Phytotoxicity of Fusaric Acid and Analogues to Cotton

Fusaric acid has been isolated from 11 Fusarium species including Fusarium oxysporum f. sp. vasinfectum.  Although it shows low toxicity to animals, it is classified as a wilt-inducing toxin in many plants including cotton, which is particularly susceptible to this compound.  We were interested in determining the phytotoxicity of fusaric acid analogues and as such have developed a cotton cotyledonary leaf assay.  In the assay, 20µL of 0.0 µM, 0.5 µM, 1.0 µM, 2.0 µM, 4.0 µM and 8.0 µM solutions of the compounds were placed on three spots of a seven day-old cotyledonary leaf and the leaf was pierced with a needle through each drop of solution.  After two days, plants were scored by three persons giving ratings of 0 to 5, with 0 meaning no symptoms and 5 severely necrotic lesions.  Each test was replicated 4 times.  Of the compounds tested, fusaric acid and its methyl ester were among the most phytotoxic showing severe wilting at 8.0 µM and detectable necrosis at the point of piercing at 0.5 µM.  In contrast, even at 8.0µM, the analogues 3-methylpyridine, 3-ethylpyridine, 3-butylpyridine, 2-picolinamide, 2-cyanopyridine and picolinaldehyde had no detectable effect.