February 2015 Plant Disease Update

Botrytis blight on marigold.
Botrytis blight on marigold.

The appearance of a gray, fuzzy mold on flowers, leaves or stems of ornamental plants is a sign of the fungal disease, Botrytis blight, also called, for obvious reasons, “gray mold”. The gray mold fungus grows well under cool, humid conditions and can attack a wide range of plant species. It is common on greenhouse plants, such as poinsettia, geranium, and petunia, at this time of year. It can also occur on landscape plants, including tulips, peonies, and roses, in early spring. 

Botrytis blight on tulips
Botrytis blight on tulips in the landscape.

Symptoms include leaf and flower spotting or blight, stem lesions, and dieback. The fungus produces spores prolifically on both dead and recently infected tissue. Because each spore can cause a new infection, it is important to remove dead tissue, including fallen leaves and flowers, on a regular basis. In the landscape, removal of plant debris also helps to prevent overwintering of the fungus. In greenhouses, careful control of temperature and humidity, in addition to strict sanitation, helps to control the disease. Preventative fungicides are also available for control. As temperatures rise later in the growing season, the disease is typically less of a problem.