Chrysanthemum-Gray Mold

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Cause Botrytis cinerea, a fungus. Botrytis blight is a disease of greenhouse-grown chrysanthemums. The fungus survives in plant debris as sclerotia. Spores are spread by overhead irrigation and by air. The disease is favored by high humidity and temperatures of 50°F to 70°F. Warm days followed by cool nights encourages condensation on flowers which also favors disease development. Continuous wetness for 5 to 8 hours is necessary for conidial infection. Wounded or senescent tissue is most likely to be infected.

Symptoms Brown water-soaked spots on chrysanthemum petals and leaves. First symptoms usually are on outer, older florets. Later, a powdery grayish-brown mass of fungal spores covers affected parts. Infected flower buds may fail to open. Lower senescent leaves may rot, and if the fungus gets into the stem, it may girdle the stem.

Cultural control

  • Maintain a steady, relatively dry environment by keeping greenhouse humidity below 90%. Heating the greenhouse at night or venting around sunset may be necessary. Heating in the morning before sunup can also help prevent dew formation as air temperature increases faster than the temperature of plant parts.
  • Provide good air circulation-space plants apart.
  • Remove and destroy affected leaves as soon as noticed. Dispose of all plant debris promptly.
  • Avoid overhead watering.

Chemical control Use as a foliar spray in conjunction with cultural control methods. Tank-mix and/or alternate products from different groups with different modes of action to prevent the buildup of resistant fungi. Limit the use of any one group during crop production.

  • Astun at 10 to 17 fl oz/100 gal water. Group 7 fungicide. 12-hr reentry.
  • Bonide Captan 50 WP may be used in Oregon home gardens at 2 Tbsp/gal water. H
  • Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. H
  • Broadform at 4 to 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
  • Captan 80 WDG at 1.5 lb/100 gal water. Group M4 fungicide. 48-hr reentry.
  • Chipco 26019 N/G at 1 to 2.5 lb/100 gal water. Group 2 fungicide. 12-hr reentry.
  • Copper-Count-N at 1 quart/100 gal water. Discolored foliage or blooms have been noted. Group M1 fungicide. Oregon only. 48-hr reentry general or 24-hr reentry for greenhouse.
  • Daconil Weather Stik at 1.4 pints/100 gal. Avoid blooms. Group M5 fungicide. 12-hr reentry.
  • Decree 50 WDG at 0.75 to 1.5 lb/100 gal water. Group 17 fungicide. 12-hr reentry.
  • Emblem at 2 to 4 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
  • Exotherm Termil at 1 can/1500 sq ft of greenhouse. It ignites to form a vapor that condenses back on the plants. See label for details. Group M5 fungicide. 24-hr reentry with no ventilation or 12-hr reentry with ventilation.
  • Hi Yield Captan 50 W Fungicide may be used in Oregon home gardens at 2 Tbsp/gal water. H
  • Nu-Cop 50 DF at 1 lb/100 gal water. Blooms may discolor. Group M1 fungicide. 24-hr reentry.
  • Mancozeb-based products can be used as mixing partners and provide some protection. Group M3 fungicides. 24-hr reentry.
    • Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
    • Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
  • Medallion WDG at 2 to 4 oz/100 gal water. Use with oils or adjuvants may damage plant. Group 12 fungicide. 12-hr reentry.
  • Mozart TR at 1 can/4,500 sq ft of greenhouse. Group 12 fungicide. 12-hr reentry with no ventilation.
  • OHP 6672 4.5 F at 10 to 14.5 fl oz/100 gal water plus another fungicide. Group 1 fungicide. 12-hr reentry.
  • Orkestra at 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
  • Pageant at 12 to 18 oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
  • Phyton 27 at 1.5 to 2.5 fl oz/10 gal water. Group M1 fungicide. 48-hr reentry.
  • Sil-Matrix at 1 to 4 quarts/100 gal water plus a nonionic surfactant. 4-hr reentry. O
  • Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
  • Spirato GHN at 2 to 4 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
  • Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • Zyban WSB at 24 oz/100 gal water. Not to be confused with the smoking cessation drug. Group 1 + M3 fungicide. 24-hr reentry.

Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Heritage.

Biological control

  • Prestop (Gliocladium catenulatum strain J1446) at 3.5 oz/5 gal water. Do not use with other products in the tank. 0-hr reentry. O

Reference Körner, O.and Holst N. 2005. Model based humidity control of Botrytis in greenhouse cultivation. Acta Hort. 691:141-148.