Cause Golovinomyces cichoracearum (formerly Erysiphe cichoracearum) has not been reported formally for the Pacific Northwest, but it is a common garden problem on cosmos. This fungus is favored by conditions that produce high humidity but dry leaves. It is a highly specialized pathogen that forms 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. Some yellowing and premature defoliation may be observed.
Cultural control
- Space plants for good air circulation.
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.
- 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.
- 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.
- Pageant at 6 to 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
- Regime at 20.5 to 45.7 fl oz/A. Reapply if rain occurs within 12 hours of original application. Group BM01 fungicide. 4-hr reentry.
- 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.
- Thiolux (80% sulfur) at 3 to 10 lb/A. Group M2 fungicide. 24-hr reentry. O
Reference Braun, U. and Cook, R.T.A. 2012. Taxonomic Manual of the Erysiphales (Powdery Mildews). CBS-KNAW Fungal Biodiversity Center, The Netherlands.