| Summary: | <i>Biscogniauxia rosacearum,</i> recognized for the first time as a pathogen involved in grapevine trunk diseases in Paveh (west of Iran) vineyards, produced <i>meso</i>-2,3-butanediol (<b>1</b>) as the only phytotoxin. Nectriapyrone (<b>2</b>), (3<i>R</i>)-5-methylmellein (<b>3</b>), (3<i>R</i>)-5-methyl-6-methoxymellein (<b>4</b>), and tyrosol (<b>5</b>) were instead produced as phytotoxins from a strain of the same fungus isolated from oak trees in Zagros forests of Gilan-e Gharb, Kermanshah Province. They were identified comparing their <sup>1</sup>H and <sup>13</sup>C NMR, ESIMS, and specific optical rotation data with those already reported in the literature. The phytotoxicity of metabolites (<b>1–5</b>) was estimated by leaf puncture assay on <i>Quercus ilex</i> L. and <i>Hedera helix</i> L., and by leaf absorption assay on grapevine (<i>Vitis vinifera</i> L.) at a concentration of 5 × 10<sup>−3</sup> and 10<sup>−3</sup> M. Tested on grapevine, <i>meso</i>-2,3-butanediol (<b>1</b>) and (3<i>R</i>)-5-methyl-6-methoxymellein (<b>4</b>) resulted to be the most phytotoxic compounds. On <i>Q. ilex</i>, nectriapyrone (<b>2</b>) and tyrosol (<b>5</b>) showed severe necrosis at the highest concentration while none of the compounds (<b>1</b>–<b>5</b>) was active on <i>H. helix</i>. Furthermore, the phytotoxicity of compounds <b>3</b> and <b>4</b> was also compared with that of some related natural melleins to perform a structure-activity relationship (SAR) study. The results of this study were also discussed.
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