Race to halt mystery curse spreading across Southeast Asia

16 April, 2014 by (comments)
Cassava witches' broom is so-called because of the proliferation of leaves at the top of the stem

Cassava witches’ broom symptoms

The race is on to identify the mysterious cause of a devastating cassava disease sweeping through the region. Dozens of species of insects – thought to be the culprits for its spread – are currently being tested for traces of the disease, as researchers intensify efforts to stamp it out.

Cassava witches’ broom disease is so called because of the broom-shaped proliferation of leaves it causes at the top of the stem. It can cause up to 100 percent losses in farmer’s yields and a 30 percent reduction in starch content, posing a threat to the billion-dollar starch industry.

A team of researchers at Vietnam’s Plant Protection and Research Institute are currently analyzing insect samples from infected cassava fields region-wide. “We can’t halt this disease until we know what’s spreading it,” said Kris Wyckhuys, CIAT’s regional entomologist.

“It’s a long process, and there’s a lot we don’t know about the disease,” added Dr. Wyckhuys. “But all our efforts are focused on finding the cause, identifying clean planting material, exploring ways to treat infested stakes and raising farmer awareness about disease symptoms and preventative tactics. We’re getting closer,” he said.

A research team at Vietnam’s Plant Protection and Research Institute (PPRI) is currently analyzing results to reveal if the insects contain the phytoplasma, or bacteria. Lined up in petri-dishes on ice, the insects have been crushed to extract DNA, which will be analyzed by state-of-the-art equipment.

Analyzing the different species of insect

Analyzing different species of insect

Pin-pointing the culprit

Although cassava witches’ broom disease has been detected in the region for some years, tackling it is only now possible due to advanced technology. Juan Manuel Pardo Garcia, a visiting plant pathologist from CIAT’s headquarters in Colombia working with PPRI to standardize rigorous testing, explained that current technology is 1,000 times more powerful than older technology.

“Using real-time Polymerase Chain Reaction technology is an accurate and more powerful way of verifying the presence of disease” he said. Slashing the time needed to run lab samples from eight hours to fifty minutes compared with older, standard gel-based PCR technology, real-time PCR also increases the efficiency of identifying the presence of the disease, and the species of insect that transmit it, he added.

“The concentration and distribution of the disease in each sample we are working with is not regular, so this highly sensitive technology is needed to detect the phytoplasma,” he said. If results indicate positive for the pathogen, further tests will be needed to reveal whether the insect is capable of transmitting the disease and therefore its spread.

Infected or not?

Complicating awareness-raising efforts and a clamp-down by researchers, farmers and quarantine officials to slow the diseases’ spread is the fact that infected planting material can be symptomless. “Symptoms depend on the intensity of infection,” explained Trinh Xuan Hoat, PPRI’s Deputy Director.

“Cassava is a very important cash crop, especially for poor, ethnic minority groups,” he explained. Farmers will continue to trade and buy infected cassava planting material, spreading the disease further, because they cannot see the symptoms and they depend on the crop. They only find out the crop is infected when it’s too late – when it’s time to harvest and sell the roots.

At the same time as hunting for the cause of the disease, CIAT and teams of local researchers are running tests to identify clean cassava planting material in the region that can be multiplied up and certified so farmers know what they are buying.

“We are working on a clean seed system and identifying clean planting material so we can ensure disease-free planting material for cassava farmers,” added Dr. Hoat. “We are also looking for varieties tolerant to the disease.”

Extracting DNA

Extracting DNA information from insects

Market in jeopardy

Great losses loom large over the livelihoods of more than 40 million smallholder farmers in the region dependent on the crop, as well as cassava traders, processors and the growing billion-dollar starch industry, which fetched US$ 1.1 billion in exports in Vietnam alone last year.

CIAT agronomist Keith Fahrney said: “This situation can seriously impact farmers’ cassava yields and income – and also has the potential to damage the local starch industry.”

“If cassava processors go belly-up, that’s a loss of markets for farmers that may take many years to recover,” he added.

Cassava is processed for sale – much of it by small family-scale cassava processing mills in Vietnam – as either wet or dry starch, processed into foods like noodles; pharmaceutical products or ethanol fuel and industrial alcohol. Dry root chips supply the animal feed and biofuel industries.

Starch content is measured by most processors, so with reduced quantity and quality of starch, processors need to buy more cassava roots to extract the same amount of starch, and farmers are paid a lower price for their product. The disease also causes roots to become sticky, taking longer to dry and turning starch a yellow-brown color instead of industry – required white.

The results of ongoing research into potential insect-based transmission of the disease, together with economic impact assessments estimating the value of damage under various projections, are due later this year.

For more information contact: g.smith[at]cgiar.org

Photo credit: Georgina Smith.

Related links:

Witches’ Broom: A curse on cassava 

Colombia is key in the fight against Witches’ Broom 

Clamp-down launched on devastating threats to starch crop 

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