Cassava Witches’ Broom (CWB) disease, which reduces cassava yields by an average of 30–35%, is an emerging threat to the livelihoods of millions of farmers in Southeast Asia. CIAT is working with national partners to better understand the disease, improve surveillance, and help farmers manage it.
Cassava is an important cash crop for the region’s smallholders, supporting the livelihoods of about 40 million people. Cassava has historically been spared major pests and diseases in Southeast Asia, but the recent spread of CWB disease, as well as invasive cassava mealybugs, poses a major threat to the region’s farmers and economies.
The symptoms of CWB disease – such as a broom-like proliferation of leaves at the top of plants – were first described in Thailand the 1990s, yet scientists have only determined that it is caused by phytoplasmas in the past decade. The first regional surveillance of the disease was not conducted until the dry season of 2014, when CIAT and partners monitored 429 fields in five Southeast Asian countries and found CWB in 64% of the plots.
“We have only recently realized what a large-scale problem CWB is,” said Kris Wyckhuys, an entomologist who works for CIAT in Asia. “It has reached near-pandemic levels in Cambodia, Central Vietnam, Western Thailand, and the Philippines.”
Wyckhuys is collaborating with Southeast Asian partners to better understand the CWB pathogen and its insect vectors, and to design effective responses to the disease. Scientists knew the pathogen is spread by infected planting material and insect vectors, but nobody knew which insects acted as vectors. CIAT consequently collaborated with Thai, Philippine, and Vietnamese partners on a step-wise process to identify candidate insect vectors and elucidate their roles in pathogen transmission. Using a combination of field observations, laboratory feeding trials, and molecular diagnostics, researchers identified eight species of leafhopper and planthopper as probable vectors. Some of the insects are more common in rice fields, which led Wyckhuys to hypothesize that rice may serve as an asymptomatic host for the pathogen.
CIAT scientists also performed restriction fragment length polymorphism analyses on infected cassava material from Cambodia in 2016, and identified phytoplasmas belonging to ribosomal subgroups 16SrI-C. Wyckhuys worked with scientists at the Agricultural Genetics Institute in Vietnam to develop a loop mediated isothermal amplification-based, isothermal DNA amplification device (Smart Dart) to detect phytoplasmas in plant material. The device is small enough to be taken into the field to detect the disease in real time, which is especially important because infected plants are often asymptomatic. CIAT also developed a phone-based diagnostic key to facilitate rapid, field-level identification.
To strengthen surveillance across the region, CIAT produced a training video and organized training events in Cambodia, Vietnam, Laos, Thailand, and the Philippines, where ministry and research institute representatives learned how to recognize CWB symptoms and use the Smart Dart. Government officers, researchers, and companies across the region are currently using Smart Dart as well.
Because CWB pathogens are spread through planting material, CIAT is testing thermotherapy as a way of killing the phytoplasmas in infected planting material. CIAT has successfully used thermotherapy in Latin America to clean cassava planting material of phytoplasmas responsible for frogskin disease. Wyckhuys explained that the technology needs further validation in the field before it can be promoted for CWB.
Meanwhile, CIAT and partners are advocating greater surveillance and controls to slow the spread of CWB. They are also promoting a farm-level, integrated disease management approach that entails fertilizing and removing diseased plants from a portion of a farm—dubbed the ‘corner of prosperity’ —from which farmers select quality planting material for the next crop cycle. In addition to helping farmers manage the disease, this approach can improve farm productivity in general, which makes it a good way to help smallholders while enlisting them in the battle to control CWB disease.
Photo: A cassava farmer examines his field infected by cassava witches’ broom disease in Cambodia. G.Smith/CIAT