In the summer of 2000, tomato (Lycopersicon esculentum Mill.) plants in several commercial fields in southeastern and eastern Tennessee exhibited symptoms of Fusarium wilt. All cultivars on which symptoms were observed are classified as resistant to races 1 and 2 of the causal fungus, Fusarium oxysporum Schlechtend.:Fr. f. sp. lycopersici (Sacc.) W.C. Snyder and H.N. Hansen. Race 3 has been reported from several areas (1), but not from Tennessee, a major producer of fresh market tomatoes. F. oxysporum was consistently isolated from discolored vascular tissue on potato dextrose agar (PDA). Pathogenicity and race determination tests for six isolates representing three counties were conducted by inoculating cultivars susceptible to races 1, 2, and 3 (Rutgers); resistant to race 1 (Bradley, Roma VF); resistant to races 1 and 2 (Conquest, Florida 47); or resistant to races 1, 2, and 3 (Floralina). Inoculum suspensions were obtained from 1-week-old cultures grown on PDA. Seedlings were grown in commercial potting mix for 3 weeks. The roots were rinsed and submerged for 30 s in inoculum suspensions (1 × 107 conidia per ml). Seedlings were then transplanted into potting mix in metal flats and placed in a greenhouse. Natural light conditions provided a 12-h photoperiod, and day and night temperatures averaged 29 and 17°C, respectively. Within 4 weeks after inoculation, all isolates caused symptoms of Fusarium wilt in all cultivars except Floralina, indicating that the isolates were race 3. The pathogen was reisolated from the discolored vascular tissue of diseased plants. Among the cultivars most severely affected by all six isolates was Conquest, which is resistant to F. oxysporum f. sp. radicis-lycopersici, the cause of Fusarium crown and root rot.
Reference: (1) M. L. Marlatt et al. Plant Dis. 80:1336, 1996.
