By C. M. Ocamb
Cause The fungus, Alternaria alternata, and potentially other Alternaria sp. can infect leaves, petioles, stems, and flower parts of Cannabis as well as numerous other plant species. This fungus survives on residues of infected crops and weeds, producing asexual spores (conidia) when conditions are conducive and the debris is on the soil surface. The conidia can be moved by wind and water. Conidia produced on infected plant debris are considered the primary inoculum for disease outbreaks.
Symptoms Cannabis leaves and buds first turn a brown coloration on the outer surfaces which then changes to a dark brown to black coloration as sporulation occurs. Small, dark or yellow spots can develop on leaves, enlarging to circular areas that are brown to gray in color with or without concentric rings and possibly with black or purple borders or surrounded by yellow halos. The centers of leaf spots may be coated with sooty black spore masses. Leaf spot size is quite small on hemp leaves, being barely visible (0.1 inch or smaller diameter), observed in OSU surveys during 2021. Leaf spots can be larger, especially when the relative humidity is high.
Cultural control
- Improve air circulation and minimize leaf wetness.
- Remove and destroy infected leaves.
Biological 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
References 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.
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.
Scott, M., Charron, J.-B., Jabaji, S., Rani, M., and Samsatly, J. 2018. Endophytes of industrial hemp (Cannabis sativa L.) cultivars: identification of culturable bacteria and fungi in leaves, petioles, and seeds. Can. J. Microbiol. 64(10):664-680.