The fungus known as gray mold spreads easily and quickly.

Courtesy of University of Georgia, Mary Ann Hansen, Virginia Tech,

Botrytis is a fungus that can cause leaf spots, petiole blighting and stem cankers on many different annuals and perennials. It will produce large masses of spores that are most often called “gray mold.” These spores or conidia will be spread on wind currents and can travel from infected to uninfected plants in that manner. The spores can live for upwards of 21 to 24 days before they germinate on a plant.

Facts about Botrytis

Identification: Gray mold is an ascomycete fungus that produces abundant gray-colored mycelium and conidia or asexual spores. The conidial or botrytis stage is common in all nurseries and landscapes. Dead flowers and plant parts fall to the ground where sclerotia later form to insure survival of the pathogen. Sclerotia later germinate as hyphae to grow and form more conidia. The disease may affect flowers which may not open and may become covered with grayish brown fungal growth. Sometimes the disease is observed as small flecks on infected petals.

Courtesy of University of Georgia, Mary Ann Hansen, Virginia Tech,

Damage: Botrytis is a necrotrophic pathogen and relies on enzymes to rot plant parts. Infected tissues soon turn brown due to enzymatic degradation of the middle lamella, cell walls and cell contents.

Chemical control: When cool, wet storms are predicted during sensitive growing periods (times of new growth or flowering), there are many fungicides that will offer some protection to these tissues. Fungicides in the strobulurin group and many newer (such as fludioxonil) and older (such as triadimefon) fungicide active ingredients can provide control of gray mold; however, fungicides in every FRAC (Fungicide Resistance Action Category) number listing are rated as moderate to high resistance risk materials, so it is wise to alternate active ingredients in any spray program or use combination products that employ active ingredients from more than one FRAC group.

Courtesy of University of Georgia, Mary Ann Hansen, Virginia Tech,

Cultural control: Several cultural control practices will reduce the conditions that favor botrytis infections, including: reducing the relative humidity in the greenhouse or poly house below 85%; making sure plants do not remain wet for six or more hours in a 24-hour period; and if possible, heat and vent mornings and evenings for at least a half-hour or more to reduce humidity and keep plant surfaces dry. Increase plant spacing to allow for more air movement, less spore splashing and less plant-to-plant contact. This will slow the progress of botrytis during prolonged cool periods. Cleaning up rotted plant debris caused by botrytis blight or rots is essential in limiting the disease and preventing future outbreaks. While spores are mostly ubiquitous, they can concentrate in decayed plant matter, so sanitation by deadheading diseased flowers is helpful in controlling outbreaks.

Source: Michigan State University Extension

January 2023
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