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Pathogen Profile #15
Banana Boat - Fusarium oxysporum f. sp. cubense
Hello Robigalia Reader! Welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inbox!
Last week, researchers from the US and China published an article in Nature Microbiology on the evolutionary origins of a fungal pathogen that’s been devastating bananas since the 1970s. Scroll down to learn more about this fungus that might have you singing about bananas all day. If you’re a Robigalia subscriber, you also have access to my online database and today’s title song!
Gif by peacock on Giphy
Fusarium wilt of banana, caused by the soil-borne fungus Fusarium oxysporum f. sp. cubense (Foc), has plagued banana growers worldwide for over a century. In the 1950s, Foc race 1 wiped out the Gros Michel variety, leading to its replacement with the Cavendish cultivar. However, the emergence of Foc Tropical Race 4 (TR4) in the 1970s in Taiwan has posed an even greater threat, as it can infect and kill Cavendish bananas.
First detected in Southeast Asia in 1994, TR4 quickly spread across China, Malaysia, and Indonesia. The rapid spread of TR4 in Southeast Asia signalled its potential for global devastation, given the Cavendish banana's dominance in world trade. Throughout the 1990s and 2000s, TR4 devastates entire plantations in Australia's Northern Territory, forcing growers to abandon infested properties. Today, TR4 has been confirmed in over 20 countries across Asia, the Middle East, Africa, and, most recently, South America. Its arrival in Colombia in 2019, the leading banana exporter to the US, significantly impacted the industry.
The life cycle of TR4 starts with spores in the soil that germinate and enter the plant's roots through natural openings or wounds. The fungal hyphae then grow through the corm into the pseudostem's vascular system, blocking water and nutrient flow, which causes the plant to wilt. As the disease progresses, the fungus produces more spores, including long-lasting chlamydospores that can persist in the soil for years. Infected banana plants initially show yellowing at the edges of older leaves, which spreads toward the midrib. As the disease worsens, leaves turn brown, dry out, and collapse around the pseudostem, forming a "skirt." Younger leaves may remain erect but appear pale, and in severe cases, the entire plant dies.
TR4 poses a significant economic threat to the global banana industry, valued at over $52 billion annually and vital to the livelihoods of hundreds of millions. The pathogen can cause up to 100% yield loss in infested fields, as seen in Southeast Asia, where TR4 has devastated thousands of hectares of Cavendish plantations since the 1990s. In 2009, Indonesia reported losses of up to $121 million due to TR4, while Malaysia estimated $14.1 million in losses in 2013. Economic models suggest that developing TR4-resistant bananas could save billions globally by preventing catastrophic losses. If TR4 spreads to major banana-exporting regions in Latin America, the Caribbean, and West Africa, it could severely impact international trade and farmer livelihoods.
Since eradicating TR4 from the soil is impossible, the disease is likely to continue spreading globally. The best hope lies in breeding new resistant cultivars, though resistance might be temporary as the fungus can evolve. For long-term sustainability, the banana industry must diversify beyond Cavendish monocultures. Greater cultivar diversity, combined with resistant varieties and better management, is essential.
For those interested in the science behind TR4, I've summarised a few articles below. Robigalia subscribers can scroll down to access my online database, which includes full summaries of these and all other featured articles.
Fusarium oxysporum f. sp. cubense tropical race 4 papers
Virulence of banana wilt-causing fungal pathogen Fusarium oxysporum tropical race 4 is mediated by nitric oxide biosynthesis and accessory genes
This Nature Microbiology study reveals the virulence of TR4 is driven by nitric oxide biosynthesis and specific accessory genes. Genomic and transcriptomic analyses showed TR4 strains have a distinct evolutionary origin and lack accessory chromosomes, but possess virulence-related genes on core chromosomes. These findings could guide the development of targeted strategies to combat Fusarium wilt in bananas.
This review out of South America highlights the urgent threat of TR4 in the region, emphasising its rapid spread since 2019. It underscores the need for integrated management strategies, including biosecurity measures, development of resistant varieties, and enhanced soil health practices, to mitigate the disease's impact. It calls for coordinated efforts among researchers, farmers, and national organisations to ensure sustainable banana production and food security.
Development and application of a droplet digital PCR assay for the detection of Fusarium oxysporum f. sp. cubense tropical race 4 from different biological samples
Researchers have developed a droplet digital PCR (ddPCR) assay for detecting TR4. This assay demonstrates higher sensitivity and specificity than conventional methods, allowing for early detection in asymptomatic plants and complex environmental samples like soil and water. The ddPCR technology offers significant potential for managing Fusarium wilt by facilitating early detection and containment.
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Until next week,
Alyssa
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