Optimizing Cercospora Leaf Spot Control in Table Beet Using Action Thresholds and Disease Forecasting

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/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 area under the disease progress stairs compared with other fungicides tested and was most efficacious when applications began at 1 CLS lesion/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 with 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. Thus, there was substantial scope to reduce spray frequency by scheduling based 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.

Weather-Based Predictive Modeling of Cercospora beticola Infection Events in Sugar Beet in Belgium

“…Weather-based models in which continuous rainy conditions were given less importance (models M13 and M14; Table 4 ) were among the worst performing models ( Table 5 ). Although there are models relying only on temperature and relative humidity to predict C. beticola infection and CLS progress (e.g., [ 28 , 36 ]), in regions with weather conditions similar to those across the study region in Belgium, attention must be paid to continuous rainy hours before a likely infection event by C. beticola (i.e., when optimum temperature and relative humidity conditions are met). In this study, a threshold of R ≥ 0.1 mm/h during the four preceding hours, associated with optimum RH and daytime and nighttime T, was found to provide the most favorable conditions conducive to C. beticola infection events.…”

“…The main method used to control CLS in sugar beet is regular application of fungicide. Improving fungicide-based disease management (and reducing fungicide usage) requires an approach relying on weather-based disease risk modelling rather than a growth stage-based or fixed-calendar schedules [ 8 , 36 ]. The results of our study are based on three sugar beet cropping seasons.…”

“…We further analyzed the proportions of hours with dominant classes of RH and T, associated with rainy conditions (R ≥ 0.1 mm/h), to determine those favorable weather conditions conducive to CLS at the study sites. The definition of intervals of weather variables in this step was based on previously reported favorable weather conditions for CLS [ 2 , 5 , 15 , 18 , 36 ].…”

A phytopathometry glossary for the twenty-first century: towards consistency and precision in intra- and inter-disciplinary dialogues

“…A perusal of the plant pathology literature will demonstrate that many authors already use these various metrics as measures of severity as described in numerous journal articles. For example, Pethybridge et al (2020) and Cowling and Gilchrist (1980) referred to lesion counts as disease severitythe latter authors also used lesion size as a measure of severity. Quantitative ordinal scales based on intervals of the percentage scale to determine quantity of disease are well known and widely used, with the stated purpose of rating disease severity (Horsfall and Barratt 1945;Kousik et al 2018;Urrea and Harveson 2014).…”

Screening Table Beet and Swiss Chard for Resistance to Pseudomonas syringae pathovar aptata

“…Similarly stringent requirements may be found for Swiss chard production. Additionally, table beets are often harvested by top-pulling machinery (Pethybridge et al, 2017(Pethybridge et al, , 2020. If greens are too damaged by disease, the tops will be pulled off the roots so that the roots will not be harvested, leading to direct losses for growers (Pethybridge et al, 2017(Pethybridge et al, , 2020.…”