Propiconazole-based products. Group 3 fungicides.
- Banner MAXX at 5 to 8 fl oz/100 gal water. 12-hr reentry.
- Concert at 22 to 35 fl oz/100 gal water. Landscape use only. Group 3 + M5 fungicide. 12-hr reentry.
- Infuse Systemic Disease Control at 1 Tbsp/gal water. H
- 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
- Seido at 4 to 5 fl oz/100 gal water plus an adjuvant. Group 50 fungicide. 4-hr reentry.
- 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.
- Tourney EZ at 1 to 4 oz/100 gal water. Group 3 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
References Farinas, C., Jourdan, P. S. and Peduto Hand, F. 2020. Flaming Phlox and the Ubiquitous Powdery Mildew Disease. Plant Health Progress, 22:11-20.