Fungal Bioagents and Botanicals Efficacy against Alternaria alternata Responsable for Leaf Blight Disease of Stevia rebaudiana

Stevia rebaudiana, a herbaceous perennial prized for its natural sweetness, has gained global acclaim and found in various parts of India including Karnataka. An exploration into leaf spot disease (caused by Alternaria alternata (FR.) Keissler) in Stevia was conducted under the conditions of southern Karnataka to identify effective management strategies. The symptoms initially manifested as petite circular spots of a light brown colour, subsequently evolving into irregular shapes ranging from dark brown to grey. Some spots maintained their circular form, exhibiting concentric rings or zones. Severely affected leaves exhibited the merging of numerous spots, forming expansive necrotic areas. On older leaves, concentric spots were predominantly found at the tips. The diameter of the leaf spots ranged from 2 to 18 mm. The conidial dimensions varied from 10 to 40 × 6-12 mm, displaying a mid to dark brown or olive-brown color. They were short-beaked, arranged in long chains, and had an oval and bean-shaped structure with 3–5 transverse septa. Considering the adverse effects of chemical fungicides, the exploration for a safer alternative to control the pathogen became a preferable option. This led to experiments involving the use of bioagents for pathogen control. The six known bioagents were evaluated by dual culture, pathogen at periphery and pathogen at the center technique to monitor antagonistic effect. The results revealed that out of all the six bioagents used, two bioagents namely Trichoderma viride (74.77%, 69.04% and 79.45%) and T. harzianum (71.25%, 59.96% and 74.78%) showed maximum growth inhibition in dual culture, pathogen at periphery and pathogen at the center methods, respectively. among the botanicals used neem (36.63%) and ginger (36.42%) found to be effective in inhibiting mycelial growth. Unraveled the strong antagonistic effect to inhibit the mycelia growth of the pathogen significantly.