Optimal growth stage for applying fungicide on <i>Brassica carinata</i> to protect against yield loss due to Sclerotinia stem rot

Carinata (<i>Brassica carinata</i>) is an oilseed crop gaining global attention as a sustainable biofuel feedstock. Sclerotinia stem rot (SSR), caused by <i>Sclerotinia sclerotiorum</i>, has been identified as a major disease of concern for carinata in the southeastern United States. This study determined the optimal growth stage for a single fungicide application to control SSR and evaluated the relationship between SSR severity and seed yield among multiple varieties. A field trial was conducted over four seasons at the University of Florida Institute of Food and Agricultural Sciences North Florida Research and Education Center in Quincy. Treatments were arranged in a randomized complete block split-plot design with variety as the whole plot and the fungicide (Proline^®) application timing as the subplot. A disease severity index (DSI) was calculated from SSR assessments at several growth stages. The final DSI in non-treated plots increased from an average of 13.2 (± 10.1 SD) in 2018 and 12.5 (± 11.1 SD) in 2019 to an average of 49.2 (± 16.5 SD) in 2020 that remained at 40.4 (±10.9 SD) in 2021. The correlation of yield with DSI was significant (P ≤ 0.01) for varieties AGR044-321E (r = -0.49), Avanza 641 (r = -0.59), DH-040.342 (r = -0.28), and M-01 (r = -0.62). Early to full flowering was the optimal growth stage for a fungicide application to manage SSR on carinata, but seasonal differences in disease affected the magnitude of control. Effective management using well-timed fungicide applications can support the sustainability of carinata as a biofuel crop.