Gerbera Daisy-Powdery Mildew

Cause Golovinomyces cichoracearum (formerly Erysiphe cichoracearum) is a fungus that is favored by moderate temperatures (68°F to 82°F) with alternating cool night and warm days and conditions that produce high humidity (80% to 90%) but dry leaves. A closed canopy and poor air circulation also promote disease development. 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. Repeated outbreaks affect plant vigor and production.

The cultivars Snow White and Orange were the most susceptible; while Fuchsia and Hot Pink were less susceptible.

Symptoms Grayish white, powdery spots develop on leaves. These may be scattered at first but can quickly cover entire leaves giving them a frosty appearance. All aboveground plant parts can become infected including the flower stalk and petals. Discolored patches may appear on petals but may not look white and powdery, thus being confused with Botrytis or spray injury. Occasionally, a colony may develop on the underside of a leaf with a small, subtle yellow blotch on the opposite side of the leaf. Heavily infected plants lack vigor and may be reduced in size.

Cultural control

  • Space plants for good air circulation and light penetration. This will also help initiate more flower buds.
  • Heating the greenhouse at night and/or venting around sunset may be necessary to reduce humidity.
  • Scout regularly for symptoms and signs of the disease. Flag any area where it is found and scout more intensively in that area in the future.
  • Remove plant parts or plants that are infected from the greenhouse. Place in a sealed plastic bag while scouting other areas.
  • Allow plants to dry out between watering.
  • Plant resistant cultivars.

Chemical control Good coverage is essential. 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. Group 3 fungicides may have a PGR effect that might produce a more compact and darker green plant.

  • Affirm WDG at 0.25 to 0.5 lb/100 gal water. Group 19 fungicide. 4-hr reentry.
  • 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 1.25 to 5 lb/100 gal water. Oregon and Washington only. 1-hr reentry.
  • Broadform at 2 to 4 fl oz/100 gal water. Do not use on open flowers. Group 7 + 11 fungicide. 12-hr reentry.
  • Compass 50 WDG at 1 to 2 oz/100 gal water plus an adjuvant. Do not use organosilicate additives or on open flowers. 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.
  • EcoSwing at 2.5 to 3.2 oz/10 gal water. Group BM01 fungicide. 4-hr reentry. O
  • Fame SC at 1 to 4 fl oz/100 gal water. Group 11 fungicide. 12-hr reentry.
  • Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Best used before symptoms develop. Group 11 fungicide. 4-hr reentry.
  • Insignia SC at 3 to 6 fl oz/100 gal water plus surfactant. 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.
  • 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. Growth regulation effect may produce a more compact and darker green plant. Group 5 fungicide. 12-hr reentry.
  • Regalia CG at 1.5 to 5.9 tablespoons/1,000 sq ft. 4-hr reentry. O
  • Sporan EC2 at 32 to 48 fl oz/100 gal water plus an adjuvant. Do not use when temperature equals or exceeds 90F. No reentry listed. O
  • Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • Trigo at 1.5 to 2.4 oz/100 gal water. Group 3 + 11 fungicide. 12-hr reentry.
  • Trinity TR at 1 can/3,000 sq ft of greenhouse. Group 3 fungicide. 12-hr reentry without ventilation or 4-hr with ventilation.

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 Acidic-electrolyzed oxidizing water has been shown to effectively reduce the percentage of PM when sprayed twice a week and when sprayed every other week, alternating with fungicides. Weekly applications can be phytotoxic.

Reference Moyer, C. and Peres, N.A. 2008. Evaluation of biofungicides for control of powdery mildew of gerbera daisy. Proc. Fla. State Hort. Soc. 121:389-394.