Botrytis cinerea, commonly known as Gray Mold, is a pervasive fungal pathogen that causes significant damage across a vast range of hosts. This fungus is a necrotroph, meaning it primarily attacks aging, wounded, or dead plant tissue, quickly spreading its characteristic fuzzy, gray spores under favorable conditions. The visible mold is the symptom, but the true threat lies in its ability to infect and survive unnoticed until conditions allow for a rapid outbreak. Effective management relies almost entirely on preventative strategies, as treatment is difficult once the infection is established.
Environmental Management for Disease Avoidance
The fungus requires specific environmental conditions to germinate and initiate infection. Botrytis spores need moisture and a high relative humidity, often exceeding 93%, along with a film of water on the plant surface for several hours to successfully colonize the host. Optimal temperatures for active growth range from 18 to 24 degrees Celsius (64–75 degrees Fahrenheit).
Controlling humidity and eliminating surface moisture are effective strategies to prevent spore germination. Growers should aim to water plants early in the day, allowing foliage to dry completely before nightfall. This prevents leaf surfaces from remaining wet during the cooler overnight period, which can lead to condensation.
Air circulation is equally important, as stagnant conditions trap moisture and create microclimates conducive to fungal growth. Strategic plant spacing and the use of horizontal air flow fans in enclosed environments reduce boundary layer humidity around leaves and flowers. Avoiding rapid temperature drops, particularly in greenhouses, helps prevent the formation of dew or condensation on plant surfaces, which the fungus uses as a pathway for infection.
Essential Cultural Practices
Cultural practices, involving physical maintenance and meticulous hygiene, are the most impactful long-term methods for disease suppression. Botrytis frequently exploits damaged or senescing plant parts as a food source. Therefore, removing any dead, dying, or infected plant material immediately eliminates potential entry points and sources of inoculum.
Sanitation involves carefully removing old flowers, fallen leaves, and damaged fruit from the growing area without scattering the fungal spores. If infected material must be removed, it should be placed into a sealed bag to prevent the gray, dusty spores from dispersing into the air. Tool hygiene is also important, requiring regular cleaning and sanitization of pruning shears and other equipment to avoid transferring spores from diseased to healthy plants.
Pruning and canopy management should focus on increasing light and air penetration within the plant structure. Thinning dense foliage reduces leaf-to-leaf contact and promotes faster drying after watering or rain events.
Careful handling of plants during maintenance and harvest is necessary, as the pathogen often uses physical wounds as an easy entry point. Damage caused by insects, mechanical operations, or rough handling during transplanting provides a direct route past the plant’s natural defenses. Managing fertilizer inputs, particularly avoiding excessive nitrogen, can help prevent the flush of tender, susceptible new growth that the fungus readily attacks.
Targeted Fungicide and Biocontrol Applications
When environmental and cultural controls are insufficient, or during periods of high disease pressure, chemical fungicides should always be used preventatively, applied before spore germination or infection is likely to occur. The Fungicide Resistance Action Committee (FRAC) assigns codes to these chemicals based on their mode of action against the fungus.
To maintain the effectiveness of chemical treatments, growers must rotate products with different FRAC codes. Since Botrytis develops resistance easily, alternating chemical classes prevents the pathogen population from becoming resistant. Multi-site fungicides, which attack the fungus in several ways, are often used as a foundation for a robust resistance management program.
Biological control agents (BCAs) offer a natural alternative or supplement to suppress the pathogen. These biocontrols, which include certain strains of bacteria like Bacillus amyloliquefaciens and fungi such as Trichoderma harzianum, work through several mechanisms. They may compete with Botrytis for space and nutrients on the plant surface, or they may produce antifungal compounds that inhibit the pathogen’s growth.
Some yeast and bacterial BCAs, such as Aureobasidium pullulans, are effective when applied to flowers, which are highly susceptible to infection. Natural plant extracts, including those derived from thyme or Melaleuca alternifolia, can also be used as protective barriers to inhibit spore germination on the plant surface.