Systemic Acquired Resistance Induced in Cucumber Plants Against Powdery Mildew Disease by Pre-inoculation with Tobacco Necrosis Virus

Systemic Acquired Resistance (SAR) in cucumber plants against powdery mildew disease, caused by Sphaerotheca fuliginea (Schlechtend Fr.) Pollacci, was induced by localized infection in cucumber cotyledons with Tobacco Necrosis Virus (TNV). Inoculation of the two cotyledon leaves significantly reduced powdery mildew severity on cucumber true leaves similar to the effect of fungicide. TNV-local lesions developed plants still protected against powdery mildew for 16 days. The level of protection was higher in the upper true leaves when compared with the bottom ones. Antifungal protein was extracted and partially purified by DEAE-cellulose column chromatography from cucumber TNV-inoculated plants only. Bioassay detection for antifungal activity indicated that, 0.6 M NaCl fraction had the highest activity. SDS-PAGE of partially purified 0.6 M NaCl fraction protein indicated the presence of a single protein band with a molecular weight of about 30 kDa. This protein was extracted from upper TNV-uninoculated true leaves of plants inoculated on the cotyledon leaves, which acquired systemic resistance against powdery mildew challenge inoculation. In vitro study of the antifungal activity of these proteins showed that only 0.6 M NaCl fraction has direct antifungal activity towards S. fuliginea conidial spores. The induced systemic resistance was not accompanied with the activity of β-1, 3 glucanase.