An in-field heat treatment to reduce Cercospora beticola survival in plant residue and improve Cercospora leaf spot management in sugarbeet

Abstract

Introduction: Sugarbeets account for 55 to 60% of U.S. sugar production. Cercospora leaf spot (CLS), primarily caused by the fungal pathogen Cercospora beticola, is a major foliar disease of sugarbeet. Since leaf tissue is a primary site of pathogen survival between growing seasons, this study evaluated management strategies to reduce this source of inoculum.

Methods: Fall- and spring-applied treatments were evaluated over three years at two study sites. Treatments included standard plowing or tilling immediately post-harvest, as well as the following alternatives to tillage: a propane-fueled heat treatment either in the fall immediately pre-harvest or in the spring prior to planting, and a desiccant (saflufenacil) application seven days pre-harvest. After fall treatments, leaf samples were evaluated to determine C. beticola viability. The following season, inoculum pressure was measured by monitoring CLS severity in a susceptible beet variety planted into the same plots and by counting lesions on highly susceptible sentinel beets placed into the field at weekly intervals (fall treatments only).

Results: No significant reductions in C. beticola survival or CLS were observed following fall-applied desiccant. The fall heat treatment, however, significantly reduced lesion sporulation (2019-20 and 2020-21, P < 0.0001; 2021-22, P < 0.05) and C. beticola isolation (2019-20, P < 0.05) in at-harvest samples. Fall heat treatments also significantly reduced detectable sporulation for up to 70- (2021-22, P < 0.01) or 90-days post-harvest (2020-21, P < 0.05). Reduced numbers of CLS lesions were observed on sentinel beets in heat-treated plots from May 26-June 2 (P < 0.05) and June 2-9 (P < 0.01) in 2019, as well as June 15-22 (P < 0.01) in 2020. Both fall- and spring-applied heat treatments also reduced the area under the disease progress curve for CLS assessed the season after treatments were applied (Michigan 2020 and 2021, P < 0.05; Minnesota 2019, P < 0.05; 2021, P < 0.0001).

Discussion: Overall, heat treatments resulted in CLS reductions at levels comparable to standard tillage, with more consistent reductions across year and location. Based on these results, heat treatment of fresh or overwintered leaf tissue could be used as an integrated tillage-alternative practice to aid in CLS management.

Keywords: Beta vulgaris (sugar beet); Mycosphaerellaceae; integrated disease management; integrated pest management (IPM); leaf residue; propane burner; sugar beet (Beta vulgaris L.); tillage alternative.

Figure 1

Mean area under the disease progress curve (AUDPC) values from Michigan field studies collected in (A) 2020, (B) 2021, and (C) 2022, the years following fall-applied treatments. Treatments included were a non-treated control, plow with a 3-m tandem disc set to invert soil 15 cm. immediately post-harvest, heat treatment using a propane-fueled burner (Multi-Trail Enterprises LLC) calibrated to heat foliage to 649-871°C at 1.6 kmph and 4.8 kmph prior to defoliation and a desiccant (Sharpen 0.07 L/ha, methylated seed oil 1% v/v, ammonium sulfate 2037 g/L) applied seven days pre-harvest. AUDPC values were calculated according to Madden et al. (2017) using severity ratings collected at 5 timepoints; ratings were based on the KWS severity scale (0-10) in 2020 and 2021 and the Agronomica severity scale (standardized to 0-10) in 2022. Bars represent the means of four replicate plots and error bars represent standard errors. Bars with the same letter were not significantly different based on Fisher’s Protected LSD (α = 0.05). Analyses were conducted within each year.

Figure 2

Mean Cercospora leaf spot severity progression on sugarbeet in (A) 2020, (B) 2021, and (C) 2022, following fall-applied treatments evaluated in Frankenmuth, MI. Treatments included a non-treated control, plow with a 3-m tandem disc set to invert soil 15 cm. immediately post-harvest, heat treatment using a propane-fueled burner (Multi-Trail Enterprises LLC) calibrated to heat foliage to 649-871°C at 1.6 kmph and 4.8 kmph prior to defoliation, and a desiccant (Sharpen 0.07 L/ha, methylated seed oil 1% v/v, ammonium sulfate 2037 g/L) applied seven days pre-harvest. In 2020 and 2021, CLS ratings were based on the KWS severity scale (0-10) in 2020 and 2021 where 0 is 0.1% severity (1-5 spots per leaf) and 10 is 50% severity. In 2022, the CLS ratings were based on the Agronomica (Battilani et al., 1990) severity scale standardized to 0-10 scale where 0 is completely healthy foliage and 10 is completely destroyed original foliage with respect to CLS. Each point represents a mean of four replicate field plots. Date axes start at the planting date and end at the harvest date for each year.

Figure 3

Mean area under the disease progress curve (AUDPC) values from Minnesota field studies in (A) 2019, (B) 2020, and (C) 2021. Treatments included a non-treated control, tillage with a rotary tiller in the spring (prior to planting) to a depth of 10 cm to bury the residue, and propane burner heat application using a handheld Flame King Heavy Duty Propane Torch Weed Burner (Pico Rivera CA 90660). AUDPC values were calculated according to Madden et al. (2017) using severity ratings collected at 8-9 timepoints; ratings represented scores assigned by two to four raters and were based on the KWS severity scale (0-10). Bars represent the means of four replicate plots and error bars represent standard errors. Bars with the same letter were not significantly different based on Fisher’s Protected LSD (α = 0.05). Analyses were conducted within each year.

Figure 4

Mean Cercospora leaf spot severity progression in (A) 2019, (B) 2020, and (C) 2021, following spring-applied treatments evaluated in Renville, MN. Treatments included a non-treated control, tillage with a rotary tiller in the spring (prior to planting) to a depth of 10 cm to bury the residue, and heat treatment of residue using a propane-fueled using a handheld Flame King Heavy Duty Propane Torch Weed Burner (Pico Rivera CA 90660) immediately prior to planting. CLS ratings were based on the KWS severity scale (0-10) where 0 is 0.1% severity (1-5 spots per leaf) and 10 is 50% severity. Each point represents a mean of four replicate field plots. Date axes start at the planting date and end August 15 for each year as trials were not harvested.