Location: Sugarbeet and Bean ResearchTitle: Cercospora leaf spot: monitoring and managing fungicide resistance in populations of Cercospora beticola in Michigan Author
Submitted to: Newsbeet
Publication Type: Trade Journal
Publication Acceptance Date: 2/1/2014
Publication Date: 3/1/2014
Citation: Kirk, W.W., Hanson, L.E., Rosenzweig, N., Jiang, Q., Clark, G. 2014. Cercospora leaf spot: monitoring and managing fungicide resistance in populations of Cercospora beticola in Michigan. Newsbeet. Winter 2013-2014:17-18. Interpretive Summary:
Technical Abstract: Cercospora leaf spot (CLS, Cercospora beticola) is the most serious foliar disease of sugarbeet in Michigan and Ontario.Management of CLS depends on timely fungicide applications, disease forecasting prediction models and the use of CLS resistant sugar beet varieties. Fungicides have a dominant role in the control of CLS. There are significant economic barriers that prevent sugarbeet growers in Michigan from applying more than three fungicide applications to control CLS and commonly only single applications were made. One drawback to this approach has been the tendency to apply products with a single active ingredient. Several of the modern fungicides have highly specific single targets of activity. Using such products alone has lead to the development of insensitivity to some fungicides previously recommended for control of CLS, most notably thiophanate-methyl and recently the strobilurin group of fungicides. In other growing areas, insensitivity has been identied to additional fungicides, such as the sterol demethylation inhibitors (DMIs). The sensitivity of the population of C. beticola in eastern Michigan and Ontario to fungicides such as triphenyltin OH, thiophanate-methyl, pyraclostrobin, and tetraconazole has been monitored in a joint program between Michigan State University (MSU) and the Michigan Sugar Company (MSC) since 2003. The type of survey used in the MI/ON sugarbeet region is designed to test a wide range of leaf spot lesions from 100 to 120 locations/year. This method can quickly detect if there are any changes in the population of C. beticola and further analyses of the effective concentration of fungicides can then be done by EC50 determination (the concentration of fungicide required to prevent development or growth of spores or mycelium by 50%) value determination and DNA-based tools such as PCR for detection of gene changes known to confer fungicide resistance. The insensitivity response of the population of C. beticola to fungicides has fluctuated drastically from 2003 to 2013. There have been no reports of field failure of the triazole products in MI/ON although there is evidence that isolates of C. beticola with reduced sensitivity are present in the region. However, growers recently have reported loss of efficacy when strobilurin products were applied particularly in years when CLS was severe. Strobilurin-insensitivity was widespread throughout the growing area in 2011 and 2012. Current recommendations are for tank mixes and/or alternating chemistries to help slow spread of insensitivity.