Suppression of cucurbit scab on cucumber leaves by photodynamic dyes

Abstract The goal of this study was to test the ability of the photodynamic dyes bengal rose, toluidine blue and methylene blue to protect systemically cucumber plants from cucurbit scab. At the stage of one true leaf, water or aqueous solutions of the dyes were applied to the leaf as droplets. When the second leaf appeared and was fully developed, it was inoculated with a spore suspension of Cladosporium cucumerinum . It was found that all dyes (at 0.5–200 μM) significantly suppressed symptoms of the disease. The effect was preceded by development of local damage of the treated leaf. The degree of the damage as well as the disease suppression increased with dye concentration. Both indices were smaller if the plants were kept in darkness for the first day after the treatment. It is suggested that reactive oxygen photogenerated by dyes caused a local cell death, which induced resistance in distant leaves. The dyes tested, or their derivatives, may offer a base for novel disease resistance inducers.