Phytotoxicity of fusaric acid and analogs to cotton.

We developed a cotton cotyledonary leaf bioassay to test the phytotoxicity of fusaric acid (5-butylpicolinic acid), picolinic acid and related analogs. The compounds were dissolved in aqueous Tween 80, and 20 μL of the test solution was placed at three positions on the leaf, and a needle was used to puncture the leaf through each drop; the results were evaluated after 48 h. In contrast to previous studies, we found the carboxylic acid group is essential for phytotoxicity. Nicotinic acid was considerably less phytotoxic than picolinic acid and conversion of picolinic acid to the amide or N-oxide decreased phytotoxicity. Increasing the alkyl chain length at the 5-position on picolinic acid from two up to five carbons atoms increased phytotoxicity. Fusaric acid methyl ester, the most phytotoxic compound tested, is a naturally occurring compound; as such it has potential as a herbicide in organic farming.