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Not AvailablePowdery mildew caused by Erysiphe necator is one of the most important diseases of grapevine in India. It affects the grape production by causing significant reduction in yield and quality of grapes. Chitosan is a deactivated derivative of chitin and the efficacy of three chitosan formulations, Chitosan fulvate @ 2ml/L, Chitosan oligosaccharides @ 2ml/L, Chitosan @ 4ml/L were evaluated against powdery mildew. The physiological loss in weight of berries was also assessed. During the fruiting seasons of 2018-2019 and 2019- 2020, chitosan formulations were evaluated both as solo treatments and in alternation with Ampelomyces quisqualis @ 5ml/L. All Chitosan formulations effectively inhibited powdery mildew on leaves and berries when compared to untreated control. Among solo treatments, chitosan exhibited lower PDI (17.76 and 5.29) on leaves than untreated control (24.28 and 13.12) during 2018-2019 and 2019-2020 respectively. In case of bunches similar trend was observed with PDI 23.39 and 9.90 against untreated control 30.31 and 17.18 in both seasons. However, the formulations were found more effective when used in alternation with Ampelomyces quisqualis. During 2018-2019 and 2019-2020, Chitosan / Ampelomyces quisqualis treatment recorded significantly lower PDI of powdery mildew on leaves i.e.12.55 and 4.77than the untreated control with PDI 24.28 and 13.22 respectively. In case of bunches, treatment Chitosan / Ampelomyces quisqualis showed the lowest disease severity with PDI 19.28 and 8.98 against untreated control with PDI 31.60 and 17.18 in both the seasons respectively. The applications of formulation containing chitosan alternated with sprays of Ampelomyces quisqualis (5ml/L) showed least disease as well physiological loss in weight of grapes. All chitosan formulations were effective in minimizing physiological loss in weight of grapes as compared to control. Among all treatments, Chitosan / Ampelomyces quisqualis treatment recorded the lowest PLW i.e.13.94 and 13.90, whereas untreated control showed the highest PLW i. e. 15.85 and 15.87 during both seasons respectively. Results showed that chitosan can be effectively used in powdery mildew management, either alone or in alternation with the bioformulation of Ampelomyces quisqualis.Not Availabl