Michigan is the United States’ leading producer of asparagus. Purple spot, a foliar diseasecaused by the fungal pathogen Stemphylium vesicarium, renders spears unmarketable and causes premature defoliation of the fern, impacting spear yield and quality. Purple spot is managed by fungicide applications to the fern according to the TOMCAST disease forecaster at 15 disease severity values (DSVs). The goal was to improve management recommendations by i) evaluating 12 fungicides, alternated with chlorothalonil for efficacy under field conditions and ii) evaluating 10-day, TOMCAST 15 DSV and 20 DSV application timings for four fungicides, alternated with chlorothalonil, in 2022 and 2023. Relative area under the disease progress curve (rAUDPC) data indicated that all treatments limited disease compared to the control except for fluopyram + pyrimethanil (2022), and pyrimethanil (2022). For industry standards, azoxystrobin had a lower disease severity than the control. Pydiflumetofen + fludioxonil and fluxapyroxad + pyraclostrobin consistently had the lowest rAUDPC among unregistered fungicides. Data of rAUDPC indicated that all treatments limited purple spot disease compared to the control in 2022 and 2023 except for mancozeb applied according to TOMCAST 20 DSV (2022). For the industry standards, only azoxystrobin applied every 10 days or according to TOMCAST 15 DSV had less foliar disease at the final rating than the control each year. The rAUDPC data for pydiflumetofen + fludioxonil or fluxapyroxad + pyraclostrobin were similar regardless of application timing each year. The TOMCAST 20 DSV treatment received 6 (2022) or 4 (2023) applications and the 10-day treatment received 8 applications each year. The registration of these fungicides could protect asparagus fern from purple spot and reduce the number of applications by 25 to 50% when used with TOMCAST 20 DSV compared to a 10-day treatment.
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