Investigating the biochemical variations in onion leaves due to purple blotch disease and its management through induced resistance

Purple blotch (PB), caused by Alternaria porri (Ellis) ciferri, poses a significant threat to onion crop, resulting in major economic losses in both bulb and seed production globally. The incidence of this disease underscores the critical need for an effective management. In the present study, screening of onion genotypes for PB under field conditions revealed that the genotypes like Phulkara and Ceylon expressed resistant, response with 8.21 and 8.91% disease severity index (DSI) respectively, whereas Desi Black and Red Imposta were highly susceptible, with DSI of 67.38 - 79.41%. Furthermore, resistant and susceptible genotypes were also evaluated for biochemical variation analysis. Significant variations (p ≤ 0.05) in antioxidant enzymes were observed across reaction groups (inoculated and un-inoculated), types (resistant and susceptible), and onion varieties in response to A. porri infection. The analysis of variance showed significant changes in antioxidant enzymes level of onion leaves, including catalase (CAT), peroxidase (POD), and super dismutase (SOD). Results indicated that the concentration of SOD (55.37%), POD (41.87%) and CAT (37.92), respectively, in resistant plant leaves, whereas susceptible varieties showed SOD (43.71%), POD (28.3%) and CAT (26.59%). Furthermore, the amount of antioxidant enzymes was reduced in both resistant as well as susceptible varieties of onion after inoculation. Amount of SOD (68.04%), POD (57.45%) and CAT (50.87%) were recorded in un-inoculated group of onion plants that reduced to 31.05, 12.98 and 13.64% in inoculated group respectively. For the management of A. porri four different plant activators (salicylic acid, benzoic acid, citric acid and di-potassium hydrogen phosphate) at three different concentrations (0.5, 0.75 and 1%) were evaluated under greenhouse and field conditions. Among all salicylic acid was found to be most effective in controlling this disease under greenhouse and field conditions. The present study revealed that purple blotch affects the biochemical mechanism of the plants, which helps in activating the resistance process against pathogens through various enzymes. Additionally, salicylic acid demonstrated significant efficacy in controlling purple blotch in onions.