Baseline Sensitivity of Populations of Phytophthora capsici from China to Three Carboxylic Acid Amide (CAA) Fungicides and Sequence Analysis of Cholinephosphotranferases from a CAA-sensitive Isolate and CAA-resistant Laboratory Mutants

During 2006―2008, 572 isolates of Phytophthora capsici were collected from seven provinces in China, and their sensitivities to three carboxylic acid amides (CAA), dimethomorph, flumorph and pyrimorph were determined. Of these isolates, 90 isolates without a history of exposure to CAA fungicides (CAAs) were used to set up the baseline sensitivity. Baseline EC 50 values ranged from 0.122 to 0.203 (mean ± SD, 0.154 ± 0.022) μg ml ―1 for dimethomorph, from 0.301 to 0.487 (mean ± SD, 0.373 ± 0.043) μg ml ―1 for flumorph and from 0.557 to 0.944 (mean ± SD, 0.712 ± 0.082) μg ml ―1 for pyrimorph, respectively. The other 482 isolates were tested with a single discriminatory dose and were completely inhibited at 0.5 μg ml ―1 of dimethomorph. Four CAA-resistant mutants were generated by repeated exposure to dimethomorph in vitro. As compared to the parental wild-type isolate, the four CAA-resistant mutants showed similar fitness in hyphal growth, sporulation in vitro and pathogenicity in vivo. Mutants resistant to CAAs in the in vitro assay caused visible lesions on pepper stems or roots treated with the recommended dose of dimethomorph. Previous studies upon the mode of action of CAAs suggested that these fungicides maybe inhibit phospholipid biosynthesis and that the primary target could be the cholinephosphotranferase (CPT), which is referred to aminoalcoholphosphotransferases (AAPTs). We sequenced and analyzed two CPT (AAPT1 and AAPT2) genes in P. capsici. Based on the cDNA sequence, we found that the AAPT1 and AAPT2 gene span 1538 and 1459 bp and were interrupted by five and three introns, respectively. There was no difference between the parental wild-type isolate and the four CAA-resistant mutants in the amino acid sequences of AAPT1 and AAPT2 gene. So, it was assumed that the resistance to dimethomorph was not due to mutations in the amino acid sequence of these two possible target genes.

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