Efficacy of organic fungicides against hemp powdery mildew caused by Golovinomyces ambrosiae in a greenhouse in Tennessee.

Powdery mildew is a common disease of hemp in greenhouses in Tennessee. Fungicide efficacy data can support the use or approval of new fungicides. Therefore, two greenhouse experiments were conducted to assess the efficacy of nine commercial fungicides against powdery mildew. 'BaOx2' or 'Sweetened' hemp cultivars, which are susceptible to powdery mildew, were inoculated with a conidial suspension one day before or one or seven days after the first fungicide application. Two additional fungicide applications were made at 7-day intervals by thoroughly spraying the plants using a hand-held sprayer. Control plants were sprayed with water only. Weekly disease incidence and severity ratings were done three times. Disease index (DI) and the Area Under the Disease Progress Curve were calculated. All the fungicides significantly reduced powdery mildew symptoms. Compared to the control plants, disease reductions ranged from 76% to 100%. Bonide sulfur, Luna Experience, and MilStop exhibited "excellent" efficacy, reducing disease by 96% to 100%. Cinnerate, Exile, Regalia, and Sil-Matrix exhibited "very good" efficacy, reducing disease by 86% to 95%. Defguard and Stargus exhibited "good" efficacy, reducing disease by 76% to 85%. Koch's postulates were performed with the PM isolate used in this study. The isolate recovered following inoculation was found to be identical to the original isolate used in the experiments. The isolate was identified as Golovinomyces ambrosiae based on ITS, IGS, and β-tubulin sequencing and phylogenetic analysis with nucleotide sequences from closely related species. These findings provide useful information for the control of hemp powdery mildew and further research.