Chilli is one of the important spices and cultivated around the world for its peculiar hot taste. Chilli leafs prone to affect by Alternaria alternata (leaf spot). In order to overcome this problem, efforts have been made to evaluate the efficiency of the bio-control agent Trichoderma harzianum for controlling the leaf spot disease in chilli. Trichoderma harzianum was isolated from the field soil and identified through microscopic and recommended standard methods. The antagonistic activity of the Trichoderma harzianum was evaluated against the pathogenic fungi responsible for leaf spot disease. Alternaria alternata belongs to the sub-division Deuteromycotina, class Dothideomycetes, family Pleosporaceae. The mycelium of Alternari alternata is septate, brown to brownish grey in colour. The conidiophores are dark, septate, arise in fascicles, measuring 14-74 × 4-8 μm. Conidia are brownish black, obclavate, borne singly or sparingly in chains of 2-4, muriform with long beak and the overall conidial size ranges between 148-184 × 17-24 μm with 10-11 transverse and Alternaria alternata proved the efficiency to inhibit. The antagonistic activity of Trichoderma harzianum was screened in vitro against Alternaria alternata by dual culture plate technique on PDA media for 7days. The results revealed Trichoderma harzianum was shown inhibition of 60.71% over Alternaria alternata.

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