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Friday, March 9, 2012

Global march of banana fungus revealed

6 March 2012
Global march of banana fungus revealed

By Anna-Louise Taylor
Reporter, Nature News

A banana and plantain fungus which has spread across the world originated in South East Asia, new research has found.

Black leaf streak disease (Mycosphaerella fijiensis) affects leaf photosynthesis, and causes premature ripening.

It also delays harvests and can affect banana quality, size and numbers.

A Molecular Ecology Journal study found "an original and unprecedented global scenario of invasion".

It is the most important and destructive banana disease in the world, says one of the authors, Stephanie Robert.

"It starts with small flecks and spreads to the whole banana leaves - the disease can totally destroy the whole banana plant," she says.

The fungus can cause bananas to ripen prematurely

Using genetic markers, the team were able to map the streaks on 735 banana leaves from 37 different countries and identify genetic similarities.

"The historical hypothesis was that it came from South East Asia," Ms Robert says.

While the fungus was first recorded in Fiji in 1963, it was initially thought that the centre of origin could have been Papua New Guinea or the Solomon Islands.

However, the study found the whole of South East Asia could be the centre of diversity - encompassing at least Malaysia, Philippines, Indonesia and Papua New Guinea.

It says the area is home to a diverse array of wild banana and plantain species and defines "the area of banana and plantain domestication that began several thousand years ago".

But the exact point of origin of the host plant could not be pinpointed without further comparison between wild and domesticated banana plants.

Ms Robert says fungal spores cannot travel more than a few metres and are very sensitive to UV rays, but when travelling on the wind spores can be dispersed up to several hundred kilometres.

So this does not explain how the disease has travelled so far around the world, she says.

"I don't think the disease would have spread so far without human contribution.

"It's very difficult to understand exactly how the disease is dispersed - it's currently proceeding through the Caribbean and has just invaded Martinique," she says.

Originally it was thought that the fungus travelled through Africa after just a few introductions, but the research suggests it was spread through a single source near the South China Sea.

In the Americas, the fungus is thought to have been derived from mingling between genetically different sources in South East Asia and Oceania, through multiple introductions in the same place, or at different times and places.

The pathogen was first identified in Honduras in 1972 but would have been present in the 60s, it is thought.

Ms Robert says she hopes the study will help the banana industry reduce fungicide use and develop better control strategies.

"It's very important for the creation of pathogen-resistant varieties in a sustainable way because the pathogen does adapt," she says.

The study calls for more precise investigation into the disease.

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