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Late blight of potato is one of the most devastating diseases with great economic importance. Among several disease management strategies, ISR (Induced systemic resistance) plays an important role in checking the disease spread. So, in the present study, nine different SAR chemical inducers viz. Phosphoric acid, Salicylic acid, Naphthalene acetic acid(NAA), Benzoic acid, Benzothiadiazole, Dichloro-iso nicotinic acid (INA) and Kinetin were evaluated at 0.05% (C1), 0.10% (C2) and 0.15% (C3) concentrations, whereas Calcium chloride and Ascorbic Acid at 3.5% (C1), 5.0% (C2), 6.5% (C3) and 1.5% (C1), 2.0% (C2), 2.5% (C3) respectively at three different concentrations each, along with positive and negative checks of standard fungicide metalaxyl 8+ macozeb 64 WP @0.25% and control(water) respectively by spraying them at 2-3 compound leaf stage of plants which were challenged by P. infestans inoculations five days later in pots to assess disease incidence and intensity which were recorded at weekly intervals of four stages (stage I-IV). It was found that the foliar spray with benzothiadiazole (BTH) @ 0.10 to 0.15% was best in delaying the appearance of first symptoms of the disease by 11 days followed by salicylic [email protected] 0.10 to 0.15% and ascorbic acid @ 2.5% which delayed the symptom appearance by 9 days, whereas the standard fungicidal spray of metalaxyl 8 + mancozeb 64 WP at the recommended concentration of 0.25% delayed the symptom appearance by only 8 days when compared to water sprayed check. It was also found that on an average, BTH sprays completely arrested the late blight development (0% intensity) followed by salicylic acid, INA and ascorbic acid sprays exhibiting 0.46-0.74% mean blight intensity compared to 2.00 and 6.28 percent intensity recorded, respectively, on metalaxyl 8 + mancozeb 64 WP and water-sprayed plants one week after first appearance of disease in control pots (stage I). These treatments with chemical inducers showed an increase in the activity of peroxidases and polyphenol oxidases in the infected potato leaves as a result of SAR activity at all stages (stage I-IV). Average maximum peroxidase activity of 19.01-20.66 mg and polyphenol oxidase activity of 2.70-2.89 mg were recorded in the potato leaves during stage-I, sprayed with either BTH, benzoic acid, phosphoric acid or salicylic acid as compared to only (16.78 mg and 2.28 mg), (4.66 mg and 1.36 mg) recorded in (metalaxyl 8 + mancozeb 64 WP) and water-sprayed check respectively. The highest concentrations of all the test chemicals, in general, showed increased biochemical activity thereby yielding lesser blight intensity compared to their lower concentrations.
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