Biorational fungicides reduce powdery mildew development on hemp without affecting cannabinoid content

Powdery mildew, caused by the fungus Golovinomyces ambrosiae, is the among the most prevalent diseases of hemp (Cannabis sativa) in the United States, and its management is crucial to support hemp grown for cannabinoids. Several fungicides have been registered for the control of hemp powdery mildew and are important tools to manage G. ambrosiae, but to date there have been few studies of their effects on cannabinoid yields. Here, five disease control treatments were applied to susceptible hemp cultivars, and disease severity ratings (0 to 100%) were captured to calculate the area under the disease progress curve (AUDPC). Products with the active ingredients Bacillus amyloliquefaciens strain D747, potassium silicate, and a rotation between Bacillus mycoides isolate J and B. amyloliquefaciens strain D747, effectively reduced powdery mildew severity compared to the untreated control in the two consecutive field studies. An analysis of the cannabinoid composition of extracts from all the fungicide-treated plots, as well as uninoculated controls, revealed that neither biological fungicide treatments nor G. ambrosiae infection affected the profile of the cannabinoids Δ9-tetrahydrocannabinol (THC), cannabidiol (CBD), cannabigerol (CBG), and cannabichromene (CBC). These results suggest that fungicides may provide an effective strategy to control G. ambrosiae without compromising end-use product quality.