Advancing control of Stemphylium vesicarium on asparagus fern using TOMCAST and new fungicides

Stemphylium vesicarium, the causal agent of purple spot disease on asparagus, renders spears unmarketable and causes premature defoliation of the fern, reducing subsequent spear yield. The fungicides azoxystrobin, mancozeb, and chlorothalonil are registered to control purple spot on the fern and may be applied according to the TOMCAST disease forecaster or a calendar-based schedule. Registered fungicides were compared to the non-registered fungicides pydiflumetofen + fludioxonil and fluxapyroxad + pyraclostrobin, each alternated with chlorothalonil, and were applied according to TOMCAST at a threshold of 15 or 20 disease severity values (DSV) or every 10 days or not treated (control). The 10-day treatment received eight applications each year, TOMCAST 15 DSV received eight (2022) or six (2023); TOMCAST 20 DSV treatment received six (2022) or four (2023). Mancozeb applied every 10 days (2023 only) or azoxystrobin applied every 10 days or according to TOMCAST 15 DSV had less disease than the control at the final assessment. Pydiflumetofen + fludioxonil and fluxapyroxad + pyraclostrobin had less disease than the control at the final assessment at all intervals. Relative area under the disease progress curve (rAUDPC) data indicated that all treatments limited purple spot disease compared to the control in 2022 and 2023 except for mancozeb applied at TOMCAST 20 DSV (2022). The rAUDPC data were similar at all application timings for pydiflumetofen + fludioxonil and fluxapyroxad + pyraclostrobin each year. Results indicate TOMCAST 20 DSV can be used with locally systemic, highly efficacious, fungicides to optimize spray applications to protect asparagus fern.