Novel secondary metabolite from a new species of Hypoxylon saxatilis sp. nov. for suppressing bacterial wilt in tomato

The bacterium <i>Ralstonia solanacearum</i> 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 <i>R. solanacearum</i> 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 <i>R. solanacearum</i> was subsequently examined using the agar plug diffusion test. Two isolates, <i>Hypoxylon</i> sp. NGPM and <i>Hypoxylon saxatilis</i> KKU-KHP 01, were identified from the midrib of the <i>Acanthus ebracteatus</i> and the petiole of the <i>Gardenia saxatilis</i> (Geddes), respectively. Furthermore, the biologically active compounds extracted from both fungal endophytes inhibited the pathogen in the fungal fermentation broth. Notably, <i>H. saxatilis</i> 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 <i>R. solanacearum</i>. 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 <i>H. saxatilis</i> 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.