The appearance of Cercospora helicola isolates with insensi tivity 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

more »

... m 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 10f,.lg mL-' TPTH and 5f,.lg mL-' BM was compared to growth in the absence of fungicide. Percent inhibition of radial growth in the presence of If,.lg mL-' TPTH ranged from 17% to 100% in 1998 and 37% to 100% in 1999_ More inhibition of radial growth was observed as the concentration ofTPTH increased. Results for 5.0f,.lg mL-' 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. In sensitivity to BM is more pronounced in Colorado and Ne braska when compared to Montana, presumably due to greater fungicide use in the more southerly production areas. Data reported here establishes baseline characteris tics of the current C. helicola population.