Biochemical basis of systemic acquired resistance in muskmelon induced by different SAR elicitors in response to challenge inoculation by downy mildew pathogen

In India, vegetable crops are known to be attacked by many bacterial, fungal and viral pathogens causing various diseases. Out of the various diseases, downy mildew of cucurbits caused by Pseudoperonospora cubensis belonging to oomycete has become quite severe in the past recent years.  The present study was conducted to reduce fungicide load and work out alternate method for control of this disease.  Different SAR compounds were tested and exogenous foliar sprays of  different conc. of Salicylic acid,  Jasmonic acid and Bion (Benzothiadiazole-BTH) @ 50µM, 250µM, 500µM, 1000µM and  β- amino butyric acid of 20 mM, 30mM, 50 mM, 100mM were given for inducing resistance in mushmelon against downy mildew pathogen. Concentration of Salicylic acid, Jasmonic acid and Bion @ 500 µM, and β- amino butyric acid @ 50 mM gave best control of disease among all tested concentrations. Protein content of treated muskmelon plants varied from 10.5 to 12.7 mg/g fresh weight compared to 5.2 mg/g fresh weight in control. Induction of proteins and defense enzymes was systemic in nature in response to all the four elicitors. The inducers also stimulated  the activities of pathogenesis related proteins (Pr- proteins) i.e. β-1,3 glucanase, Peroxidase (POD), and defense related proteins i.e. Polyphenol oxidase (PPO), Phenylalanine ammonia lyase (PAL) from 18 to 180 % indicating induced resistance in treated muskmelon plants as compared to control. Electrophoretic protein profiling of treated muskmelon plants also confirmed the induction of pathogenesis-related proteins ranging from 15- 75 kDa along with some other proteins. Total chlorophyll and carotenoids also showed spike of 3% to 55 % in response to elicitors. Salicylic acid gave best results with 77.27 % disease control followed by Jasmonic acid with 76.6%; whereas Bion and Βeta amino butyric acid were almost at par with each other and gave 66 % disease control as compared to control plants. Thus integration of disease tolerance and salicylic acid spray resulted in effective and eco-friendly and very economical control of downy mildew of muskmelon.