Botrytis blight, or gray mold, is a fungal disease that is widespread in the United States and globally. This fungus spreads via spores in the air and can result in economic losses if not managed early. In North Carolina, this pathogen is most threatening in the spring when temperatures are cool.

Botrytis blight is caused by many species of Botrytis, the most common of which is Botrytis cinerea.

This fungus has the capacity to affect a wide variety of ornamental plants including calibrachoa, geranium, annual vinca, poinsettia, cineraria, and many others.

Botrytis cinerea has the capacity to affect essentially every above ground part of the infected plant when conditions are favorable for disease development and preventative measures are not taken. Gray mold is often found on leaves in the densest part of the canopy. It may also be found sporulating on dead flowers or other senescent tissue (Figure 1 and Figure 2). This is often where Botrytis gets its start, later spreading to younger tissue in contact with the dead blossoms. Symptoms of a Botrytis infection may include lesions on stems or petioles, leaves that may appear dark-colored or gray, and—on flowers—white or light-colored flecks may appear on petals. Additional symptoms include soft rots, and irregular tan to gray lesions that form prior to sporulation on leaves and stems. Under high humidity, a gray fuzzy mold appears on the affected plant parts (Figure 3). This is the sporulation of the fungus. Botrytis also can cause damping off on newly emerging seedlings.

Overwintering structures (sclerotia) can also form on affected plant tissue (Figure 4). Sclerotia are black in color and appear flat or raised.

Figure 1. Dead petunia flowers infected with Botrytis sp.

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Figure 1. Dead petunia flowers infected with Botrytis sp.

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Figure 2. Annual vinca plants with a heavy infection of Botrytis blight

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Figure 2. Annual vinca plants with a heavy infection of Botrytis blight

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Figure 3. Sporulation of Botrytis sp. on a hellebore stem (captured at 8X magnification).

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Figure 3. Sporulation of Botrytis sp. on a hellebore stem (captured at 8X magnification).

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Figure 4. Sclerotia of Botrytis sp. formed on hellebore stem (captured at 8X magnification).

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Figure 4. Sclerotia of Botrytis sp. formed on hellebore stem (captured at 8X magnification).

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Certain symptoms of Botrytis blight can resemble those of other conditions and diseases that affect greenhouse ornamentals, including Myrothecium leaf spot (Figure 5). Myrothecium roridum is a fungal pathogen that causes gray to brown leaf spots often with concentric rings that are initially similar to the leaf spots caused by Botrytis. However, these two pathogens can be differentiated based on signs of the pathogen: M. roridum produces tiny, globular structures (</= 2 mm diameter) with a dark black or olive green center (this is the mass of spores) surrounded by a white mat (mycelium) (Figure 6) whereas Botrytis forms dark colored spores that arise from mycelium (the vegetative thread-like structures) and appear as a “gray fuzz” (Figure 2, Figure 3, and Figure 6). On stems, symptoms of Botrytis could be confused with those caused by Rhizoctonia, Sclerotinia, or even Berkeleyomyces. In the absence of sporulation, these other diseases may be difficult to differentiate from Botrytis blight, but Botrytis can be encouraged to sporulate by placing in a humid chamber (plastic box with some moisture for humidity) kept at room temperature for 24 to 48 hours.

Figure 5. Impatiens with Myrothecium leaf spot.

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Figure 5. Impatiens with Myrothecium leaf spot.

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Figure 6. Sporodochia (blackish colored structures with white mats) at the base of a pansy plant with Myrothecium crown rot.

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Figure 6. Sporodochia (blackish colored structures with white mats) at the base of a pansy plant with Myrothecium crown rot.

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Resident populations of B. cinerea in a greenhouse can remain dormant until conditions and suitable host material are available. It can also come in with infected plants. Sclerotia overwinter on plant debris, leading to Botrytis blight outbreaks in early spring. Optimum conditions for the growth of B. cinerea are temperatures from 60-70°F coupled with high relative humidity (>85%). When temperatures are greater than this range and/or when humidity is low, gray mold does not develop. Spores of B. cinerea germinate under optimal conditions and can gain entry through wounds and natural openings or mycelial growth from infected plant tissue. After a plant is infected, mycelia (vegetative thread-like structures that obtain nutrients) grow inside the host tissue causing lesions and tissue death. Under favorable conditions, the fungus sporulates, and these spores spread via air and water splash to infect other susceptible plants. After B. cinerea has become successfully established, it is easy for it to infect an entire production greenhouse. Botrytis cinerea cannot be eradicated because of its ablity to survive in greenhouses in the absence of host plants and because it is ubiquitous in the environment.

