Cause Golovinomyces cichoracearum (formerly Erysiphe cichoracearum) has been reported in Washington, and Podosphaera fusca (formerly Sphaerotheca macularis) has been reported in Washington and Idaho. The classification in the rest of the world has Podosphaera pericallidis. The 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.
Symptoms Grayish white, powdery spots develop on leaves, stems, and sometimes flower buds. Infected plant parts may be dwarfed and stunted. Some premature defoliation may be observed.
Cultural control
- Space plants for good air circulation.
- Remove and destroy infected leaves and plant debris.
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.
- 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.
- Bicarbonate-based products. 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 (85% potassium bicarbonate) 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
- Seido at 4 to 5 fl oz/100 gal water plus an adjuvant. Group 50 fungicide. 4-hr reentry.
- Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Braun, U. and Cook, R.T.A. 2012. Taxonomic Manual of the Erysiphales (Powdery Mildews). CBS-KNAW Fungal Biodiversity Center, The Netherlands.