Botrytis Gray Mold
by Josh Gibson, Sarah Williams & Francesca Peduto Hand
Gray mold, caused by the fungus Botrytis cinerea, is the most common disease of herbaceous ornamentals. It can affect almost every type or variety of floral crop, as well as many other types of plants. Depending on the host and the conditions under which the crop is grown, this disease can either be a common nuisance or an economic disaster. Symptoms of gray mold vary depending on the host, the environmental conditions associated with the host, as well as the plant’s growth stage and may include:
- pre- and post-emergence damping off
- leaf/flower spots
- leaf/flower blights
- bud rots
- stem cankers
- stem and crown rots
- cutting rots
- plant death (extreme cases)
Necrotic tissues are an excellent substrate for the prolific sporulation of the fungus, which, when conditions of high relative humidity (at or above 85 %) prevail, appears as a fuzzy gray mold.
For assistance in identification, contact the C. Wayne Ellett Plant and Pest Diagnostic Clinic.
Botrytis Gray Mold is often considered a disease of bad management. Appropriate crop management techniques and manipulation of the greenhouse environment can reduce the impact of this disease in most cases without the need of fungicide applications.
- Cultural Practices: Sanitation practices before, during, and after each cropping cycle are the first important step to achieve good control. Plants with wounds should be removed from the greenhouse, as the wound is the perfect environment for the fungus to initiate the infection process. Petals falling from hanging basket plants may encourage the growth of the fungus on plants at ground level. Senescing flowers and leaves and infected plant material should be removed from the greenhouse so that it is not a source of inoculum for the rest of the house. Infected plant material should not be allowed to sit in trash cans within the house as the fungus will continue to grow and sporulate on the dead and dying tissue. Subsequent opening and closing the trash cans will produce enough air movement to release spores out into the greenhouse. Maintaining an environment within the greenhouse that will not permit the fungus to grow and sporulate is essential for control. To this extent, excellent control can be achieved by keeping the relative humidity below 85%. Proper plant spacing is important to allow better air circulation and to reduce relative humidity within the plant canopy. Formation of free moisture on plant surfaces should be avoided, so fans should be used to provide good air movement above the canopy. Overhead watering is discouraged, as the water droplets will cause the spores to become airborne allowing for further infections to occur.
- Chemical Control: A variety of fungicides are available to control gray mold in the greenhouse. However, because some Botrytis populations have developed resistance to certain chemicals, it is recommended not to rely on the use of a single chemical or on multiple chemicals with the same mode of action (check label for FRAC group number). Fungicides with different modes of action can be mixed simultaneously or used in rotation so that the fungus does not develop resistance to one particular chemical. You should contact Extension specialists in your county for the chemical treatment that may be right for your particular situation.
- Biological Control: Several biological control agents are also available and are known to be effective to control gray mold in the greenhouse. These include Bacillus subtilis, Streptomyces lydicus, Streptomyces griscoviridis, and Trichoderma harzianum.