Fungicide sensitivity characteristics of Cercospora beticola isolates recovered from the high plains of Colorado, Montana, Nebraska, and Wyoming. 1. Benzimidazole and triphenyltin hydroxide

The appearance of Cercospora beticola isolates with insensitivity to triphenyltin hydroxide (TPTH) or benzimidazole (BM) fungicides is a problem in many sugarbeet growing areas of the world including the United States. In 1998 and 1999 sugarbeet growing areas in northeastern Colorado, southeastern Montana and southwestern Nebraska were surveyed to determine if insensitive isolates were present. One field in southeast Wyoming was included in the 1999 survey. There were 328 isolates recovered from 110 fields in 1998 and 305 isolates from 101 fields in 1999. Radial growth of isolates on potato dextrose agar amended with 1, 5, and 10μg mL -1 TPTH and 5μg mL -1 BM was compared to growth in the absence of fungicide. Percent inhibition of radial growth in the presence of 1 μg mL -1 TPTH ranged from 17% to 100% in 1998 and 37% to 100% in 1999. More inhibition of radial growth was observed as the concentration of TPTH increased. Results for 5.0μg mL -1 BM revealed that 72 of the 328 isolates grew with 20% or less inhibition in 1998 and 87 of 305 isolates grew with 40% or less inhibition in 1999. Insensitivity to BM is more pronounced in Colorado and Nebraska when compared to Montana, presumably due to greater fungicide use in the more southerly production areas. Data reported here establishes baseline characteristics of the current C. beticola population.