Identification and Characterization of Carbendazim-Resistant Isolates of Gibberella zeae

Sensitivity of Gibberella zeae to carbendazim was determined by measuring mycelial growth in fungicide-amended media. Among 1,529 isolates tested, 31 isolates showed a high level of resistance (HR) to carbendazim (fungicide concentration that results in 50% inhibition of mycelial growth [EC50] of 10.35 to 30.26 mg a.i. liter(-1)) and 10 isolates were moderately resistant (MR) (EC50 of 4.50 to 7.28 mg a.i. liter(-1)). The remaining 1,488 isolates were sensitive to carbendazim and were unable to grow on potato dextrose agar amended with carbendazim at 2 mg a.i. liter(-1). Analysis of DNA sequences of the beta 2-tubulin (Tub2) gene showed that all 10 MR isolates had a point mutation at codon 198 causing a replacement of glutamic acid by glutamine. At the codon position 167, the amino acid phenylalanine was replaced by tyrosine in 28 of 31 HR isolates. The remaining three HR isolates had a point mutation at codon 200 which converted phenylalanine to tyrosine. Based on these point mutations in the Tub2 gene, allele-specific polymerase chain reaction primers were developed for rapid detection of the point mutations. The rapid molecular method will be a valuable tool for the monitoring of carbendazim resistance in G. zeae. Additionally, deletion of the beta 1-tubulin gene (Tub1) in the HR isolate GJ33 resulted in increased resistance to carbendazim. These results indicate that Tub1 plays a role in the sensitivity of G. zeae to carbendazim.