Red-Light-Induced Resistance in Broad Bean (Vicia faba L.) to Leaf Spot Disease Caused by Alternaria tenuissima

The effect of different light wavelengths on the development of lesions induced by Alternaria tenuissima in broad bean leaves was investigated. Lesion development was completely suppressed in red-light-irradiated broad bean leaflets, irrespective of isolate or spore concentration. Pre-treatment of leaflets with red light for 24 h before inoculation also suppressed lesion development. Alternaria tenuissima failed to produce infection hyphae in red-light-irradiated broad bean leaflets. These results indicate that disease suppression in broad bean leaflets is due to light-induced resistance. Spore germination fluid (SGF) of A. tenuissima allowed non-pathogenic Alternaria alternata to infect wounded and unwounded broad bean leaflets kept in the dark, results suggesting that SGF induced susceptibility. Red light suppressed susceptibility induced by A. tenuissima SGF; thus, lesion formation and development were suppressed when leaflets inoculated with the spores of A. alternata suspended in A. tenuissima SGF were kept under red light. From these results, we conclude that red light induced resistance in broad bean to leaf spot disease caused by A. tenuissima, and that SGF induced susceptibility of broad bean leaflets to a non-pathogenic isolate of A. alternata