Fungicide Sensitivity Characteristics of Cercospora beticola Isolates Recovered from the High Plains of Colorado, Montana, Nebraska, and Wyoming. 2. Mancozeb, Propiconazole. and Azoxystrobin

Abstract

Sugarbeet (Beta vulgaris L.) growing areas in northeastern Colorado, southeastern Montana and southwestern Nebraska were surveyed in 1998 through 2001 to determine if insensitive Cercospora beticola isolates were present in fields. One field in southeast Wyoming was included in the 1999 and 2000 surveys. A total of 328 isolates recovered from 110 fields in 1998, 305 isolates recovered from 101 fields in 1999, 255 isolates recovered from 72 fields in 2000, and 256 isolates recovered from 85 fields in 2001 was tested in this study. The radial growth of each isolate on potato dextrose agar (PDA) was compared to its radial growth on PDA amended with 5 μg mL -1 mancozeb, 1 and 10 μg mL -1 propiconazole and 1 and 10 μg mL -1 azoxystrobin. The overall percentages of isolates that grew in the presence of 5 μg mL -1 mancozeb were 92% in 1999, 88% in 2000 and 90% in 2001. Overall percentages of isolates that grew in the presence of 1 μg mL -1 propiconazole were 2% in 1999, 0.4% in 2000 and 0% in 2001. The overall percentages of isolates that grew in the presence of 1 μg -1 mL azoxystrobin were 99.7% in 1998 and 98% in both 2000 and 2001. Data reported here establishes baseline characteristics of the current C. beticola population infecting sugarbeet in the High Plains regions of Colorado, Montana, Nebraska and Wyoming.