- Witches’ Broom, a devastating fungal disease, has spread for the first time from Southeast Asia to Latin America, arriving in French Guiana in 2023 and has now infiltrated northern Brazil.
- Cassava is a vital crop for food security in South America and Africa, and a critical cash crop in Southeast Asia, where the fungal disease is spreading rapidly. More than 500 million people worldwide rely on cassava for their dietary needs.
- The pathogen has already caused massive cassava losses in Southeast Asia, with infection rates in some fields near 90%, and now it threatens food security in Latin America. Climate change is helping the fungus thrive and spread, as wetter conditions create an ideal environment for infection.
- Brazil has launched emergency measures, including funding research and farmer training, but scientists warn that without swift containment, cassava production across the tropics could face severe declines.
As smallholder farmers in French Guiana walked their fields in 2023, they noted a disturbing pattern. Their healthy cassava plants, once green and thriving, had begun to turn yellow. Leaves wilted, stalks withered, and small shoots looking like broomsticks sprouted from the plants. When they dug up their harvest, instead of unearthing large foot-long bunches of cassava, they found only stunted roots.
The farmers reported the crisis to agricultural authorities, desperate to find a cure for their threatened crop. When international scientists arrived in 2024 to investigate the blight, they recognized their foe immediately. The witch had arrived in Latin America.
Witches’ broom disease is a fungal pathogen, Ceratobasidium theobromae, that has been wreaking havoc on cassava crops in Southeast Asia for more than a decade, crippling yields and fueling economic hardship across that region.
Plant scientists had hoped to contain the fungal pathogen to Asia. But since its arrival in South America in 2023, the disease — which attacks the plant’s vascular system — has spread into at least three Amazonian nations: It arrived first in French Guiana, is likely in Suriname, and is now sweeping through cassava production regions in the neighboring northern Brazilian state of Amapá, where it has decimated entire fields, and is also present in Pará state.
“It’s really worrying because we know what it is capable of doing,” said Wilmer Cuellar, a plant virologist with the International Center for Tropical Agriculture (CIAT) who confirmed the diagnosis through DNA sequencing of infected plants in French Guiana.
Plant infection rates in some parts of Southeast Asia — where cassava is a major cash crop — are now as high as 90%, according to Cuellar, and some farmers’ yields have dropped by more than half as the fungus continues to spread. The plant’s starch is used as a binding agent in food processing as well as in papers and textiles, with annual exports from Asia bringing in about $4 billion, making the root vegetable a vital economic lifeline in Southeast Asia.
The spread of witches’ broom disease to a new continent, scientists say, is particularly troubling because South America — along with Africa — rely on cassava for its calories. The U.N. Food and Agriculture Organization estimates that about 800 million people worldwide depend on cassava as a staple food, and it’s considered the third largest source of carbohydrates in the tropics, after rice and maize. The arrival of witches’ broom could therefore inflict severe repercussions on food security in South America.
Climate change playing role in spread?
Cuellar, who leads CIAT’s cassava protection team, said scientists are still trying to figure out exactly how the pathogen was introduced to Latin America. It’s clear, however, that climate change has helped to fuel the spread of witches’ broom across Southeast Asia, making it more likely that infected stem cuttings could have been transported across the ocean.
“There are some special conditions of high humidity that will induce the production of spores of the fungus. That explains why several outbreaks of the disease were reported after long rainy seasons,” Cuellar said of the spread in Asia.
While fungal pathogens are often overlooked compared to viruses and bacteria as causes of disease, they can pose serious threats to environmental health — especially because few treatment options exist for crops. Scientists have found that climate change is giving fungi more opportunities to adapt and survive in higher temperatures, helping them expand into new areas, while heavier rains can spread fungal spores more widely. In Southeast Asia, climate change has intensified the region’s rainy season in many places, leading to longer, wetter periods with more frequent flooding and extreme rain events.
“The problem with the Amazon is that it’s raining almost all year round, compared to Southeast Asia,” Cuellar said. “Probably the fungus will like it even more here.”
A fungal disease with Asian roots
The fungus that causes witches’ broom disease first emerged in Southeast Asia’s cacao crop in the 1970s, leading to a plant ailment known as vascular-streak dieback. That blight hit the region’s cacao farmers hard, but it took decades for the pathogen to infect Southeast Asia’s cassava plantations.
Cassava witches’ broom disease was first reported in Thailand’s Rayong province in 2008. Scientists theorize more intense rainy seasons may have led to higher spore production, enabling the fungus to move from cacao into a new host. Still, it was not yet seen as a major threat.
“We had little outbreaks over the years, but people tended to be able to manage it during … those periods,” said Jonathan Newby, CIAT’s cassava program leader based in Laos. When mosaic disease — another ailment that impacts cassava — was discovered in Southeast Asia in 2015, agricultural researchers were distracted from seeing C. theobromae as a major threat.
“Witches’ broom was cooking away in these areas, and it was getting worse, but we had funding to work on a different issue,” Newby said, recalling his efforts with CIAT, a research and development organization focused on reducing poverty and malnutrition.
That diversion, combined with climate change-driven shifts in rainfall patterns and unhelpful agricultural practices, led to an eruption of the sleeper cell fungal disease across Southeast Asia in the latter half of the last decade.
“Often we’ll see that when the price for cassava is really low, the better-off farmers will decide not to harvest and leave their crop in the field,” Newby explained. “Then you have an infected plant that stays in there building up more inoculum. When the next season gets planted, it already has infected plants around it.”
