Evaluation of cultural practices to manage Cercospora leaf spot of sugar beet in Michigan, 2019-20

Authors: HERNANDEZ, ALEXANDRA - Michigan State University; BLOOMINGDALE, CHRIS - Michigan State University; BUBLITZ, DANIEL - Michigan State University; Hanson, Linda; WILBUR, JAIME - Michigan State University

Submitted to: Plant Disease Management Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2021
Publication Date: 6/28/2021
Citation: Hernandez, A., Bloomingdale, C., Bublitz, D.M., Hanson, L.E., Wilbur, J.F. 2021. Evaluation of cultural practices to manage Cercospora leaf spot of sugar beet in Michigan, 2019-20. Plant Disease Management Reports. 15. Article V008. https://doi.org/10.1094/PDMR15.
DOI: https://doi.org/10.1094/PDMR15

Interpretive Summary: Three methods were tested for potential to assist in managing Cercospora leaf spot (CLS) of sugar beet. Beets were inoculated with the causal agent, the fungus Cercospora beticola, in 2019. At the end of the season, these beets were given one of four different treatments: a no treatment control, leaves were plowed into the soil, leaves were given a heat treatment with a propane burner, or leaves were treated with a chemical hypothesized to promote leaf break down. At harvest samples showed significantly lower lesion sporulation and fungal viability from the heat treatment than the other samples. No differences were detected at later time points during the winter. However, for both live spore traps and on leaves of beets planted in the same field locations in 2020, the 2019 heat treatment resulted in significantly fewer lesions than all other treatments. The treatments did not significantly reduce yield, percent sugar, or recoverable white sugar per ton. This preliminary report indicates that the treatments can be used without lowering yield or sugar, and the reduced CLS severity in the year following treatment may offer some new options for disease management. Note that both of the cultural management methods, which include plowing and heat treatment, could be used in either organic or conventional agriculture.

Technical Abstract: Three methods were examined for their potential to impact the survival of the leaf spot pathogen Cercospora beticola in the field. A randomized complete block design was set up at the Saginaw Valley Research and Education Center in Frankenmuth, MI with treatments replicated four times. Plots were four rows wide (30 in row spacing) by 60 ft long. Beet plants were inoculated with a C. beticola spore solution in July 2019. Plots were scouted regularly to monitor pest populations in the field and to track Cercospora leaf spot (CLS) progression. In October, the four treatments were applied. These included a non-treated control, plowing plant residue after defoliation and harvest, a heat treatment using a propane-fueled burner prior to defoliation, or a desiccant applied seven days pre-harvest. Root subsamples from each plot were collected at harvest and analyzed by Michigan Sugar. Just prior to harvest, but after all treatments except plowing had been applied, leaf samples were collected and used to evaluate fungal sporulation and viability at 0-, 45-, 90-, and 135-days post-harvest (DPH). Leaves in mesh bags were incorporated into the soil to mimic soil conditions after treatment application, 6 in. for plow-treated plots and <1 in. for all other plots. Following harvest, winter wheat was planted in the 10-ft buffer zones surrounding each plot and maintained during the 2020 season. In 2020, beets were planted in the plots. In addition, a highly susceptible sugar beet, F1042, was used as a live spore trap to estimate inoculum levels weekly from May until late July. The spore trap beets were placed in enclosures. Total CLS lesions were counted after spore trap beets were incubated in a controlled environment. CLS ratings were conducted until end of July using a rating scale of 1-10 where 0= no visible spots and 10=50% or more of leaf area affected. The ratings were used to calculate area under the disease progress curves (AUDPC) for CLS. AUDPC values were significantly different among treatments (P < 0.01), and the heat treatment resulted in significantly lower CLS than all other treatments. Significant treatment differences were detected in percent lesion sporulation (P < 0.001) and percent C. beticola viability (P < 0.05) from at-harvest leaf samples. No significant differences were detected in leaf samples evaluated at 45-, 90-, and 135-DPH. Heat treatment resulted in significantly fewer lesions on sentinel beets from CLS than all other treatments (P < 0.05). No significant differences were detected in yield, percent sugar, or RWST.