The Evolution of Bananas: From Wild Fruit to Vulnerable Crop

The yellow banana in nearly every supermarket is a single variety, and its plants are essentially copies of one another. That means a disease able to kill one plant can, in principle, kill them all. It has already happened once, to the banana your grandparents ate, and a new strain of the same soil fungus is now spreading.1PubMed Central. Fusarium Wilt of Banana: Current Knowledge on Epidemiology and Research Needs Toward Sustainable Disease Management2PubMed Central. Fusarium Tropical Race 4 in Latin America and the Caribbean: status and global research advances towards disease management

How a Seedy Wild Fruit Became the Banana We Eat

A wild banana is small and packed with hard, pebble-like seeds, with very little flesh around them. The tiny black specks in the middle of a store banana are what remain of seeds that never developed.

The edible bananas we know come from crosses between wild banana species and subspecies in Southeast Asia.3PubMed Central. Unravelling genomic drivers of speciation in Musa through genome assemblies of wild banana ancestors Their DNA is a patchwork from nine wild ancestral groups, two of which scientists hadn’t known about until recently.4PubMed Central. Unravelling genomic drivers of speciation in Musa through genome assemblies of wild banana ancestors Two wild species did most of the work: Musa acuminata, coded ‘A’, and Musa balbisiana, coded ‘B’.5PubMed. Identification of cytoplasmic ancestor gene-pools of Musa acuminata Colla and Musa balbisiana Colla and their hybrids by chloroplast and mitochondrial haplotyping6PubMed. Genetic Diversity in Musa acuminata Colla and Musa balbisiana Colla and some of their natural hybrids using AFLP Markers

Seedlessness came from a couple of lucky quirks. Some wild bananas could form fruit without being pollinated, which gave flesh without seeds.7ProMusa. Domestication of the banana Sometimes one parent also passed on a double set of its chromosomes, producing offspring with three sets instead of two.8ProMusa. Domestication of the banana Those plants make almost no working seeds or pollen, so their fruit stays seedless.9ProMusa. Domestication of the banana10PubMed Central. Going wild in banana breeding enables Fusarium-resistant hybrids with improved fruit quality The only way to multiply them is to replant shoots, so every plant is a copy of its parent.

Most seedlings from wild crosses are seedy and unappealing, so farmers would have noticed the rare one with sweet, seedless fruit. The only way to keep it was to dig up a shoot that grows from the base of the parent, and replant it. Each named variety today, such as Cavendish or a plantain type, traces back to one such lucky plant multiplied over generations. Since the plants rarely reproduce by seed, new varieties arose mainly from rare fresh crosses or small mutations in a shoot, not from regular breeding. Farmers could keep the best plants exactly as they were for millennia, but with no new mixing of genes there were no natural new defenses against disease.

People started early. In the highlands of Papua New Guinea, they were intensively growing bananas at least 6,500 to 7,000 years ago.11PubMed. Origins of agriculture at Kuk Swamp in the highlands of New Guinea They burned forest and transplanted banana plants into open grassland there.12Quaternary International. The palaeoenvironments of Kuk Swamp from the beginnings of agriculture in the highlands of Papua New Guinea Bananas were domesticated across New Guinea, Southeast Asia and India, and reached East Africa about 2,000 years ago.13Environment and History. Food Traditions and Landscape Histories of the Indian Ocean World: Theoretical and Methodological Reflections

The Banana That Disappeared, Gros Michel and Panama Disease

Gros Michel was the main export banana in the first half of the 20th century.14ProMusa. Gros Michel By around 1900 it made up most banana exports to the United States.15ProMusa. Gros Michel It was widely described as creamier and more strongly flavored than today’s banana, and artificial banana flavor is often said to resemble it.

Panama disease, also called Fusarium wilt, is caused by a fungus that lives in the soil and attacks the plant’s roots and water-carrying tissue.16PubMed Central. Fusarium Tropical Race 4 in Latin America and the Caribbean: status and global research advances towards disease management The leaves yellow and wilt as the water supply is choked off, and the plant eventually collapses and dies. In the mid-20th century a strain called Race 1 wiped out the Gros Michel industry in Central America.17PubMed Central. Fusarium Wilt of Banana: Current Knowledge on Epidemiology and Research Needs Toward Sustainable Disease Management

The soil is what makes the disease so hard to fight. The fungus leaves residue that infects future crops planted there,18Food Ingredients First. Scientists unlock genetic resistance to combat Fusarium wilt disease in bananas and its spores can sit dormant for decades, waiting for a susceptible banana to be planted. Growers couldn’t simply replant Gros Michel on the same land. Because the fungus lives in the soil and gets in through the roots, fungicide sprays on the leaves cannot stop it. The practical answer was to abandon infected fields and clear new land, which is why plantations moved. That only worked for a while, since moving infected plants or soil carried the disease to new farms.

