See:
Greenhouse Plants, Ornamental - Powdery Mildew
Cause Podosphaera fuliginea and Podosphaera macularis (formerly Sphaerotheca macularis) , fungi reported from the PNW. The conidia are readily disseminated by wind. Chasmothecia develop within the external hyphae and form appendages late in their development. The disease is favored by shade or low light, warm days (60°F to 80°F) and cool nights. Overcrowding of plants is also favorable for disease development.
Symptoms White powdery growth on both sides of the leaf and sometimes on stems. Leaves may be somewhat deformed if they are infected before they have fully developed. New infections are typically bright white then becoming gray with age. Severe infections will cause necrosis and death of the leaves.
Cultural control
- Space plants for good air circulation.
- Maintain high light levels and good air movement around the foliage.
- The cultivars Bingo Deep Purple, Crown Yellow Splash, Delta Pink Shades, Delta Pure Rose, Delta Pure Yellow, Majestic Giant White, and Maxim Orange are reported to be 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.
- 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.
- 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
- 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. Do not use during early growth stages. 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.
- Disarm 480 SC at 1 to 4 fl oz/100 gal water. 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 2 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-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.
- Postiva at 10 to 28 fl oz/100 gal water. Group 3 + 7 fungicide. 12-hr reentry.
- Seido at 4 to 5 fl oz/100 gal water plus an adjuvant. Group 50 fungicide. 4-hr reentry.
- Sil-Matrix at 1 to 4 quarts/100 gal water plus a nonionic surfactant. Do not use on open blooms. 4-hr reentry. O
- 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.
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
Note: Although Trinity TR is registered it is not recommended due to phytotoxicity issues.
Reference Chase, A.R., Daughtrey, M.L., and Cloyd, R.A. 2018. Compendium of Bedding Plant Diseases and Pests. St. Paul, MN: APS Press.