Tom Creswell, Plant Disease Diagnostician, Botany & Plant Pathology Dept, Purdue University
Rapid blighting and rot of flowers and leaves often signals damage from gray mold (aka Botrytis blight). The fungus causing the blight, Botrytis cinerea, is known to attack almost 3000 kinds of plants worldwide; including annuals, perennials, fruits and vegetables. We typically see it causing damage to flowers and soft fruit or leaf tissues when humidity is high. The fungal spores move easily on air currents and can also be spread by splashing water from plant to plant. Infection requires temperatures between 55-75 degrees F and a relative humidity of 93% or higher.
In greenhouses you can reduce the chance of infection by removing infected material and lowering relative humidity. Increasing air movement along with venting will help lower excessive humidity and prevent dew formation. Botrytis has developed resistance to some fungicides, namely those containing thiophanate methyl and iprodione, so rotation of active ingredients within a spray program is very important.
For further information:
Botrytis Blight of Greenhouse Crops - UMass Extension
Click image to enlarge
Cyclamen flowers rapidly succumbing to Botrytis blight
Botrytis blight attacking Kalanchoe flowers
Poinsettia bracts may be blighted just at the time the plants are ready for sale.
Courtesy of NC State
Spores of Botrytis producing copious spores on an infected Petunia stem. The olive-green to gray color of the mass of spores shows why it's also called gray mold.