Effects of thiophanate-methyl and azoxystrobin on the composition of Cercospora kikuchii populations with thiophanate-methyl-resistant strains

Journal of General Plant Pathology (Impact Factor: 0.97). 10/2006; 72(5):292-300. DOI: 10.1007/s10327-006-0285-9


Azoxystrobin was recently registered in Japan for the control of purple seed stain of soybean caused by Cercospora kikuchii, because the pathogen has developed resistance to thiophanate-methyl. To investigate the effects of these fungicides on the
frequency of C. kikuchii strains resistant to thiophanate-methyl and on the genotype structure of the population, we sowed purple-stained seeds, approximately
40% of which were infected with resistant strains, as inocula with asymptomatic seeds and applied thiophanate-methyl and azoxystrobin
during the reproductive growth of soybeans. The isolation frequency of resistant strains increased more than 99% by thiophanate-methyl
but was not significantly increased by azoxystrobin. In amplified fragment length polymorphism (AFLP) DNA fingerprinting,
genotypic diversity was significantly decreased by thiophanate-methyl but was not affected by azoxystrobin. In addition, the
similarity of the AFLP genotype structure was increased by thiophanate-methyl but not by azoxystrobin. These results suggest
that thiophanate-methyl selectively inhibited the proliferation of sensitive strains, which resulted in a small number of
genotypes, most of which were resistant strains. Azoxystrobin was found to nonselectively inhibit proliferation of the pathogen,
which retained a large number of genotypes including thiophanate-methyl-sensitive or thiophanate-methyl-resistant strains
or both.

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