Rapid Tomato Seedling Assay for Virulent Isolates of Fusarium oxysporum F. sp. Radicis‐lycopersici (FORL), the Tomato Crown and Root Rot Pathogen

AbstractA rapid tomato seedling assay was developed for determining the relative wilt capacity of isolates of Fusarium oxysporum f. sp. radicis‐lyco‐persici (FORL), a virulent strain of the tomato pathogen. The procedures for the assay require that 5‐day‐old cv. Bonny Best tomato seedlings be dipped in 30‐day cell‐free concentrated culture filtrates of FORL isolates, which were grown in Czapek‐Dox medium with 2% Bacto‐casamino acids (CDA). The seedlings in the culture filtrates were then incubated at 30 C under artificial light (1200 ftcandles) at 28% relative humidity in a wind stream of 100–150 m min. The relative pathogenicity of the isolates was determined by inoculating the roots of 18‐day‐old seedlings with cultures of FORL isolates. The pattern of cell‐free filtrate wilt among the isolates was the same as that for the disease caused by cultures of the isolates. The seedlings treated with the filtrate from the most virulent isolate (Harrow HRS‐182) wilted in 20 min. The filtrates from less virulent isolates took progressively longer. up to 90 min. to cause comparable wilt. Isolate HRS‐082 was the first isolate also to induce disease in 10‐day‐old seedling assay. Both assays indicate three levels of wilt and disease capacities amongthe isolates examined. The utility of the assay in research and breeding for resistance is discussed.