Fusarium wilt of banana, caused by the soilborne phytopathogen Fusarium oxysporum f. sp. cubense (Foc), is considered one of the most damaging diseases of agricultural crops. In recent years Foc tropical race 4 (TR4), a strain that severely affects Cavendish banana and several other dessert and cooking banana varieties, has spread to many banana-growing countries worldwide (Viljoen et al. 2020). Foc TR4 was first reported from two countries in the Indian subcontinent, India and Pakistan, but only Foc race 1 was previously reported from Nepal (Pant et al. 2023). Banana is considered the third most important fruit crop in and is cultivated on more than 26,000 ha by approximately 37,000 farmers. In June and September 2023, typical Fusarium wilt symptoms with progressive leaf yellowing and the reddish-brown discoloration of pseudostem vascular tissue were observed on Cavendish cv Grand Naine bananas in Sudurpaschim (28.50°N and 81.1333 E) and Lumbini (27.32°N and 83.40°E) provinces. Pseudostem samples were collected from symptomatic plants and discolored vascular strands of 5 cm each were surface sterilized with 1% sodium hypochlorite under laboratory conditions, washed twice with sterile water, and air-dried before it was placed onto potato dextrose agar (PDA) plates for isolation. After five days of incubation at 25°C with a 12-hr light/darkness illumination, colonies showing typical cultural and morphological characteristics of F. oxysporum on PDA and carnation leaf agar, respectively, were observed (Leslie and Summerell, 2006). These characteristics included white aerial mycelia with a purple center, short monophialides with microconidia produced in false heads, and an abundance of chlamydospores. All purified isolates were then stored on silica gel at the Nepalese Type Culture Collection at NARC-National Plant Pathology Research Center with reference number RBK 01-09. Nine representative F. oxysporum isolates were selected for molecular and vegetative compatibility (VCG) testing.. For molecular identification, DNA was extracted and PCR conducted with two sets of Foc TR4-specific primers (Dita et al. 2010; Li et al. 2013). After gel electrophoresis respective amplicon sizes of 463 bp and 452 bp were observed, confirming that the isolates tested positive as Foc TR4. For VCG testing, nit-mutants were generated for each Nepalese isolate and paired with Foc VCG 01213/16 tester strain which is associated with Foc TR4 (Ploetz, 2006). The identities of the Nepalese isolate were confirmed when it formed stable heterokaryons with VCG 01213/16 tester strains. Pathogenicity testing was conducted by inoculating five tissue culture banana plants (4-5 leaf stage) of Cavendish cultivar Grande Naine with a 20-ml of a spore suspension (1×105 conidia ml-1) per plant. Control plants were each drenched with 20 ml sterile water. Typical Fusarium wilt symptoms developed six weeks after inoculation with the Foc isolates from Nepal and a Foc TR4 reference isolate, while the water controls remained free of symptoms. Re-isolations were made and identified using the Foc TR4-specific PCR primers developed by Dita et al. (2010) to confirm Koch’s postulates. These results thus confirm the presence of the Foc TR4 in Nepal for the first time. Given that Cavendish cultivars account for about 70% of banana production in Nepal, the incursion of TR4 represents a critical biosecurity threat with far-reaching implications for national food and nutritional security.
Khadka et al. (Thu,) studied this question.
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