Managing Botrytis or Gray Mold in the Greenhouse

Bedding plants are very susceptible to Botrytis, so growers should aggressively scout for signs of infection and be proactive in managing this common disease.

Botrytis cinerea can survive as a saprophyte on leaves, flowers, and stems or on plant debris that has accumulated under the benches in the greenhouse. Fungal spores produced on decaying tissues can be spread by splashing water and on air currents in the greenhouse. Botrytis infection is encouraged by the presence of free moisture on plant tissues for 8–12 continuous hours, high relative humidity (greater than 93% in the canopy), and cool temperatures (55 to 65°F).

Condensation can form on the greenhouse glazing as it cools and then drips onto the foliage and flowers below. This source of free moisture, coupled with elevated relative humidity levels in the growing environment, can create the perfect conditions for gray mold infection and development.

Most floriculture crops are susceptible to Botrytis infection throughout the production cycle. Scouting for Botrytis starts when the first box of cuttings or plugs is opened and continues until the finished crops are loaded up for delivery or are sold to the end consumer.

Growers unfamiliar with scouting for Botrytis infection in the greenhouse should consider the following tips:

• Examine all cuttings for blighted leaves and/or decaying tissues upon receipt from the propagator.
• Examine mature plants weekly and look for brown lesions on the leaves or for the presence of blighted foliage. Also, examine the stems and look for brown, sunken cankers (especially where blighted leaves have been observed). Scout for signs of Botrytis cinerea infection in areas of the greenhouse where plants are overcrowded, where air circulation is poor, or where condensation appears to accumulate and drip.
• If you cannot confirm botrytis infection through visual inspection, consider placing the blighted plant parts in a sealed plastic bag with a damp paper towel. Light gray to tan cobweb-like mold will develop on the infected plant parts if Botrytis cinerea is present. If you still are unsure about the diagnosis, please submit a disease sample to the Penn State Plant Disease Clinic. Sample submission guidelines and forms can be found on the Penn State Plant Disease Clinic website.

Good air movement and humidity management coupled with good sanitation practices to remove blighted plant tissues from the greenhouse are critical to the prevention of Botrytis infection. While fungicides can be successfully utilized to manage this disease, fungicide-resistant strains of Botrytis cinerea are known to exist which can make fungicide selection and use more difficult. Growers utilizing fungicides to prevent/manage gray mold in the greenhouse should rotate FRAC (Fungicide Resistance Action Committee) codes or modes of action to limit the development of resistant strains or biotypes.

Resistant strains of Botrytis cinerea to fungicides containing fenhexamid, thiophanate methyl, and iprodione are well documented. The threat of resistance to FRAC Group 11 materials (strobilurins) by strains of Botrytis cinerea is recognized by plant pathologists so growers should avoid applying Group 11 fungicides successively.

Some of the fungicides that are labeled for use in managing Botrytis cinerea in the greenhouse can be found in the table below. (Source: New England Greenhouse Floriculture Guide).

Before mixing and using any pesticide please read the label carefully and follow the label as directed. *The table below should be used only as a guide when selecting products for Botrytis cinerea management. Listed application rates were derived from the 2021–2022 New England Greenhouse Floriculture Guide. Prior to applying a fungicide for Botrytis cinerea management, determine the actual application and mixing rates directly from the pesticide label.