The key to managing Botrytis blight is avoiding the conditions that are conducive to this disease: high humidity (85% or higher) and cool temperatures (60-70°F). Outside of these conditions, Botrytis blight does not develop well, if at all. In greenhouses in NC, January through April appears to be “peak season” for Botrytis blight based on samples submitted to the NC State Plant Disease and Insect Clinic. However, Botrytis blight can occur anytime of the year when conducive conditions are present (e.g. following several days of rain).

Avoid Excessive Periods of Leaf Wetness and High Humidity

Drip irrigation is recommended to avoid prolonged periods of leaf wetness and spore dispersal via water splash. If you must overhead irrigate, do so at a time when foliage will dry quickly. Botrytis blight can be managed and reduced by avoiding prolonged periods of leaf wetness and avoiding periods of high humidity longer than 3 hours per day.

Ensure Good Air Circulation

Adequate air circulation, even when humidity is high, can greatly reduce the development of gray mold. Ensure there are no “dead air” spots in the greenhouse by adjusting fan placement.

Inspect Plants Often

Scouting plants regularly helps catch and manage disease problems early.

Remove Dead Plant Tissue

Botrytis tends to gain entry into the plant through necrotic areas or dead flowers, so care should be taken to remove dead portions of the plants and dispose of them outside of the greenhouse or facility. Because dead plant tissue provides a substrate for the fungus to sporulate and survive long-term, it is important to remove dead plant tissue to prevent an outbreak during the season and in the following season.

Maintain Healthy Plants

Botrytis blight development primarily occurs on unhealthy, wounded, stressed, or inactive plants, as they are the most susceptible to infection. Maintaining vigorous plants using appropriate fertilization and irrigation practices can reduce the development of gray mold.

Host Resistance

Host resistance to Botrytis blight is uncommon because of the “generalist” nature of the pathogen and the frequent changes in cultivar demand; however, Shrestha and Hausbeck (2021) identified less susceptible varieties of geranium: ‘Horizon Coral Spice’, ‘Pinto Premium Orange’, and ‘Ivy Tornado White’. Although susceptibility of cultivars within other plant species is largely unknown, greenhouse growers may observe differences in susceptibility among varieties or plants they grow.

Preventative Fungicide Program

A rotation of fungicides can help to successfully manage Botrytis blight if the above cultural practices are employed. Fungicides alone will not effectively manage this disease. In addition, Botrytis species are known to develop resistance to multiple FRAC groups, so it is extremely important to rotate fungicides.

Conventional growers should incorporate the management practices listed above in addition to a fungicide spray program. Please note: Many strains of Botrytis cinerea have developed resistance to multiple fungicide modes of action listed below. It is extremely important to rotate FRAC numbers to avoid resistance in this pathogen. In general, FRAC groups with “M” are broad-spectrum and the risk of fungicide resistance to these fungicides is low. On the other hand, FRAC groups with a single number (no “M") are single-site fungicides and the risk of fungicide resistance is high. Based on a recent study by Lukasko and Hausbeck (2023), fungicide resistance is more common to FRACs 1, 2, 7, and 11. See pest alert “Fungicide resistance in Botrytis: A reminder for floriculture crop producers” from March 2024 for more details.

The most effective way for growers to avoid Botrytis blight is to employ the cultural management practices mentioned above. Various biorational products including Serenade Opti, Serifel, Botector, Zio, and Ecoswing have been proven effective in reducing the number of blighted leaves, foliar lesions, and leaves with B. cinerea sporulation when combined with the least susceptible geranium cultivars (e.g., Pinto Premium Orange) (Lukasko and Hausbeck, 2023). PreStop and Regalia (both OMRI-listed) have shown some efficacy as reported in research reports by the IR-4 Project (Environmental Horticulture), but results were variable among experiments.

The NC State Extension Plant Disease and Insect Clinic provides diagnostic services and management recommendations.

The NC State Extension Plant Pathology Portal provides information on crop disease management including pest news and alerts.

The NC Agricultural Chemicals Manual is updated annually and provides pesticides labeled for use on ornamentals.