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​​Figure 1 Advanced symptoms of gray mold include brown, dead tissue, especially in buds.  Gray mold is identified by the gray masses of spores produced in dense growth or on damaged or senescing tissue.  Gray mold is different from powdery mildew in color and tissue affected.

Figure 2 Stem infections and damping off disease of seedlings is often the result of wounds such as pruning cuts or insect damage.  The botrytis pathogen is opportunistic, and it requires an entry site to infect.

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A Scientific Look at Diseases of Kentucky Hemp


Disease Management Guide to Gray Mold in the Greenhouse 

                 By:  Dr. Nicole Gauthier

Botrytis cinerea

Hemp (Cannabis sativa x indica) is grown both outdoors and in greenhouses.  High humidity greenhouses and other closed environments are often ideal for fungal diseases such as gray mold.  Outdoor-grown hemp is susceptible during periods of high humidity or rain.

Symptoms.  Gray mold develops primarily in flower buds and tightly-packed plant parts, and it is recognized by its gray moldy growth.  Masses of mycelia (fungal strands) and clusters of conidia (asexual spores that are produced in large numbers) make up the dense gray mats.  The fungus is commonly found in and between buds where microclimates are humid and air flow is limited.  Severe infections can lead to yield loss as buds and flowers become infected.   

While moldy buds are the most common disease phase, stems, petioles, and growing tips can also be affected.  Pruning cuts, insect damage, and other openings are ideal for the opportunistic fungus.  Once the fungus colonizes, it can girdle stems and cause them to break at the point of infection.  Seedling infections and damping off can also result under humid conditions and in greenhouses with high levels of fungal contamination.  Damping off in seedlings causes plant collapse.

Causal Agent.  The fungus Botrytis cinerea is the causal agent of gray mold.  It is ubiquitous, in that it is ever-present.    Infection by the Botrytis fungus is dependent upon a wound or opening in plant tissue.  Once infection takes place, though, the fungus can move from cell to cell, killing tissue as it spreads. 

An single bud can host up to 1 million infective spores as humidity rises above 85%.  These spores are carried by air currents to healthy plant material; fans, wind, and mechanical means can move spores, setting off a repeating disease cycle.