Wayne F. Wilcox, and
Michael G. Milgroom
First and fourth authors: School of Integrative Plant Science, Section of Plant Pathology and Plant-Microbe Biology, Cornell University, Ithaca, NY 14853; second author: United States Department of Agriculture, Agricultural Research Service Grape Genetics Research Unit, Geneva, NY 14456; and third author: School of Integrative Plant Science, Section of Plant Pathology and Plant-Microbe Biology, Cornell University, Geneva, NY 14456.
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Accepted for publication 16 September 2014.
We studied the mechanisms of azole resistance in Erysiphe necator by quantifying the sensitivity to myclobutanil (EC50) in 65 isolates from the eastern United States and 12 from Chile. From each isolate, we sequenced the gene for sterol 14α-demethylase (CYP51), and measured the expression of CYP51 and homologs of four putative efflux transporter genes, which we identified in the E. necator transcriptome. Sequence variation in CYP51 was relatively low, with sequences of 40 U.S. isolates identical to the reference sequence. Nine U.S. isolates and five from Chile carried a previously identified A to T nucleotide substitution in position 495 (A495T), which results in an amino acid substitution in codon 136 (Y136F) and correlates with high levels of azole resistance. We also found a nucleotide substitution in position 1119 (A1119C) in 15 U.S. isolates, whose mean EC50 value was equivalent to that for the Y136F isolates. Isolates carrying mutation A1119C had significantly greater CYP51 expression, even though A1119C does not affect the CYP51 amino acid sequence. Regression analysis showed no significant effects of the expression of efflux transporter genes on EC50. Both the Y136F mutation in CYP51 and increased CYP51 expression appear responsible for azole resistance in eastern U.S. populations of E. necator.
This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2015.