RESUMO
Banana cultivars with the AAB genome group comprise diverse subgroups, such as Plantain, Silk, Iholena, and Pisang Raja, among others, which play an important role in food security in many developing countries. Some of these cultivars are susceptible to Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4), the most destructive pathogen threatening banana production worldwide, and some of them are still largely unknown. We evaluated the resistance of 37 banana genotypes, including Plantain, Silk, Iholena, Maia Maoli/Popoulu, Pisang Raja, Pome, and Mysore, to Foc TR4 under both greenhouse and field conditions. Genotypes from the Silk and Iholena subgroups were highly susceptible to Foc TR4. Pome and Mysore showed resistance and intermediate resistance, respectively. However, Pisang Raja ranged from susceptible to intermediate resistance. One cultivar from the Maia Maoli/Popoulu subgroup was highly susceptible, while the other displayed significant resistance. Most Plantain cultivars exhibited high resistance to Foc TR4, except two French types of cultivar, 'Uganda Plantain' and 'Njombe N°2', which were susceptible. The susceptibility to Foc TR4 of some of the AAB genotypes evaluated, especially Plantain and other cooking bananas, indicates that growers dependent on these varieties need to be included as part of the prevention and integrated Foc TR4 management strategies, as these genotypes play a crucial role in food security and livelihoods.
RESUMO
Fusarium wilt, caused by Fusarium oxysporum f. sp. cubense (Foc), has been considered as the most devastating disease affecting bananas (Musa spp.) worldwide. A highly virulent strain of Foc, known as tropical race 4 (TR4), has been detected in the southeast Asia in the 1990s, and has since spread to western Asia, Australia, the Middle East, southern Africa, and South America (Viljoen et al. 2020). Foc TR4 can cause severe yield losses in Cavendish (AAA), Gros Michel (AAA), Silk (AAB), Pisang Awak (ABB) and Bluggoe (ABB) bananas (Ploetz et al. 2006). However, cooking bananas such as plantain (AAB) and Matooke (AAA) bananas, appear to be resistant (Zuo et al. 2017). Iholena bananas (AAB), a subgroup of varieties related to plantains (also known as Pacific plantains), is an important staple food in the Pacific Islands where it was domesticated. It is also popular in Peru, probably due to its nutritional value (Kepler et al. 2011) and is wildly cultivated in other South American countries (Dita et al. 2013). In December 2019, typical symptoms of banana Fusarium wilt were observed on Iholena accession 'Pacific Plantain' (ITC0210) in experimental fields located in Dongguan, Guangdong Province of China. The symptoms included leaf yellowing and pseudostem splitting. The vascular tissue inside the pseudostems was dark red to brown, and the inner rhizomes yellowish-brown. Vascular tissues from three diseased plants were sampled aseptically and placed on potato dextrose agar (PDA) containing 0.05 g/liter kanamycin. Fungal colonies typical of F. oxysporum developed rapidly, with purple-tinged white aerial mycelia and an abundance of microconidia borne in false heads on short microconidia (Nelson et al. 1983). Chlamydospores were produced singly or in pairs in hyphae and macroconidia. Molecular identification was performed using Foc race 4-specific primers (Lin et al. 2009), Foc TR4-specific primers (Dita et al. 2010), Foc race 1 and Foc STR4-specific primers (Ndayihanzamaso et al. 2020). Amplicons of expected sizes were obtained for Foc TR4 and race 4, but not for Foc race 1 and STR4. Sequencing of the ITS and 18S rDNA from the three Iholena isolates and BLAST result showed a 100% similarity to the Foc TR4 reference sequences in GenBank (Foc II5, PRJNA73539 and PRJNA56513) to prove that the isolates were Foc TR4. Pathogenicity of the three isolates from Iholena bananas was determined by infecting 4-month-old Cavendish cv. 'Grand Nain' bananas and three Iholena accessions, 'Pacific Plantain' 'Tigua' and 'Uzakan', under greenhouse conditions by root immersion in a Foc conidial suspension and soil drenching at 106 conidia/ml (Dita, 2010). Control plants were treated with sterile distilled water. Three replications of five plantlets were used for each accession. After 35 days, the inoculated plantlets developed typical Fusarium wilt symptoms such as yellowing of the older leaves and discoloration of the inner rhizome. The control plants did not develop symptoms. To complete Koch's postulates, the fungus was re-isolated from inoculated plants and identified as Foc TR4 by PCR (Dita et al, 2010). The susceptibility of 'Tigua' and 'Uzakan' was also confirmed in Foc TR4-infested field trials, with both accessions developing severe Fusarium wilt symptoms. The susceptibility of Iholena bananas to Foc TR4 is of significant concern to all countries where this subgroup is cultivated for major food source, including Peru and other South American countries.
