By N. K. Osterbauer and C. M. Ocamb
Cause The fungus, Botrytis cinerea (teleomorph: Botryotinia fuckeliana), is an opportunistic pathogen that easily invades weak, damaged, or senescing tissue. This fungus is very common in the Pacific Northwest and occurs worldwide in greenhouses. Cannabis can be killed within one week. Spores (conidia) are produced throughout the growing season under a wide range of temperatures and humidity; and stem infections are a durable inoculum source, producing conidia throughout bloom. Cool temperatures (68°F is optimal but the fungus can grow between 50°F and 80°F), high humidity, and free water on plant surfaces favor the disease, as do close plant spacing and irrigation practices that keep plants wet for a longer time. Durable sclerotia overwinter in soil.
Symptoms The pathogen causes brown, water-soaked spots on buds or chlorotic areas on stems. Buds eventually rot producing a gray-brown mass of spores; infections start within buds. Botrytis cankers develop to the point of limb breakage or stem splitting, especially if plant canopies are dense and heavy. Small, black sclerotia can develop within stem tissues. Like the buds, cankers produce gray-brown masses of spores. Seedlings can also be killed.
Cultural control
- Limit irrigation during and after bloom. Irrigate in the morning so plants do not stay wet more than 12 hours.
- In greenhouses, maintain the relative humidity below 50%, temperatures warm, and high light intensity. Filtering out UV light may prevent epidemics since sporulation requires UV light.
- Avoid over-fertilization with nitrogen.
- Canopy management is the best preventive method.
Chemical control
- LifeGard WG (Group P6) at 1 to 4.5 oz/A on 3- to 14-day intervals for activating plant resistance. Refer to label for appropriate rate per application volume. Preharvest interval is 0 days. 4-hr reentry. O
- Regalia (Group P5) at 1 to 4 quarts/A on 7- to 10-day intervals. Does not benefit from the addition of an adjuvant. Studies by OSU show efficacy of this formulation for gray mold control. Preharvest interval is 0 days. 4-hr reentry. O
References Bates, T.A., M. Dietrich, and C.M. Ocamb. 2021. Evaluation of biofungicides for gray mold on hemp in Oregon, 2020. Plant Disease Management Reports: Report No. 15:V048.
Punja, Z.K., Collyer, D., Scott, C., Lung, S., Holmes, J., and Sutton, D. 2019. Pathogens and molds affecting production and quality of Cannabis sativa L. Front. Plant Sci. 10:1120. DOI:10.3389/fpls.2019.01120
Ward-Gauthier, N., Beale, J., Amsden, B., and Dixon, E. 2015. Greenhouse hemp in Kentucky exhibits many common diseases. American Phytopathological Society Annual Meeting, Poster 502-P.