Submitted to: Journal of Sugarbeet Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2000
Publication Date: N/A
Interpretive Summary: Sugarbeet production in the north central U.S. typically employs fungicides to control leaf spot disease caused by Cercospora beticola. If the fungus develops tolerance or resistance to the fungicides used to control the disease, then fungicide application becomes less effective in controlling the disease. Historically, sugarbeet production in Michigan has relied on natural leaf spot resistance in the varieties used as a means to control the disease. Increasingly, however, higher yielding sugarbeet varieties that are less resistant to leaf spot disease are being grown in Michigan. The subsequent increase in leaf spot disease has led to an increase in the use of fungicides to control the disease. As some growers reported the ineffectiveness of benzimidazoles at controlling leaf spot, an assay of C. beticola isolates was made to determine if resistance to the fungicide could be detected. The results indicate that resistance to the benzimidazole fungicides is present in C. beticola in Michigan.
Technical Abstract: Leaves of sugarbeet (Beta vulgaris L.) were collected in 44 fields in Michigan following reports of inadequate control of Cercospora leaf spot disease with benzimidazole fungicides. Standard fungus isolation techniques were combined with fungicide resistance testing to determine if isolates of Cercospora beticola Sacc. resistant to thiophanate methyl (TM), a representative benzimidazole fungicide, could be obtained from the leaves. Resistance was assayed by measuring radial growth of fungal mycelia on potato dextrose agar (PDA) amended with 5 ppm TM. Conidia of C. beticola were recovered from 556 individual leaf spots; 102 of these isolates, from 21 of the 44 fields, were resistant to TM. Resistant isolates were recovered from most of the counties in Michigan where sugarbeet is grown. The results indicate a need to monitor C. beticola in Michigan for increases in the proportion of isolates exhibiting resistance to TM and for the existence of resistance or tolerance to other fungicides.