Role of salicylic acid in disease resistance against Alternaria spp in tomatoes (Lycopersicon esculentum L.)

Early blight and Alternaria leaf spot are among the major diseases limiting productivity of tomatoes. Evolvement of pathogen into resistance form and detrimental effect of fungicides have spawned efforts to develop management strategies that are economic and environmentally friendly. Salicylic acid is a natural plant hormone known to induce plant defense against various biotic and abiotic stresses. Therefore, the main focus of this study was to evaluate the role of salicylic acid on tomato crop against Alternaria spp infection. A range of salicylic acid concentrations (1.0, 1.5, 2.0, 2.5 and 3.0 mM) and distilled water (control) were laid in a randomized complete block design. Peroxidase (POD), phenylalanine ammonia lyase (PAL) and polyphenol oxidase (PPO) enzyme activities were recorded from one to four weeks after inoculation with virulent Alternaria spp spores. The results showed that plants treated with 2.5 mM responded rapidly and significantly showed higher induction of POD, PAL and PPO enzyme activities. Plants sprayed with salicylic acid at 3 mM showed phytotoxic symptoms and reduced enzyme activity was recorded. In addition, foliar application of salicylic acid at 2.5 mM significantly (p<0.001) reduced disease severity (25.5 %) compared to plants treated with distilled water (85.46 % disease severity). These results showed that 2.5 mM of salicylic acid is safe and can be used in inducing tomato plant disease resistance.