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Witches' broom - a curse on cassava

The disease is named after the broom-like leaf proliferation it causes at the top of cassava plants (© Dr Trinh Xuan Hoat, Plant Protection Research Institute, Vietnam)
The disease is named after the broom-like leaf proliferation it causes at the top of cassava plants
© Dr Trinh Xuan Hoat, Plant Protection Research Institute, Vietnam

Scientists are urging renewed efforts to contain and clamp-down on emerging pests and diseases threatening cassava production in Southeast Asia. A key threat, cassava witches' broom disease, threatens up to 40 million smallholder farmers in the region who depend on the crop for their livelihood. Named after the broom-like leaf proliferation it causes at the top of cassava plants, the disease has already swept through Vietnam, Thailand, Cambodia, Lao, China and the Philippines, resulting in dramatic reductions in cassava root starch content and in parts of Vietnam, massive 80 per cent yield losses.

Cassava is of growing importance in the region as a food security crop. It is either consumed directly (i.e. Indonesia and Cambodia), or cultivated as a source of income for the poor, thereby improving their capacity to buy food. It is also increasingly valued as an industrial and export crop, which can be processed into a wide range of higher-value food or industrial products from noodles to textiles, pharmaceuticals, cardboard and glue. In Vietnam alone, 3.1 million tons of cassava fetched US$1.1 billion in exports last year, with the bulk supplied by smallholder farmers.

Much of the crop's value lies in its starch content, and since witches' broom reduces starch content by an estimated 30 per cent, the disease has a direct impact on farmer incomes. "It's difficult to gauge the exact impact of novel plant health threats such as witches' broom," says Kris Wyckhuys, lead entomologist in Asia for the International Center for Tropical Agriculture (CIAT). "But it is very clear that urgent action is necessary to prevent these threats further biting into smallholder incomes."

With cassava a relatively recent introduction to Southeast Asia, it has, until recently, encountered very few pest and disease problems. However, since witches' broom disease was first reported in Thailand in 2008 it has spread across the region - a trend attributed by researchers to unchecked, cross-border movement of infected cassava planting material. Treatment options are limited. "A farmer is doomed once his crop has cassava witches' broom," says CIAT's molecular biologist Manabu Ishitani. "The most reliable option is crop elimination, by burning," he adds.

Stopping the spread

Yield losses of up to 80% have been recorded in Vietnam (© Dr Trinh Xuan Hoat, Plant Protection Research Institute, Vietnam)
Yield losses of up to 80% have been recorded in Vietnam
© Dr Trinh Xuan Hoat, Plant Protection Research Institute, Vietnam

Further complicating the fight against the disease is the mystery shrouding its spread. Although it is caused by phytoplasma - microscopic pathogenic agents - how the disease is transferred between plants remains to be understood. Much of the problem lies in the prohibitive cost of molecular tools to detect microorganisms and identify diseases before it's too late. Quarantine measures to restrict the movement of infected plants are therefore crucial, and national quarantine systems need to be further reinforced.

In late 2013, CIAT and partners in Colombia and Vietnam organised a series of hands-on training workshops in Cali and Hanoi to train researchers from Cambodia, China, India, Indonesia, Thailand, the Philippines and Vietnam in disease-free certification techniques. The scientists learned about clean seed propagation systems, practices for integrated disease management, tools for detection, analysis and prevention including disease characteristics and symptoms, taxonomy, transmission, molecular diagnosis and DNA sequencing.

"Governments in these countries are extremely concerned about this disease. The adoption of clean seed-system certification will drastically enhance the reliability and rapidity of pest and disease diagnosis," says Elizabeth Alvarez, a CIAT plant pathologist who led the training workshop in Colombia. "It's essential that all the actors involved in cassava production are trained in not only mitigating or counteracting the effects of cassava witches' broom in affected areas, but also in preventing its introduction into new regions," she adds.

Farmer field training activities include workshops, where tips on disease detection and prevention of spread will be provided and posters displaying symptoms and control measures will be distributed. "In the meantime, it's vital for us to develop clean cassava stake systems for every country in Southeast Asia, so that clean tissue can be multiplied and certified disease-free, for transfer to farmers in the field," says Ishitani, who led the training in Vietnam. "That is why we will be working with industry and local associations, to ensure that farmers have access to clean planting material," he adds.

Staying ahead of the threat

Roots skins exude sticky liquid in infected cassava (© Dr Trinh Xuan Hoat, Plant Protection Research Institute, Vietnam)
Roots skins exude sticky liquid in infected cassava
© Dr Trinh Xuan Hoat, Plant Protection Research Institute, Vietnam

Beyond immediate concerns with witches' broom, strengthening cassava pest and disease response systems, both in Southeast Asia and other regions, is increasingly recognised as a key priority. "Once we detect new pests and diseases we don't want to spend several years trying to figure out what to do with those organisms," says Soroush Parsa, CIAT's entomologist in Colombia. "We need plans in place to respond swiftly - an evolving research strategy is essential."

A four-fold response strategy being developed includes a list of high-risk pest and pathogen threats to cassava, tools and techniques to improve their early detection, and rapid response deployment of integrated pest management systems to slow their spread. "One challenge is to transform informal reports of new pests or diseases from concerned farmers in their fields into data-rich input used by researchers to track and monitor threats," says Parsa. In Southeast Asia, this process is already beginning through farmer field training.

Date published: January 2014

 

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