Sensitivity of Fusarium graminearum isolates to pyraclostrobin

Fusarium graminearum causes Fusarium head blight (FHB) of wheat as well as ear and stalk rots of corn. Pyraclostrobin is one of the most frequently applied fungicides for control of foliar diseases of wheat and corn in Nebraska. The objective of this study was to determine if Nebraska isolates of F. graminearum differed in sensitivity to pyraclostrobin. The isolates were collected from wheat fields and elevators in 2007 following severe epidemics of FHB in the south central and eastern parts of the state. Potato dextrose agar was amended with salicylhydroxamic acid (SHAM, dissolved in methanol) at 100 kg/ml of PDA, then with technical grade pyraclostrobin (95 percent) dissolved in acetone at 0, 0.001, 0.01, 0.1, 1.0, and 10.0 kg/ml. A 5-mm-diameter PDA mycelial plug from an actively growing edge of each of 15 F. graminearum isolates was placed, mycelial face down, at the center of the amended PDA plates which were then incubated at 25C in 12 hr light and 12 hr dark. An alpha lattice randomized design with 3 replications was used. EC sub(50) values calculated from mycelial area measured after 10 days ranged from 0.063 kg/ml to 1.585 kg/ml for 12 isolates. However, isolates NE90, NE101, and NE91 had EC sub(50) values of 15.85, 100.0, and 398.1 kg/ml, respectively. These preliminary results indicate the development of resistance to pyraclostrobin in some Nebraska populations of F. graminearum.