Field resistance of Cavendish somaclonal variants and local banana cultivars to tropical race 4 of Fusarium wilt in the Philippines

Recent epidemics of Fusarium wilt, caused by the virulent strain tropical race 4 (TR4) of Fusarium oxysporum f. sp. cubense (Foc), pose a serious threat to the banana industry. Four Cavendish (AAA genome) somaclonal variants selections from Taiwan ('GCTCV-105', 'GCTCV-119', 'GCTCV-218' and 'GCTCV-219') and three important Philippine local cultivars ['Latundan' (AAB genome, Silk subgroup), 'Lakatan' (AAA genome, Lakatan subgroup) and 'Saba' (ABB genome, Saba subgroup)], were compared to the commercial Cavendish 'Grand Naine' (AAA genome, Cavendish subgroup) in a heavily Foc-infested soil in the southern Philippines. Experimental plots were planted with 10 tissue culture-derived seedlings, replicated 10 times, and arranged in a completely randomized design. Disease incidence was assessed weekly by monitoring early symptoms of yellowing of older leaves and/or other symptoms such as pseudostem splitting. Onset of disease symptoms was recorded. The infected plants were allowed to develop severe typical symptoms of Fusarium wilt (leaves showing clear yellowing, and marginal necroses appear on older leaves) and then checked for vascular necroses as confirmation. The commercially-grown cultivars 'Grand Naine' and 'Lakatan' showed susceptibility with a disease incidence of 64 and 76%, respectively, in the primary crop; and 79 and 92%, respectively in the ratoon crop. Disease incidence in the GCTCV cultivars ranged from 0-6% in the primary crop and 0-8% in the ratoon crop. 'Saba' showed 0% incidence in the primary as well as in the ratoon crop. Foc VCG 01213/16, the Foc strain associated with TR4, was confirmed in the infected plants using a Polymerase Chain Reaction (PCR)-based diagnostic tool. Results confirm the stability of resistant somaclones to Foc TR4, even in agro-ecosystems differing from those where these were first developed. They also show that selecting favorable somaclonal variants is a feasible approach in cultivar improvement, and they have a potential role in integrated Foc TR4 management. The study identifies the TR4-resistant cultivar 'Saba, showing that TR4 does not severely affect all cultivars.