Optimizing Cercospora leaf spot control in table beet using action thresholds and disease forecasting

Authors: PETHYBRIDGE, SARAH - Cornell University; SHARMA, SANDEEP - Cornell University; HANSEN, ZACHARIAH - University Of Tennessee; KIKKERT, JULIE - Cornell University; OLMSTEAD, DANIEL - Cornell University; Hanson, Linda

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2020
Publication Date: 3/4/2020
Citation: Pethybridge, S.J., Sharma, S., Hansen, Z., Kikkert, J.R., Olmstead, D.L., Hanson, L.E. 2020. Optimizing Cercospora leaf spot control in table beet using action thresholds and disease forecasting. Plant Disease. 104:1831-1840. https://doi.org/10.1094/PDIS-02-20-0246-RE.
DOI: https://doi.org/10.1094/PDIS-02-20-0246-RE

Interpretive Summary: Cercospora leaf spot (CLS) caused by the fungus, Cercospora beticola, is the dominant foliar disease affecting table beet production in New York. CLS epidemics occur annually and if uncontrolled will rapidly lead to defoliation. In broad-acre production, season-long maintenance of healthy leaves is important to facilitate harvest by top-pulling. Fungicides are the dominant means of CLS control. Currently treatments are started at an action threshold of 1 CLS lesion per leaf. After this first application, additional treatment is done on a calendar basis, without consideration for disease risk. The current study was to examine both the efficacy of the action threshold with different fungicides and to assess the potential to use a risk-based model to improve fungicide timings. For two of the tested fungicides, there was significantly improved CLS control if treatment was done before the pathogen was present. For the third fungicide tested, the current application threshold was the most effective. Two risk thresholds (moderate and high) were tested with a weather-based risk model from sugar beet and compared to the calendar-based spray schedule. Applications of the fungicides pydiflumetofen + difenoconazole were reduced from three to two by using the forecaster at either risk threshold compared to calendar applications without reducing CLS control in two years and six locations. For propiconazole, the moderate risk threshold provided CLS control equivalent to calendar-based applications and saved one spray per season, but the high-risk threshold showed reduced CLS control. This shows there is an opportunity to reduce spray frequency by scheduling on weather-based risk rather than calendar applications, but different fungicides vary in the optimal risk threshold. The action thresholds and risk thresholds need to be assessed for each fungicide to be used for optimal disease management, but there are opportunities for improving disease control and potentially reducing fungicide use with these tactics.

Technical Abstract: Cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, is the dominant foliar disease affecting table beet production in New York. CLS epidemics occur annually and if uncontrolled will rapidly lead to defoliation. In broad-acre production, season-long maintenance of healthy leaves is important to facilitate harvest by top-pulling. Fungicides are the dominant means of CLS control and applications are initiated at an action threshold of 1 CLS lesion per leaf. Regular fungicide application occurs thereafter without regard for scheduling based on weather-based risk. The current action threshold was evaluated with selected fungicides in two replicated field trials. Copper oxychloride + copper hydroxide and propiconazole significantly improved CLS control if initiated prior to infection. Pydiflumetofen + difenoconazole significantly reduced AUDPS compared to other fungicides tested and was most efficacious when applications began at 1 CLS lesion per leaf. Six replicated field trials also evaluated the utility of scheduling fungicides on weather-based risk rather than a calendar approach. Two risk thresholds (moderate and high) integrating the accumulation of daily infection values based on temperature and relative humidity from a forecaster for CLS in sugar beet were evaluated. Applications of pydiflumetofen + difenoconazole were reduced from three to two by using the forecaster at either risk threshold compared to calendar applications without affecting CLS control. For propiconazole, the moderate risk threshold provided CLS control equivalent to calendar applications and saved one spray per season. There also therefore substantial scope to reduce spray frequency by scheduling on weather-based risk rather than calendar applications. The optimal risk thresholds for pydiflumetofen + difenoconazole and propiconazole were high and moderate, respectively. In these trials, periods of high risk occurred less frequently than moderate risk, increasing the reapplication intervals and hence represented a less conservative approach to disease management.