Early Cercospora beticola detection and germination in commercial sugarbeet fields.
Cercospora leaf spot (CLS) cause by Cercospora beticola (Cb) continues to be an important sugarbeet disease in all production areas in the world. Most CLS forecasting models predict conditions favorable for disease spread and application of fungicides after disease is present, but do not include conditions influencing spore production and germination that may influence early fungicide application before infection by Cb. Because we observed spore germination in Cb cultures stored at 10C, we conducted laboratory experiments to determine how temperature and moisture affect spore germination. Across all treatments, spore germination is higher in free water (95%) compared to 100% RH (30%). Germination begins in two hours at 10°C and increases with time and temperature. Across all treatments, spores from fungicide resistant isolates have a lower percent germination compared to fungicide sensitive isolates, and significantly lower percent spore germination at temperatures <14°C but not at >18°C. This implies there may be fitness penalty for spore germination of resistant isolates at colder temperatures that disappears at warmer temperatures. Preliminary studies in sugar beet fields in ND and MN show that earlier fungicide applications improves CLS control. Field experiments in 2021 (a dry spring) and 2022 (a wet spring) to were conducted to study timing of Cb spore detection and infection using spore traps (Spornado) and weather stations with leaf wetness sensors in six commercial sugarbeet fields. Filter cartridges collected three times per week from early May to early August were tested for the presence of Cb DNA by PCR. From early June to late July of 2021, asymptomatic leaves from 57 commercial fields were tested for the presence of Cb DNA by PCR. Some sites were monitored for appearance of CLS spots. In 2021, first Cb spores detection in some field traps was May 3, Cb DNA first detected in asymptomatic leaves June 14 and 17, and CLS spots first observed June 29. In 2022, a wet time, Cb first spore detection was May 23 in all spore traps and continued in all locations until completion of trapping on July 8. Spore DNA was tested for sensitivity/resistance to QoI and DMI fungicides. Based on our studies, we conclude that forecasting models for CLS should include spore detection and early weather conditions, and adjusted to recommend fungicide applications earlier in the growing season before infection by Cb. This study also confirms the utility of Cb spore trapping and PCR detection of infection by Cb before CLS is present.