Lack of specificity in plant extracts and chemicals as inducers of systemic resistance in cucumber plants to anthracnose

Canola, lettuce, green bean, dwarf pea, tobacco, tomato, poplar, corn, wheat, and cucumber leaves were extracted by boiling with 95% ethanol. The ethanol was removed from the ethanol-water extract, and the water-soluble fraction was centrifuged and filtered to remove compounds with a molecular weight > 1 000 D. The undersides of the first true leaf of a cucumber plant (leaf 1) and the leaf above (leaf 2) were sprayed with each plant extract (induction), and 7 days later leaf 3 was inoculated with a conidial suspension of Colletotrichym lagenarium (challenge). Extracts from all plants tested induced systemic resistance in leaf 3 to C. lagenarium. Commercially available compounds were screened to determine structural similarities among he compounds which can induce systemic resistance. All compounds which induced resistance in leaf 3 caused some damage to leaves 1 and 2, but not all compounds which damaged leaves 1 and 2 induced resistance in leaf 3. Galacturonic acid, glucuronic acid, salicylic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, gamma-resorcylic acid, protocatechuic acid, gallic acid, hemimelletic acid, trimelletic acid, trimesic acid, phloroglucinol, and glycine induced systemic resistance, whereas catechol, resorcinol, hydroquinone, pyrogallol, hydroxyhydroquinone, benzoic acid, phthalic acid, alpha-resorcylic acid, beta-resorcylic acid, gentisic acid, 2,3-dihydroxybenzoic acid, benzaldehyde, 2,3-dihydroxybenzaldehyde, beta-resorcaldehyde, protocatechuic aldehyde, benzyl alcohol, salicyl alcohol, myo-inositol, phytic acid, pinitol, and quebrachitol did not. Structural similarities were not evident among either compounds which induced systemic resistance or those that did not.