Bananageddon: researchers race against time

About 100 million tons of bananas are produced annually in nearly 120 countries, making the banana one of the world’s top five staple foods.

But advancing fungal diseases pose such a serious threat that the global banana industry could be wiped out in just five to 10 years, devastating millions of small-scale farmers who depend on the fruit for food, fibre and income.

Three diseases in one

Sigatoka — a three-fungus disease complex — has already reduced banana yields by 40%. Its three fungal diseases — yellow Sigatoka (Pseudocercospora musae), eumusae leaf spot (Pseudocercospora eumusae) and black Sigatoka (Pseudocercospora figiensis) — emerged as destructive pathogens in just the last century. Eumusae leaf spot and black Sigatoka are now the most devastating, with black Sigatoka posing the greatest constraint to banana production worldwide. The constant threat of the disease requires farmers to make 50 fungicide applications to their banana crops each year to control the disease.

“30 to 35% of banana production cost is in fungicide applications,” explained Ioannis Stergiopoulos, plant pathologist at the University of California, Davis. “Because many farmers can’t afford the fungicide, they grow bananas of lesser quality, which bring them less income.” And for those growers who can afford fungicide, the applications pose environmental and human-health risks.

To further complicate matters, the bananas most commonly found in supermarkets and greengrocers are of the Cavendish variety, and unlike plants like tomatoes or beans, which are grown from seeds, bananas are grown from shoot cuttings.

“The Cavendish banana plants all originated from one plant and so as clones, they all have the same genotype — and that is a recipe for disaster,” Stergiopoulos said, noting that a disease capable of killing one plant could kill them all.

But the fight against Sigatoka has received a boost, with Stergiopoulos and colleagues from UC Davis, together with researchers in the Netherlands, discovering how three fungal diseases have evolved into such a lethal threat to the world’s bananas.

The discovery, reported in the online journal PLOS Genetics, better equips researchers to develop hardier, disease-resistant banana plants and more effective disease-prevention treatments.

“We have demonstrated that two of the three most serious banana fungal diseases have become more virulent by increasing their ability to manipulate the banana’s metabolic pathways and make use of its nutrients,” said Stergiopoulos.

Probing the genomes for solutions

Stergiopoulos and colleagues sequenced the genomes of eumusae leaf spot and black Sigatoka, comparing their findings with the previously sequenced yellow Sigatoka genome sequence.

They discovered that this complex of diseases has become lethal to banana plants not just by shutting down the plant’s immune system but also by adapting the metabolism of the fungi to match that of the host plants. As a result, the attacking fungi can produce enzymes that break down the plant’s cell walls. This allows the fungi to feed on the plant’s sugars and other carbohydrates.

“Now, for the first time, we know the genomic basis of virulence in these fungal diseases and the pattern by which these pathogens have evolved,” Stergiopoulos said.

“This parallel change in metabolism of the pathogen and the host plant has been overlooked until now and may represent a ‘molecular fingerprint’ of the adaption process,” he said.

Image caption: Plant pathologist Ioannis Stergiopoulos. Credit: Gregory Urquiaga/UC Davis