Efficacy of Organic Fungicides for Management of Hemp Septoria Leaf Spot

Septoria leaf spot, caused by the fungus Septoria cannabis, is among the most common and devastating diseases of field-grown hemp in New York State. Fungicide field trials were conducted in 2023 and 2024 to test products previously deemed efficacious and to test products of interest to New York State hemp growers. Five disease control products were tested in 2023, and 10 treatments were tested in 2024. Hemp cultivar TJ's CBD, with known susceptibility to S. cannabis, was inoculated with a single-spore S. cannabis isolate. The products were applied according to their labeled rate for 5 weeks, and five disease severity ratings (0 to 100%) were captured to calculate the area under the disease progress curve (AUDPC). Azoxystrobin provided the most effective control of Septoria leaf spot, with significantly lower AUDPC values than the untreated, inoculated control in both years. Other products had either variable efficacy or no efficacy against S. cannabis in either year. Cultural management strategies and host resistance may be key to controlling S. cannabis.