Fungus that attacks cocoa plants reproduces by cloning, study says

A fungal disease that poses a serious threat to cocoa plants reproduces clonally, according to researchers from Purdue University.

The fungus Moniliophthora roreri is believed to be the cause of frosty pod rot - a disease that has decimated cocoa plantations through much of the Americas - reproduced sexually because M. roreri belongs to a group of fungi that produces mushrooms.

But a study by Purdue mycologists Catherine Aime and Jorge Díaz-Valderrama shows that M. roreri generates billions of cocoa pod-destroying spores by cloning, even though it has two mating types -- the fungal equivalent of sexes -- and seemingly functional mating genes.

The findings could help improve cocoa breeding programmes and shed light on the fungal mechanisms that produce mushrooms, according to the researchers.

"This fungus is phenomenally unusual -- it has mating types but doesn't undergo sexual reproduction," said Díaz-Valderrama, doctoral student in mycology. "This knowledge is biologically and economically valuable as we seek better insights into how mushrooms come about and how we can reduce this disease's damage to the cocoa industry."

"Fungi usually start reproducing via cloning when they're very well suited for their environment," Aime said. "In terms of resources, sex is expensive while cloning is a cheap and easy way to produce a lot of offspring.

In addition, while both mating types were found in South America, only one type was found in Central America. This finding supports the hypothesis that the fungal disease originated in South America and was more recently introduced into Central America, where it rapidly spread via cloning.

Frosty pod rot has wreaked havoc on cocoa plantations in much of the Americas, dropping cocoa yields by up to 100 percent in some areas, and forcing many farmers to abandon their plantations. Brazil is currently the only cocoa-producing country in the continental Americas to remain untouched by the fungal disease. Much of the world's cocoa production has relocated to West Africa, partly due to the spread of frosty pod rot.

But although frosty pod rot remains a very real threat to cocoa plants, chocolate lovers shouldn't panic just yet, according to Díaz-Valderrama.

"We're working on identifying biochemical components that could be useful for controlling frosty pod rot and protecting vulnerable cacao-growing regions,” he said.