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First Report of Fusarium oxysporum f. sp. cubense Tropical Race 4 (VCG 01213/16) Associated with Cavendish Bananas in Laos
- Chittarath, K., Mostert, D., Crew, K. S., Viljoen, A., Kong, G., Molina, A. B., Thomas, J. E.
- Plant disease 2018 v.102 no.2 pp. 449
- DNA, Fusarium oxysporum f. sp. cubense, Fusarium wilt, Musa, ambient temperature, bananas, conidia, crop losses, crop production, culture media, discoloration, disease outbreaks, farmers, fruits, fungal culture, income, leaves, pathogenicity, plantains (fruit), plantations, polymerase chain reaction, potting mix, pseudostems, rhizomes, soil fungi, staining, streptomycin, wilting, China, Laos, Myanmar, Vietnam
- The popular Cavendish banana (Musa spp., AAA) constitutes about 45% of all bananas grown worldwide (Lescot 2015). Mainland China is one of the leading producers of Cavendish bananas, and to satisfy local demand for the fruit, commercial production is expanding into neighboring countries such as Laos, Myanmar, and Vietnam. In Laos, local banana varieties such as “kuay nam” (Silk, AAB), “kuay hom” (local Cavendish, AAA), and “kuay klai” (Horn Plantain, AAB) are preferentially grown for food and income generation. The expansion of Cavendish production from China into Laos, therefore, presents significant risks to these varieties, such as the introduction of Fusarium oxysporum f. sp. cubense (Foc) tropical race 4 (TR4; vegetative compatibility group [VCG] 01213/16), a soil-borne fungus that causes severe epidemics and crop losses in Cavendish plantations in southern Chinese provinces. In November 2016, Laos Department of Agriculture staff reported wilting plants from Luang Namtha and Borkeo provinces. In February 2017, yellow leaf symptoms typical of banana Fusarium wilt were observed in commercial Cavendish plantations in Luang Namtha and Vientiane provinces of Laos. When plants were cut open, the pseudostems displayed a dark-red to brown discoloration of the vascular tissue, and the inner rhizome revealed a ring of yellow-brown staining. Infected vascular strands were collected from five plants (three from Luang Namtha province and two from Vientiane province) for morphological, molecular, and VCG identification. Infected vascular strands were plated out onto potato dextrose agar containing 0.04 g/liter of streptomycin. The fungal cultures were single-spored and identified as F. oxysporum based on cultural characteristics and spore morphology (Nelson et al. 1983). Total DNA was extracted from each fungal culture for molecular identification. Amplicons of the expected sizes for Foc TR4-specific primers (Dita et al. 2010) and Foc race 4-specific primers (Lin et al. 2009) were obtained for all five samples. VCG testing (Puhalla 1985) established that the fungal isolates were all compatible with VCG 01213/16. For two isolates collected from Luang Namtha, pathogenicity testing was performed by adding a 30-ml spore suspension (10⁸ conidia/ml) onto the potting soil in which 4-month-old “kuay hom” plants were grown. Each isolate was inoculated onto three plants, and the control plants were treated with sterile distilled water. The infected plants were then incubated in a screened shade house under ambient temperature conditions. After 8 weeks, the Foc TR4-inoculated plants produced typical wilting and internal discoloration symptoms of Fusarium wilt. Fusarium spp. were reisolated from the inoculated plants and identified as Foc TR4/VCG 01213/16 by PCR (Dita et al. 2010), thereby completing Koch’s postulates. Many large- and small-scale Cavendish plantations of less than 5 years old are affected by Fusarium wilt in Luang Namtha and Vientiane provinces, sometimes at a high incidence. The incursion of Foc TR4 in Laos threatens not only the rapidly expanding Chinese Cavendish banana production but also susceptible popular varieties grown by small farmers for local markets in Laos.