In vitro Evaluation of Trichoderma Isolates and Botanical Extracts Against Early Blight of Tomato Caused by Alternaria solani

Early blight, caused by Alternaria solani, is an important fungal disease that serves as a serious limitation to tomato production, mainly in high-humidity and warm areas of Nepal. The present study was conducted to explore the in vitro efficacy of Trichoderma isolates and botanical extracts against A. solani. The pathogen was isolated from infected tomato plants and subjected to dual culture and poisoned food assays in a completely randomized design with five replications. Three species of Trichoderma (T. asperellum, T. harzianum, and T. viride) and four selected plant extracts (Azadirachta indica, Ocimum basilicum, Cymbopogon citratus, and Psidium guajava leaves) were tested for their effectiveness to inhibit A. solani mycelial growth over a period of 10 days. All the treatments significantly inhibited the mycelia growth in comparison to the control. T. asperellum showed the highest inhibitory activity (0.21), followed by T. harzianum and T. viride, thus showing their good antagonistic potential against A. solani. Similarly, among botanical extracts, Azadirachta indica leaf extract was most effective in inhibiting the mycelia growth (83.78), followed by O. basilicum and C. citratus extracts, with P. guajava leaf extract being comparatively less effective. The results indicate that T. asperellum and A. indica leaf extract had the highest mycelial growth inhibition capacity and proved to be effective in in vitro conditions. Therefore, these options must be evaluated further in field conditions against early blight of tomato, which can be a promising eco-friendly approach alternative to synthetic chemical fungicides.