Phytotoxic Isocassadiene-Type Diterpenoids from Tomato Fusarium Crown and Root Rot Pathogen f. sp. .

Fusarium crown and root rot (FCRR) has emerged as a highly destructive soil-borne disease, posing a significant threat to the safe cultivation of tomatoes in recent years. The pathogen of tomato FCRR is f. sp. (). To explore potential phytotoxins from , eight undescribed diterpenoids namely fusariumic acids A-H () were isolated. Their structures were elucidated by using spectroscopic data analyses, quantum chemical calculations, and X-ray crystallography. Fusariumic acids A () and C-H () were typical isocassadiene-type diterpenoids, while fusariumic acid B () contained a cage-like structure with an unusual 7,8-seco-isocassadiene skeleton. A biosynthetic pathway of was proposed. Fusariumic acids A () and C-H () were further assessed for their phytotoxic effects on tomato seedlings at 200 μg/mL. Among them, fusariumic acid F () exhibited the strongest inhibition against the hypocotyl and root elongation of tomato seedlings, with inhibitory rates of 61.3 and 45.3%, respectively.