Cassava in south-east Asia under threat from witches' broom disease

A mealybug attacks a plant in Indonesia. Parasitic wasps have been deployed to combat the bug in some areas, with mixed results. Photograph: Georgina Smith/Ciat

Cassava, which is originally from South America, is now south-east Asia’s third largest source of calories after rice and maize. An estimated 40 million people in the area depend on the plant for their livelihoods and the crop forms the basis of a $5bn (£3.5bn) regional market in starch, which is used to make products ranging from paper to biofuel.

But researchers say the crop’s viability is now at risk as more intense dry spells and rains bring about conditions in which pests and diseases can flourish.

Scientists who gathered data from more than 400 sites across the region found symptoms of witches’ broom disease or mealybug in at least two-thirds of the cassava fields they studied.

The former, which has already reached the Philippines, Vietnam, Cambodia, Laos and Thailand, causes leaves to discolour and bunch into a shape resembling a witch’s broomstick.

The latter, which menaced crops in Africa after it was accidentally introduced from South America in the 1970s, destroys yields by leeching essential nutrients from the plant. The bug is now moving into parts of Indonesia where cassava is central to food security.

 “Our data suggests that a number of factors have triggered an explosion in pests and diseases in south-east Asia’s cassava fields, including globalisation, climate variability and the changing frequency of droughts,” said Dr Kris Wyckhuys, an entomologist at the International Center for Tropical Agriculture (Ciat).
“We also found that some pests and diseases are far bigger problems than we previously thought, and alarmingly, they’ve already spread further than we thought.”
The study, published in Pest Management Science, says that while infestation levels vary from country to country, urgent action is needed to tackle witches’ broom disease in Cambodia and central Vietnam, and mealybug in Indonesia.
The authors call for a rethink of crop management practices – which are exacerbating the problems and undermining cassava’s ability to withstand new threats – and suggest that biological control methods, such as insect “enemies”, be used to contain the outbreaks.
“What we’re aiming for is practical, environmentally and economically sustainable options for farmers, so they don’t need to spend much money but get long-term results,” said Dr Ignazio Graziosi, a Ciat research fellow specialising in invasive species ecology.
Two years ago, scientists began deploying tiny, parasitic wasps to control mealybugs in Vietnam and Indonesia. The minute insects lay their eggs in the bugs; the hatching larvae then consume their hosts from the inside, mummifying and killing them. Despite success in some areas, however, researchers noticed that mealybugs appeared to gain the upper hand over the interlopers in regions with prolonged drought and low soil fertility.