By C. M. Ocamb
Cause The fungi, Aspergillus flavus, A. niger, A. ochraceus, and other Aspergillus species can infect leaf, stem, petiole, and flower parts of cannabis as well as numerous other plant species including corn, cottonseed, peanut, and tree nuts. Aspergillus species can also be associated with seeds of plants, including hemp. Certain species, including A. flavus, can produce mycotoxins known as aflatoxins. Aflatoxins are poisonous to humans and other animals including birds, cats, cattle, dogs, sheep, and swine. These fungi are common in soil and survive on residues of infected plants. They produce asexual spores (conidia) and sclerotia on or in plant material when conditions are conducive; sometimes ascospores (sexual spores) are also formed. Sclerotia eventually end up in the soil as infected plant material decomposes in the field. Under conducive environmental conditions, sclerotia can activate and produce conidia or ascospores. Conidia can be moved by wind and water.
Symptoms Cannabis leaves and buds typically have a brown coloration which then changes to a dark green to black coloration as sporulation occurs.
Cultural control
- Trim flowers after the harvest (vs wet trimming).
- Discard insect- and pathogen-infested materials because these materials may contain Aspergillus and may promote pathogen growth.
- Use appropriate drying method that provides uniform drying (ex. hot air drying).
- Do not store unprocessed samples in an uncontrolled environment. Incorporating HEPA filtration and UV lighting in HVAC systems can be helpful. Regular maintenance and monitoring of air quality are important.
- Avoid using plastic materials for storing and packaging to reduce pathogen growth.
References McPartland, J.M., Clarke, R.C., and Watson, D.P. 1999. Hemp Diseases and Pests: Management and Biological Control. CABI Publishing, New York, New York.
Punja, Z.K. 2018. Flower and foliage-infecting pathogens of marijuana (Cannabis sativa L.) plants. Can. J. Plant Path. 40(4):514-527. https://doi.org/10.1080/07060661.2018.1535467
Punja, Z.K. 2021. The diverse mycoflora present on dried cannabis (Cannabis sativa L., marijuana) inflorescences in commercial production. Can. J. Plant Path. 43(1):88-100. https://doi.org/10.1080/07060661.2020.1758959
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. Frontiers Pl. Sci. 10:1120.
Punja, Z.K., and Ni, L. 2021. The bud rot pathogens infecting cannabis (Cannabis sativa L., marijuana) inflorescences: symptomology, species identification, pathogenicity and biological control. Can. J. Plant Path. 43(6):827-854. https://doi.org/10.1080/07060661.2021.1936650