Why the Cavendish Took Over and Why That Is Risky

Cavendish bananas began taking over the trade in the late 1950s because they resisted Race 1.19Promusa. Cavendish subgroup They also yield heavily and stay green for weeks after harvest, so they survive long shipping.20U.S. International Trade Commission. Yes, We Have No Bananas? (Again!) Cavendish fruit grows in tidy, uniform bunches that pack neatly into boxes, and the plants are shorter than many bananas, so wind is less likely to knock them over. Growers could replace Gros Michel without rebuilding the supply chain, because the new fruit fit the existing system.

Today Cavendish supplies 99% of banana exports.21PubMed Central. Fusarium Wilt of Banana: Current Knowledge on Epidemiology and Research Needs Toward Sustainable Disease Management Cavendish also makes up more than 40% of all bananas grown worldwide, including much local trade.22PubMed. The Vulnerability of Bananas to Globally Emerging Disease Threats That uniformity keeps prices low and quality predictable. But Cavendish is highly susceptible to the diseases now spreading, so a disease that beats one plantation can beat them all, an “extreme” genetic vulnerability.23PubMed. The Vulnerability of Bananas to Globally Emerging Disease Threats

Exported bananas are picked green and ripened later with ethylene, a natural ripening gas.24PubMed Central. Induced Ripening Agents and Their Effect on Fruit Quality of Banana The same gas explains why your bananas ripen fast at home. Bananas release their own ethylene as they ripen, so storing them in a bag traps it and speeds things up. Keeping them apart from other ripening fruit, which also gives off ethylene, slows it down.

TR4, The New Fungus Threatening the Cavendish

Tropical Race 4, or TR4, is a strain of the same soil fungus that causes Panama disease.25PubMed Central. Fusarium Tropical Race 4 in Latin America and the Caribbean: status and global research advances towards disease management Resistance is specific to a strain. Cavendish was never immune to Panama disease; it simply wasn’t affected by the particular strain that destroyed Gros Michel. TR4 gets past that protection, so the resistance that saved the industry in the 1950s does not protect against it.26PubMed. The Vulnerability of Bananas to Globally Emerging Disease Threats27PubMed Central. Fusarium Tropical Race 4 in Latin America and the Caribbean: status and global research advances towards disease management

TR4 has intensified in recent years and is spreading rapidly, including into Latin America and the Caribbean.28PubMed Central. Fusarium Tropical Race 4 in Latin America and the Caribbean: status and global research advances towards disease management It moves between farms in soil on shoes, vehicles and tools, and infected planting material carries it to new regions. Once it is in a field, there is no known way to remove it from the soil.

Even so, bananas are not about to vanish from stores. The spread is farm by farm, not overnight. Detection comes first: a farm that finds yellowing plants can have them tested so the fungus is identified early. Infected plantations can be fenced off, with footbaths and restrictions on who and what enters. Governments in unaffected countries can inspect imports of plants and soil to try to keep the fungus out. Cleaning boots and tools slows it further.

Other Diseases Bananas Face

Black Sigatoka, a leaf fungus, attacks most edible banana varieties worldwide.29PubMed Central. Mycosphaerella fijiensis, the black leaf streak pathogen of banana: progress towards understanding pathogen biology and detection, disease development, and the challenges of control It starts as small streaks and spots that darken and merge, so a field can look scorched even while the plants are alive. It cuts the leaves’ ability to make food, which reduces the amount and quality of fruit and makes it ripen too early.30PubMed Central. Mycosphaerella fijiensis, the black leaf streak pathogen of banana: progress towards understanding pathogen biology and detection, disease development, and the challenges of control Spores travel through the air and in rain splash, so the fungus is hard to keep out of any region.

Sterile bananas can’t easily be bred for resistance, so growers rely on heavy fungicide spraying.31PubMed Central. Mycosphaerella fijiensis, the black leaf streak pathogen of banana: progress towards understanding pathogen biology and detection, disease development, and the challenges of control Large export plantations may spray many times a year, which adds cost and pollution. The pesticides harm ecological and human health on top of the crop losses.32PLANTS, PEOPLE, PLANET. The long road to a sustainable banana trade

Small farmers are hit hardest. Many growers in tropical countries raise bananas to feed their own families and cannot afford repeated spraying. Without it, they lose a large share of the harvest to leaf disease, which hits their food supply directly.

