Optimizing fungicide application timing for spring dead spot based on soil temperature and season

Spring dead spot (SDS) ( Ophiosphaerella spp.) is the most detrimental disease to warm‐season turfgrasses in areas with cold‐induced dormancy. Fungicide applications do not provide consistent SDS suppression. One reason for this inconsistency is the use of solely calendar‐based fungicide applications instead of considering both calendar date and soil temperature. A field study was conducted at three separate hybrid bermudagrass ( Cynodon dactylon (L.) Pers. × transvaalensis Burtt Davy) locations in Virginia to determine the optimal soil temperature and timing for SDS suppression with tebuconazole and isofetamid. Tebuconazole (1.5 kg a.i. ha −1 ) and isofetamid (4.1 kg a.i. ha −1 ) were applied at 11 different timings throughout the year based on soil temperatures at a 0‐ to 10‐cm depth. Plots were assessed for SDS severity in the spring and early summer of 2021 and 2022. Two in vitro studies were also conducted with Ophiosphaerella herpotricha and Ophiosphaerella korrae isolates to (1) determine the optimal temperature for growth on potato dextrose agar (PDA) placed on a thermogradient table (13–33°C) and (2) compare the daily growth rate of O. herpotricha and O. korrae isolates at 11, 19, and 27.5°C on PDA. In the field study, isofetamid suppressed SDS more than tebuconazole. Fall applications when soil temperatures were 13°C consistently provided the best SDS suppression. For the in vitro studies, both species grew optimally between 24 and 25°C, yet O. korrae and O. herpotricha growth rates differed at 11°C. Fungicide applications for spring dead spot should be made based on both soil temperature and time of year. The most effective fungicide application timing varies between the Ophiosphaerella species that cause spring dead spot. Isofetamid suppresses spring dead spot more than tebuconazole. Spring dead spot is a damaging disease of bermudagrass. Fungicides are an effective tool for management of the disease, but optimal application timing can be challenging. This research provides information on proper fungicide application timing for spring dead spot suppression in bermudagrass. Our findings show that applications made in the fall when soil temperatures are near 13°C provide the greatest disease control.