See:
Hydrangea - Bud and Flower Blight
Cause Botrytis cinerea (sexual: Botryotinia fuckeliana), fungi that colonize dead, dying, and wounded plant parts. From these infections they can attack healthy tissues. A moist, humid environment is ideal for pathogen sporulation and spread. It is found everywhere plants are grown and has a wide host range wide host range including African violet, ageratum, ajuga, aphelandra, aster, begonia, caladium, calceolaria, carnation, chrysanthemum, coleus, cyclamen, fuchsia, geranium, gloxinia, hosta, Iberis, impatiens, lily, lobelia, marigold, orchid, pansy, Pericallis hybrids, petunia, poinsettia, primrose, rose, snapdragon, verbena, viburnum, zinnia, and zygopetalum. Flower petals are most susceptible at any stage of development. Could be a secondary problem moving in after death of tissues from bacterial blight.
Symptoms Blighting of blossoms is the most common symptom. Initial infections, particularly of petals, appear as water-soaked brown to tan spots. In humid conditions, a fuzzy fungal growth, brown to gray, appears around the infected plant part. Blossom infections may cause petals to fade and die prematurely.
Cultural control An integrated strategy combining environmental management, cultural practices, and fungicides will most effectively manage this disease.
- Thoroughly clean and sanitize the greenhouse before production.
- Remove all dead and dying plant parts (particularly blossoms) on and around plants and take them out of the greenhouse. Many growers use closed containers to minimize the amount of conidia.
- Maintain a steady, relatively dry environment by keeping greenhouse humidity below 90%, increasing spacing between plants for good air circulation, and taking care not to splash water on foliage during watering. Heating the greenhouse at night (especially for zero or negative DIF) 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. If using DROP or DIP for size control, do not let humidity rise above 90%.
- Spacing plants also allows better light penetration, reducing senescence of the lower leaves and thus helping reduce the amount of highly susceptible plant tissue.
- Place fans in greenhouse to get better air circulation.
Chemical control Fungicides do not compensate for poor sanitation or environmental controls. Resistance to many different and multiple fungicide groups has been documented worldwide. It is common for an isolate to be resistant to 4 different FRAC groups and isolates resistant to 7 different FRAC groups at once has been detected. To prevent developing resistant fungi, alternate or tank-mix materials from different groups with different modes of action. Also, limit applications from any specific group to two (2) or fewer sprays. Be sure to check labels for crop safety before application.
- Concert at 22 to 35 fl oz/100 gal water. May cause injury to buds, blooms or tender new growth. Landscape use only. Group 3 + M5 fungicide. 12-hr reentry.
- Daconil Weather Stik at 1.38 pints/100 gal water. Although registered, other compounds would likely be more effective. Group M5 fungicide. 12-hr reentry.
- Mural at 4 to 7 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.
- Orkestra at 6 to 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
Reference Pscheidt, J. W., and Moorman, G. W. 2001. Chapter 56. Lilac Diseases p. 221-224 in Diseases of Woody Ornamentals and Trees in Nurseries, R. Jones, and M. Benson (eds.). St. Paul, MN: APS Press