Watch Out for Botrytis Gray Mold in Your Greenhouse

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Botrytis gray mold sporulating on geranium

Botrytis gray mold sporulating on geranium (S. Sharpe)

As weather begins warming up and greenhouse annuals are in production, Botrytis gray mold will also be on the rise. Botrytis gray mold, also called gray mold or Botrytis blight, is a fungal disease that can progress rapidly under cool, moist or humid conditions and has a large host range, including many common ornamentals and vegetables. Temperatures around 70°F and humidity above 85% are ideal conditions for gray mold to proliferate. These conditions also commonly occur within a greenhouse, making management of gray mold especially important in spring and fall.

What does gray mold look like?

Spots on petunia flower caused by Botrytis gray mold

Spots on petunia flower caused by Botrytis gray mold (NC State PDIC)

Under ideal conditions, gray mold will produce spores that will look like a gray (sometimes light brown), fuzzy mass on foliage, stems, and/or flowers. However, when the fungus is not sporulating, it can easily be confused with other diseases. Symptoms include elongated tan or brown lesions on stems and petioles, irregular shaped lesions on leaves, and small spots may occur on flower petals. In severe cases, gray mold can cause part of or a whole plant to collapse. Foliar infections often start where a spent flower touches a leaf.

How do I control gray mold?

Botrytis gray mold on annual vinca stems

Botrytis gray mold on annual vinca stems (NC State PDIC)

Gray mold can be controlled by modifying the environment within the greenhouse and ensuring plants are receiving proper nutrition and moisture.

  • Maintain healthy plants. Excessive nitrogen makes plants more susceptible to gray mold.
  • Control weeds and remove plant debris between and during production. Gray mold will attack dead or dying tissue first, and then spread to adjacent healthy tissue.
  • Improve air circulation within the greenhouse and within the plant canopy. Air movement, even if it is humid air, prevents the fungal spores from germinating and will inhibit disease.
  • Heating and ventilating will inhibit gray mold. Condensation on plants overnight provides ideal conditions for gray mold. Heating and ventilating will prevent these conditions.
  • Use a preventative fungicide spray program. Botrytis is at high risk for developing fungicide resistance (insensitivity). Resistance to iprodione (FRAC group 2) and thiophanate-methyl (FRAC Group 1) has been reported in the pathogen. The pathogen is at high risk of developing insensitivity to FRAC group 11 fungicides (which includes Heritage and Compass) so it is important to rotate FRAC groups in the spray program.

You can find more information on products to manage Botrytis gray mold in the NC Agricultural Chemicals Manual.