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

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 Fusarium oxysporum f. sp. radicis-lycopersici ( Forl ). To explore potential phytotoxins from Forl , eight undescribed diterpenoids namely fusariumic acids A-H ( 1-8 ) were isolated. Their structures were elucidated by using spectroscopic data analyses, quantum chemical calculations, and X-ray crystallography. Fusariumic acids A ( 1 ) and C-H ( 3-8 ) were typical isocassadiene-type diterpenoids, while fusariumic acid B ( 2 ) contained a cage-like structure with an unusual 7,8-seco-isocassadiene skeleton. A biosynthetic pathway of 2 was proposed. Fusariumic acids A ( 1 ) and C-H ( 3-8 ) were further assessed for their phytotoxic effects on tomato seedlings at 200 μg/mL. Among them, fusariumic acid F ( 6 ) exhibited the strongest inhibition against the hypocotyl and root elongation of tomato seedlings, with inhibitory rates of 61.3 and 45.3%, respectively.