EVALUATION OF EFFECTS OF Trichoderma asperellum STRAINS IN BIOCONTROL OF CHARCOAL-ROT CAUSING Macrophomina phaseolina (Tassi) Goidanich IN COWPEA

Abstract

Macrophomina phaseolina, a soil-borne fungal pathogen, is responsible for loss in cowpea at both seedling and adult stages. Synthetic fungicides, although proven effective in M. phaseolina control, lead to severe cowpea poisoning and pose significant health risks to humans and animals. Trichoderma are efficient bioagents for managing plant pathogens, representing a promising strategy for managing M. phaseolina. This study evaluated the antifungal efficacy of some Trichoderma asperellum strains against M. phaseolina. The Trichoderma isolates obtained from soil were macroscopically, microscopically and genotypically identified based on the Internal Transcribed Spacer genes. Bioactive compounds were isolated from selected Trichoderma species and evaluated using Gas Chromatography-Mass Spectrometry (GC-MS). The inhibitory potential of the suspensions of selected Trichoderma species against M. phaseolina was tested singly in vitro and in combination in vivo. The in vivo experiments were carried out in the screenhouse across pre-planting, post-germination and seed treatment for eighty days, and plant growth indicators such as plant height, stem girth and leaf number were assessed at ten-day intervals. Disease incidence and severity were also determined following standard methods. After the termination of the in vivo experiment, yield assessment was carried out to determine the cowpea pod number, pod weight, and seed number, as well as the fresh and dry weights of leaves, stems and roots. A histopathological assessment was carried out on the lower stems of the cowpea plants to determine the effect of M. phaseolina on the cowpea tissues. Phenotypic, microscopic and genotypic characterisation identified all the Trichoderma isolates as Trichoderma asperellum. Among the Trichoderma species isolated and tested, T. asperellum (Tric13), T. asperellum (Tric4) and T. asperellum (Tric12) exhibited significant inhibitory potential (p<0.05) against M. phaseolina in vitro, with inhibition values of 82.51%, 82.41%, and 81.95%, respectively. The production of volatile organic compounds varied among the Trichoderma species. Specifically, Tric13 yielded terpenoids, ketones, sesquiterpenes, cycloalkanes, and alcohols. In contrast, Tric4 produced fatty acids, aldehydes, alkanes, terpenoids, and aromatic and bicyclic compounds, while Tric12 generated carboxylic acids, terpenoids, phenolics, bicyclic and aromatic compounds. The in vivo results showed that Trt7 (plant height = 49.8000 cm; stem girth = 0.53625 cm; leaf number = 21.028), followed by Trt3 (plant height = 49.8250 cm; stem girth = 0.49986 cm; leaf number = 18.611), recorded higher cowpea biomass across the Trichoderma formulations than the negative control. Zero disease incidence was observed in Trt3 and Trt7 (0%) upon treatment with the Trichoderma formulations. Also, zero disease severity was noted in Trt3 and Trt7 (0%), compared to the negative controls, which displayed 100% incidence and severity. The cowpea yield assessment showed that the highest pod weight (11.3 g) and pod number (8) were recorded in Trt7 (pre-planting), and seed number (32) was recorded in Trt5 (seed treatment). The histopathological assessment of the cowpea showed the efficacy of the treatments in inhibiting the growth of M. phaseolina, minimising its tissue entry, and xylem vessel occlusion. This study established the efficacy of Trichoderma in M. phaseolina biomanagement and the synergistic potential of different Trichoderma asperellum strains in its biocontrol.