Cassava: the staple crop that can kill you

Easy to cultivate and drought-tolerant, the cassava plant is eaten by a billion people around the world every day. But its popularity is also its weakness, as attacks by pests and diseases have a potentially huge impact on food security.

It’s also the only staple crop that can kill you or cause chronic neurological disease if it’s not processed, potentially producing fatal levels of cyanide when drought-stressed.

To help improve strategies for breeding disease-resistant and climate-tolerant varieties of this root crop, an international research team, including scientists from Monash University, has for the first time mapped cassava’s genetic diversity.

Monash biologist Associate Professor Ros Gleadow explained the significance of this latest research, published in Nature Biotechnology.

“Until now, very little research has been done at the DNA level — the genome — of cassava, the world’s fifth most important crop,” Associate Professor Gleadow said.

“To maintain food security, it’s critical that cassava is resistant to disease and resilient to climate change. Our new research has revealed the genetic relationships between different cassava varieties from across the world and compared them with other species in the same group, such as rubber trees. We have also captured the global genetic diversity of 53 varieties of cultivated cassava and its wild relatives — from South America, where it was first grown 6000 years ago, to Africa, Asia and Oceania.”

The Monash research team coordinated the sequencing of cassava varieties from Oceania: Fiji, Vanuatu and Australia. The findings indicated genetic differences between the varieties and will therefore help inform the production of more resistant and safe cassava crops using these and other favourable varieties from across the world.

For the Monash team, the new research also provides a way to better understand why cassava produces toxins under conditions such as drought and how that can be safely reduced in the growing plant without jeopardising pest resistance.

“The sequencing of these different genotypes allows us to look at how cyanide and other traits are inherited and why some varieties produce more cyanide than others, enabling us to develop new strategies to produce less toxic varieties that are still disease-resistant,” Associate Professor Gleadow said.

The research team found that the genetic diversity of cassava used in current breeding efforts has been greatly reduced in Africa, where viruses such as the cassava mosaic disease and the cassava brown streak disease have affected crop yields in many nations. This poses a real threat that a new disease could wipe out an entire crop. The team was able to detect the genetic signature of past cassava improvement programs going back to the 1930s, which interbred cassava and Ceara rubber; the persistence of these Ceara rubber regions in the best cassava varieties suggests they offer positive attributes. The team also discovered relationships between many cultivated cassava varieties, which could help breeders maximise genetic diversity in improvement programs.