The disease can be hard to control because farmers use cassava stem cuttings from the previous harvest to propagate the next crop, often trading cuttings with other farmers, but infected stems can lack visible signs of the disease to the untrained eye. This helps witches broom’ easily enter new agricultural areas undetected.
Challenges posed by the epidemic
Cassava — a woody shrub known as manioc or yuca or by other regional names — was originally native to South America, but was introduced to Southeast Asia in the 16th century. It has since become a key commercial crop there due to its ability to thrive under myriad growing conditions, including degraded soils, with few inputs. The FAO notes that cassava can also be an important fallback food during times of political unrest, war, and famine. Its starchy root can be boiled and eaten, or processed into tapioca flour.
“In Vietnam, they talk about it as a crop that got through them through war,” Newby said. “It was important along the Ho Chi Minh Trail because rice fields are easy to observe [from aircraft] and for people to destroy.”
Today’s witches’ broom epidemic across Southeast Asia has already reaped severe economic consequences for farmers who sow the crop. When cassava production is good, it offers rural communities some hope of a sustainable future, helping keep young people living on farms, Newby explained. But just one year after taking hold, witches’ broom can inflict a substantial fall in yields.
The disease is especially impactful for farmers because it drastically reduces the cassava’s starch content. In some cases, starch content can fall by up to 25%. “If you lose that productivity, you lose market share to global maize or global wheat,” Newby noted.
But purchasing new, clean cultivating material is also expensive, costing a farmer hundreds of dollars. “These prices make cassava uneconomic to grow. And that’s a challenge, because there is not a whole heap of other options for poor farmers,” he said.
The impact of witches’ broom disease on Southeast Asia’s cassava crop has already helped drive migration into cities, and farther afield as agricultural returns diminish and people seek new economic opportunities,” Newby told Mongabay. “We hear stories about people migrating to Thailand from Laos and Cambodia, but also beyond to the Middle East.”
What, then, could the future hold for South America and the millions of people who depend on cassava to thrive and survive?
Amazon frontline
Agricultural authorities are rapidly mobilizing in Brazil in hopes they can still stop the spread of the fungus within, and beyond, Amapá and Pará states. Pará, which had its first confirmed case of the disease in May 2025, is the largest producer of cassava in Brazil. If the disease gains a strong foothold, there are currently no easy remedies to vanquish it. In French Guiana, some farmers are already reporting they’ve lost half their yield, Cuellar said.
Training farmers to identify signs of witches’ broom to stop people accidentally moving stalks and stem cuttings into new areas of cultivation is key, Cuellar said. “Some of the Amazon communities had reported weird symptoms, but in the beginning it was not seen as such a problem,” so the disease spread, he said.
Brazil’s Ministry of Agriculture, Livestock and Supply (MAPA) announced in mid-October that it would invest 2.2 million Brazilian reals ($405,000) on emergency measures for agricultural defense in the Brazilian state of Amapá, an outbreak hotspot which borders French Guiana. The disease has already affected 10 municipalities in the state, according to MAPA, reducing agricultural production and food security for Indigenous and non-Indigenous communities.
“Brazil is a global leader in agricultural protection, and this health emergency is receiving our full attention,” said Brazil’s Minister of Agriculture Carlos Fávaro in the press release. He noted that the government is investing money for research into potential cassava varieties that are tolerant or somewhat resistant to the disease, and will also be distributing healthy seedlings to farmers, which could guarantee food for the people of Amapá until a fully resistant cassava variety becomes available.
“These Amazon communities are really attached to this crop,” Cuellar explained, likening Brazil’s cassava-reliant farmers to Peruvian farmers who depend heavily on varieties of potatoes. “It’s part of their identity. We were in Belem six months ago, and there were representatives of the Amazonian communities asking for help from the scientific community.”
Plant scientists with CIAT are also working to help safeguard cassava varieties in a gene bank to serve as a critical backup should witches’ broom decimate varieties in the wild.
“The conditions in the Amazonas region are much more favorable for the fungus than in Southeast Asia, and it has already spread quite quickly, there” Cuellar said.
And there’s an added problem: U.S. cuts to international development and conservation funding have left little money for preventative research on the fungal pathogen, further fueling the unfolding cassava crisis.
A threat to tropical food security
Based on his experience watching the spread of witches’ broom in the fields of Southeast Asia, Newby has advised Brazil to do everything it can to prevent the disease from getting into its commercial agricultural system.
“Once it’s in a bigger, commercial area where stems are moving around — and Brazil has many more large producers — then it can get out of hand and you miss the opportunity for eradication.”
Because there currently is no cure for the fungal disease, Brazil’s cassava farmers stand out as a critical frontline of defense to staunch its spread across South America, and also prevent it from spreading into other crops and even farther afield.
Scientists are particularly concerned about Brazil’s already beleaguered multibillion-dollar cacao industry, with climate impacts leading to skyrocketing prices. Because C. theobromae first emerged in Southeast Asia’s cacao crop, researchers know that this commercially valuable plant is also susceptible to the disease in South America.
Africa, which has already been contending with other cassava diseases, could be next. “With the movement to French Guiana and then to Brazil, it shows that it’s not a hypothetical. This will make it to Africa. It’s just a matter of time,” Newby said.
Banner image: A cassava farmer checks his crops in the Juma Reserve in the Brazilian Amazon, circa 2011. Cassava serves as both an important commercial crop and a subsistence crop by Indigenous and traditional peoples. Image by CIAT via Wikimedia Commons (CC BY-SA 2.0).
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