Temporal population monitoring of fungicide sensitivity in Cercospora beticola from sugarbeet (Beta vulgaris) in the Upper Great Lakes

Authors: ROSENZWEIG, NOAH - Michigan State University; Hanson, Linda; JIANG, QIANWEI - Michigan State University; MAMBETOVA, SALTANAT - Michigan State University; GUZA, COREY - Michigan Sugar Company; STEWART, JAMES - Michigan Sugar Company; SOMOHANO, PAULA - Michigan State University

Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2019
Publication Date: 1/17/2020
Citation: Rosenzweig, N., Hanson, L.E., Jiang, Q., Mambetova, S., Guza, C., Stewart, J., Somohano, P. 2020. Temporal population monitoring of fungicide sensitivity in Cercospora beticola from sugarbeet (Beta vulgaris) in the Upper Great Lakes. Canadian Journal of Plant Pathology. https://doi.org/10.1080/07060661.2019.1705914.
DOI: https://doi.org/10.1080/07060661.2019.1705914

Interpretive Summary: Over the past five growing seasons, there has been a steady increase in frequency of finding isolates of the fungus Cercospora beticola with resistance to two widely used fungicide classes from commercial production fields in Michigan, USA and Ontario, CAN. This is a concern since C. beticola is the cause of the most serious foliar disease of beet, Cercospora leaf spot (CLS), and fungicides have been an important disease management tool. The current study reports on a multi-year sensitivity monitoring program of C. beticola populations in the Upper Great Lakes sugarbeet production region, to these two fungicide classes, demethylase inhibitors (DMI) and organo-tin fungicides. Isolates of C. beticola were categorized into five sensitivity types based on mean concentrations for 50% reduction in growth (EC50) values.Significant changes were detected in the frequencies of sensitivity types to DMI and organo-tin fungicides from over the period of the study, with an overall increase in isolates in the more resistant categories and fewer sensitive isolates. In addition, isolates with cross resistance to both of these fungicide classes were identified. Fungicide sensitivity monitoring indicates that an integrated approach, combining fungicide efficacy trials and pathogen biology is essential in future fungicide resistance management recommendations as well as effective disease management strategies.

Technical Abstract: Over the past five growing seasons, there has been a steady increase in frequency of Cercospora beticola, the cause of Cercospora leaf spot (CLS) in sugarbeet (Beta vulgaris), isolates with demethylase inhibitor (DMI) and quinone outside inhibitor (QoI) fungicide resistance collected from commercial production fields in Michigan, USA and Ontario, CAN. More recently individual C. beticola isolates with resistance to fungicides with multiple modes of action have been recovered from commercial production areas. The current study was a multi-year sensitivity monitoring program of C. beticola populations in the Upper Great Lakes sugarbeet production region, to two classes of fungicides, DMI and organo-tin fungicides and with an aim to determine spatial and temporal frequency of fungicide sensitivity phenotypes from 2014 through 2017. A spiral gradient plater was used to determine effective concentration (mg L-1) in inhibiting growth by 50% (EC50) to six fungicides from the two fungicide classes. Isolates of C. beticola were categorized phenotypically into five sensitivity categories based on mean EC50 values. Significant spatial variability and temporal shifts were detected in the frequencies of sensitivity phenotypes to DMI and organo-tin fungicides from 2014 - 2017. Individual isolates of C. beticola were recovered with cross resistance to these fungicides. There was a shift towards populations with higher levels of insensitivity to both classes of fungicides, and thus an increase in frequency of recovered isolates of C. beticola with lower sensitivity to five DMI fungicides as well as triphenyltin OH. Fungicide sensitivity monitoring indicates that an integrated approach, combining fungicide efficacy trials and pathogen biology is essential in developing fungicide resistance management recommendations as well as effective disease management strategies.