Systemic Resistance to Downy Mildew and Appearance of Acid Soluble Proteins in Cucumber Leaves Treated with Biotic and Abiotic Inducers.

Abstract

Spraying cucumber leaves with salicylic acid (SA), 7-methoxycarbonyl benzo-1, 2, 3-thiadiazol and 2-chlorethylphosphate (ethephon) (abiotic inducers) reduced the diseased area caused by Pseudoperonospora cubensis by more than 50% in the sprayed first leavesand also in the upper second leaves provided challenge inoculation was made 3 to 6 days but not one to 24hr after treatment. Localized infection of cotyledons with P. cubensis (biotic inducer) also reduced the diseased area caused by the same pathogen by more than 50% in the upper leaves challenge-inoculated 6 days after inducer inoculation. Plants acquired systemic resistance when inoculated cotyledons remained attached for at least 3 days after inoculation. Protection by both abiotic and biotic inducers was more prominent in the second leaves which expanded after induction than in the first leaves which expanded before induction. The present results show that systemic resistance induced in cucumber plants by either biotic or abiotic inducers was effective in controlling infection by P. cubensis whose cell walls contain no chitin. Electrophoretic analysis of extracted proteins on polyacrylamide gel showed that both the SA treatment and localized infection with P. cubensis induced several novel acid soluble proteins in the treated and the upper untreated leaves in correlation with induced resistance.