The fight to
save the banana just
got more challenging. A fungus that threatens the world’s most popular
fruit is spreading, according to a new study. And it’s doing so despite
massive efforts to stop it.
At risk is the sweet “Cavendish”
banana sold in North America and Europe. But that's not all. There are
400 edible varieties of banana, and many of them also are susceptible to
killing by Panama disease. (That's the name of the disease that this
fungus causes.) Indeed, many of those other types of bananas are
important sources of food for millions of people outside the United
States and Europe.
The killer fungus is known as Tropical Race 4,
or TR4 for short. It's the fourth type of fungus to cause Panama disease
that scientists have identified. It spreads slowly but easily, wiping
out banana crops wherever it goes.
To date, TR4 has shown up in
China, Indonesia, Malaysia and Australia. It’s reached the Philippines
and southern Africa. And it has been discovered in the Middle East and
Asian countries of Jordan, Lebanon and Pakistan.
For their new
study, Gert Kema and his team examined fungi from each of these
locations. Kema is a plant pathologist at Wageningen University and
Research Centre in the Netherlands. (A pathologist studies disease.) The
researchers wanted to identify strains of TR4 infecting the plants. So
they analyzed TR4 DNA from bananas in those countries.
These banana fields in the Philippines are infected with the fungus that causes Panama disease.
Maricar Salacinas
Normally,
small variations in DNA will occur when cells copy it as they
reproduce. Over time, those variations can add up and become quite
dramatic. But not for TR4. The DNA sampled from each location was almost
an exact match of all of the others, the researchers now report. And
that's because each spore is a
clone of the original fungus
that invaded Taiwan, Kema explains. That means they are genetically
identical. There was none of the variation the researchers expected to
find. Moreover, these clones are well-matched to the bananas they
infect.If the genes in some bananas were quite different from
those in others of their type, some plants might be able to resist the
fungi. Unfortunately, Cavendish bananas lack any diversity. That's
because they too are all clones.
The new findings were published November 19 in
PLOS Pathogens.
Fungal stalker can lay in wait a very long time
For years, scientists have been scouting for TR4 wherever bananas are grown. But this
pathogen
is almost impossible to see coming. That's because it attacks from the
soil. After invading a plant's roots, the fungus grows long
hyphae (HY-fee). These thread-like filaments worm their way into the plant's
vascular system.
This system of tubes carries water and nutrients up from the soil and
into plant leaves. The plant responds to the invasion by blocking off
the tubes. Over time, less and less water can reach the leaves, and the
plant starts to wilt.
No cure for plants infected with TR4 exists.
All invariably die. Meanwhile, the fungus releases spores into the
soil. These tough, seed-like structures lie in wait until another banana
is planted nearby. Then they attack it. That means a farm or plantation
that has been infected will stay infected. And live spores can hang on
in the soil for decades, waiting for their next victim.
What's
more, irrigation water and equipment can spread the disease. If machetes
and other tools are not properly cleaned after being used, they can
carry bits of fungus from infected plants to healthy ones. Even dirt on
the soles of shoes can track spores from one spot to another. All of
this helps to explain how TR4 has been spreading since its original
outbreak in Taiwan.
Because Cavendish bananas lack seeds, growers
at some large plantations generate new plants from suckers, or the
offshoots of adult plants. Another way is to expose clumps of cells to a
nutrient bath. These clumps grow into new plants that are genetically
identical to their parents. It's an efficient way to grow thousands of
plants. But as clones, every single plant is as vulnerable as all its
neighbor to the same disease-causing organisms, Kema says. Worse, all of
these clones are planted together, covering hundreds of hectares
(thousands of acres). Such
monocultures are extremely
vulnerable to disease, Kema notes. Once one plant is infected, the
disease rapidly spreads through the entire crop.
His team
identified the TR4 clones through a DNA-sequencing technique. It allows
rapid identification of an organism’s genes. Traditional methods are
much slower, Kema says. Farmers also can use a different type of DNA
test to check their soil, he notes. This would allow them to detect the
pathogen before their plants show symptoms. Doing so could help to slow
the spread of TR4, he says. The hope is that over time researchers may
be able to develop bananas that are resistant to the fungus.
Altus
Viljoen isn't sure the new method is the best way to monitor for TR4.
This TR4 expert works at Stellenbosch University in South Africa and was
not involved with the new study. The DNA-sequencing method Kema's group
used simply isn't available in many areas, he says. Regardless, Viljoen
agrees that the absence of genetic diversity in bananas and in TR4 is a
concern. It means that clones grown as a monoculture can be “severely
damaged by diseases and pests,” he says. If one plant is vulnerable to
the fungus, every other one will be too.
It's a race against the
clock for banana growers and scientists. And it will take a world-wide
effort to stop the steady march of TR4 and save the banana.
Power Words
clone
An organism that has exactly the same genes as another, like identical
twins. Often a clone, particularly among plants, has been created using
the cell of an existing organism.
DNA (short for
deoxyribonucleic acid) A long, double-stranded and spiral-shaped
molecule inside most living cells that carries genetic instructions. In
all living things, from plants and animals to microbes, these
instructions tell cells which molecules to make.
DNA sequencing The
process of determining the exact order of the paired building blocks —
called nucleotides — that form each rung of a ladder-like strand of DNA.
There are only four nucleotides: adenine, cytosine, guanine and thymine
(which are abbreviated A, C, G and T). And adenine always pairs up with
thymine; cytosine always pairs with guanine.
fungus (plural:
fungi) One of a group of single- or multiple-celled organisms that
reproduce via spores and feed on living or decaying organic matter.
Examples include mold, yeasts and mushrooms.
gene (adj.
genetic) A segment of DNA that codes, or holds instructions, for
producing a protein. Offspring inherit genes from their parents. Genes
influence how an organism looks and behaves.
genetic diversity The range of genes types — and traits — within a population of some organism.
hypha (plural: hyphae) A tubular, threadlike structure that makes up part of many fungi.
infection A disease that can spread from one organism to another.
monoculture Large areas planted with a single type of crop.
pathogen An organism that causes disease.
pathology
The study of diseases and/or conditions that lead to death. People who
work in this field are called pathologists. They look at what causes a
disease, how symptoms develop and may examine all stages of an illness.
spore
A tiny, typically single-celled body that is formed by certain bacteria
in response to bad conditions. Or it can be the single-celled
reproductive stage of a fungus (functioning much like a seed) that is
released and spread by wind or water. Most are protected against drying
out or heat and can remain viable for long periods, until conditions are
right for their growth.
strain (in biology)
Organisms that belong to the same species that share some small but
definable characteristics. For example, biologists breed certain strains
of mice that may have a particular susceptibility to disease. Certain
bacteria or viruses may develop one or more mutations that turn them
into a strain that is immune to the ordinarily lethal effect of one or
more drugs.
sucker A shoot from the base of a plant.
Tropical Race 4 (TR4)
The fourth identified strain of the fungus that causes Panama disease.
TR4 is the deadliest strain, currently threatening nearly 85 percent of
the world’s bananas.
tropics The region near Earth’s equator. Temperatures here are generally warm to hot, year-round.
vascular system
A system of plant tissues and fibers that carries water and nutrients
from the soil up into the leaves, and food from the leaves to other
parts of the plant.
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