Banana bunchy top virus stunts plants, curls and shrivels the leaves, and can kill them.33PubMed Central. RNAi technology for management of banana bunchy top disease Aphids carry it locally, while infected planting material carries it over long distances.34PubMed Central. Alternative hosts of banana bunchy top virus in the Philippines and the first evidence of seed transmission of BBTV

Can Wild Bananas and Gene Editing Save the Crop

Wild bananas are inedible, but they are useful. They still make seeds and can be crossed, and their genes evolved against local diseases. A wild banana called Calcutta 4 carries a natural resistance trait against a form of Panama disease.35Food Ingredients First. Scientists unlock genetic resistance to combat Fusarium wilt disease in bananas In a wild Malaysian banana, a single gene region protects against Race 1 and gives partial protection against TR4.36PubMed Central. Genetic mapping of Fusarium wilt resistance in a wild banana Musa acuminata ssp. malaccensis accession A resistant wild banana switched on over three times as many defense genes against TR4 as Cavendish did.37PubMed. Deep RNA-seq analysis reveals key responding aspects of wild banana relative resistance to Fusarium oxysporum f. sp. cubense tropical race 4

Finding such a gene means growing resistant and susceptible plants in infected soil and comparing which survive. Researchers then look for the stretch of DNA that differs between survivors and the plants that die. Once that region is identified, breeders can screen seedlings for it rather than waiting for the plants to grow and be tested.

Even so, breeding is slow. After nearly a century of effort, it remains hard because three-chromosome-set bananas are highly sterile.38PubMed Central. Going wild in banana breeding enables Fusarium-resistant hybrids with improved fruit quality Crossing a wild banana with an edible one mixes thousands of genes, so most offspring inherit the wild plant’s seedy fruit and poor taste. Breeders must cross the offspring back toward edible types again and again, and each generation takes many months to flower and fruit. Still, crosses with wild bananas have produced hybrids resistant to Fusarium with improved fruit quality.39PubMed Central. Going wild in banana breeding enables Fusarium-resistant hybrids with improved fruit quality

Gene editing with CRISPR offers a shortcut, because it can add useful traits without bringing in unwanted foreign genes.40PubMed Central. Using Genetic Engineering Techniques to Develop Banana Cultivars With Fusarium Wilt Resistance and Ideal Plant Architecture It tweaks the banana’s own genes, whereas older genetic modification inserted genes from other organisms, and rules and public acceptance differ between the two. Researchers in Kenya used editing to protect bananas against bacterial wilt and a streak virus.41PubMed Central. Application of CRISPR/Cas-based gene-editing for developing better banana

Meanwhile, the world’s largest banana genebank, in Belgium, holds about 1,600 varieties from 38 countries, backed up frozen.42Crop Trust. Demand and Use Assessment of Genetic Plant Material from the Bioversity’s International Musa Germplasm Transit Centre (ITC) in Africa, Asia and the Pacific43CABI Agriculture and Bioscience. Safeguarding and using global banana diversity: a holistic approach It sends disease-free plants to breeders and farmers who request them.44Crop Trust. Demand and Use Assessment of Genetic Plant Material from the Bioversity’s International Musa Germplasm Transit Centre (ITC) in Africa, Asia and the Pacific

The Many Bananas Beyond the Cavendish

Hundreds of banana types exist, and the letter codes describe which wild ancestors contributed the three chromosome sets. AAB, for instance, means two sets from acuminata and one from balbisiana. More B genes tend to go with sturdier plants that cope better with drought and suit cooking. The codes capture ancestry only, so two AAA bananas can still differ in taste and size.

Plantains hold more starch and less sugar than dessert bananas, so they taste starchy rather than sweet, especially when unripe. They can be fried, boiled or roasted, and their flavor shifts as the peel turns from green to yellow to black. In many tropical regions they are a staple food, not a side fruit. You can try them, or red and finger bananas, in larger grocers. Cook plantains rather than eating them raw.

Some bananas are bred for nutrition. Many staple bananas in regions with vitamin A deficiency are low in pro-vitamin A.49Grand Challenges. Grand Challenges Retrospective Field-grown “golden” bananas engineered with a banana gene reached pro-vitamin A levels many times higher.50PubMed Central. Golden bananas in the field: elevated fruit pro‐vitamin A from the expression of a single banana transgene