Abstract
The bacterium Ralstonia solanacearum is an important pathogen that causes wilt disease in tomatoes, leading to a reduction in tomato yield. Therefore, this study aimed to investigate the antibacterial activity of endophytic fungi isolated from medicinal plants against the wilt-causing bacterium R. solanacearum in tomatoes under both laboratory and greenhouse conditions. Forty-eight fungi were isolated from 20 medicinal plant species using the surface sterilization technique. The antagonistic activity against R. solanacearum was subsequently examined using the agar plug diffusion test. Two isolates, Hypoxylon sp. NGPM and Hypoxylon saxatilis KKU-KHP 01, were identified from the midrib of the Acanthus ebracteatus and the petiole of the Gardenia saxatilis (Geddes), respectively. Furthermore, the biologically active compounds extracted from both fungal endophytes inhibited the pathogen in the fungal fermentation broth. Notably, H. saxatilis KKU-KHP 01 is a newly identified fungal species and the first within its genus to produce tetrahydrofuran, a novel compound not previously reported as a natural secondary metabolite. This compound exhibits inhibitory activity against the phytopathogen R. solanacearum. Scanning electron microscopy revealed that exposure to pure tetrahydrofuran caused structural damage to the bacterial cell wall, resulting in visibly wrinkled cells. Additionally, under greenhouse conditions, the crude extract of H. saxatilis KKU-KHP 01 could reduce the severity of wilt disease by up to 83.33%, effectively caused by tetrahydrofuran. Therefore, tetrahydrofuran can be applied to control wilt disease in tomatoes under field conditions and can be further developed as a commercial biocontrol agent.