Banana wilt disease caused by Fusarium oxysporum f. sp. cubense is one of the most significant threats to banana production worldwide. Strains of F. oxysporum f. sp. cubense have been grouped into race-1, -2, or -4 on the basis of differential virulence among different genotypes of banana. In India, though the disease is reported among susceptible varieties of races 1 and 2, the disease is not reported from Cavendish cultivars, which are the differential host to race-4. Recent surveys of the Cumbum areas (Theni District, Tamil Nadu) revealed symptoms (e.g., yellowing and drooping of leaves around the pseudostem and longitudinal splitting of pseudostem) on cv. Grand Naine (Cavendish group -- AAA). F. oxysporum f. sp. cubense was recovered and single-spore isolates had characteristic white-to-purple aerial mycelia producing single-celled, oval microconidia in false heads on branched monophialides and sickle-shaped macroconidia with an attenuated apical cell and a foot-shaped basal cell. Pathogenicity was demonstrated on cv. Grand Naine by inoculation with sand maize meal inoculum (20 g per pot containing 106 spores per g). Vegetative compatibility, using 33 nit-M testers of all known vegetative compatibility groups (2), showed that nit-1 mutants generated from a wild strain of F. oxysporum f. sp. cubense isolated from cv. Grand Naine formed robust heterokaryons with nit-M tester 0124 of the Department of Employment, Economic Development and Innovation, Brisbane, Australia and also with nit-M tester obtained from an isolate of F. oxysporum f. sp. cubense from Karpuravalli (Pisang Awak-ABB). Further characterization of this new Cavendish strain was studied on the basis of volatile odor production (3) using VCGs 0125 for race-1 (‘inodoratum group’) and 0120 for race 4 (‘odoratum group’) as positive controls and sterile medium as a negative control. This new F. oxysporum f. sp. cubense strain of Cavendish belonged to ‘inodoratum’ group of F. oxysporum f. sp. cubense. Pathogenicity was demonstrated on potted plants (10 per cultivar) of cvs. Rasthali (Silk-AAB), Karpuravalli (Pisang Awak-ABB), Ney Poovan (AB), Poovan (Mysore-AAB), Red Banana (AAA), Nendran (French plantain-AAB), Monthan (ABB), and Grand Naine (Cavendish-AAA) by inoculation with sand maize meal inoculum (20 g per pot containing 106 spores per g) in three replicate experiments. Plants were uprooted 2 months postinoculation and disease severity was estimated by rating internal vascular discoloration in the corm (1). The result showed that all cultivars, except Red Banana and Nendran, had the highest rating for disease severity, 6. To our knowledge, this is the first report of a virulent strain of F. oxysporum f. sp. cubense VCG 0124 of race-1on Cavendish banana.
References: (1) J. Carlier et al. Technical Guidelines Number 6. INIBAP, Montpellier, France, 2002. (2) J. C. Correll et al. Phytopathology 77:1640, 1987. (3) N. Y. Moore. Aust. J. Bot 39:161, 1991.