Lack of Specificity in Plant Extracts and Chemicals as Inducers of Systemic Resistance in Cucumber Plants to Anthracnose

Canoia, lettuce, green bean, dwarf pea, tobacco, tomato, poplar, corn, wheat, and cucumber leaves were extracted by boiling with 95% ethanol. The ethano! was removed from the ethanol-water extraet, and the water-soluble fraction was centrifuged and fihered to remove compounds with a molecular weight > 1 000 D. The undersides ofthe first true leaf of a cucumber plant (leaf 1) and the leaf above (leaf 2) were sprayed with each plant extraet (induction), and 7 days later leaf 3 was inoculated with a conidial suspension of Coiietotrichvm 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 the 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 I and 2 induced resistance in leaf 3. Galacturonic acid, gluctironic acid, salicyhc acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, y-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, i-resorcylic acid, ^-resorcylic acid, gentisic acid, 2.3- dihydroxybenzoic acid, benzaldehyde, 2,3-dihydroxybenzaldehyde, ^- 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 thai did not