Multiple Mutations and Overexpression in the and Genes Lead to Decreased Sensitivity of to Tebuconazole

Multiple demethylation-inhibiting (DMI) fungicides are used to control pecan scab, caused by . To compare the efficacy of various DMI fungicides on , field trials were conducted at multiple locations applying fungicides to individual pecan terminals. In vitro assays were conducted to test the sensitivity of isolates from multiple locations to various concentrations of tebuconazole. Both studies confirmed high levels of resistance to tebuconazole. To investigate the mechanism of resistance, two copies of the gene, and , of resistant and sensitive isolates were sequenced and scanned for mutations. In the gene, mutation at codon 444 (G444D), and in the gene, mutations at codon 357 (G357H) and 177 (I77T/I77L) were found in resistant isolates. Expression analysis of and revealed enhanced expression in the resistant isolates compared to the sensitive isolates. There were 3.0- and 1.9-fold increases in gene expression in the resistant isolates compared to the sensitive isolates for the and genes, respectively. Therefore, two potential mechanisms-multiple point mutations and gene over expression in the gene of isolates-were revealed as likely reasons for the observed resistance in isolates of to tebuconazole.