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article imageA fungus that causes frosty pod rot in cacao trees clones itself

By Karen Graham     Mar 25, 2016 in Science
The fungus that causes frosty pod rot in cacao trees, the source of chocolate, reproduces by cloning say researchers at Purdue University.
Moniliophthora roreri is a basidiomycete fungus that causes frosty pod rot disease, one of the most serious problems for cacao production in Latin America.
M. roreri, along with M. perniciosa, the cause of witches' broom disease, and black pod rot, caused by Phytophthora species, make up what is called the cacao disease trilogy. M. roreri has caused serious crop losses because of the ease with which its spores are spread.
Frosty pod rot on Cacao tree.
Frosty pod rot on Cacao tree.
Ruth Maloney
Perhaps because M. roreri belongs to a group of fungi that produce mushrooms, the fruit of fungal sex, it has long been thought that this fungus reproduced sexually. However, according to Science News Online, the study by Purdue mycologists showed that M. roreri generates billions of cocoa pod-destroying spores by cloning, even though the fungus has two mating types.
"This fungus is phenomenally unusual - it has mating types but doesn't undergo sexual reproduction," said Jorge Díaz-Valderrama, a doctoral student in mycology, according to a news release from Purdue University. "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."
Growing cacao trees for their cocoa can be a risky business in a lot of ways. The farms are usually small, and because of the instability of cocoa prices, and the high cost of fungicides, most growers forgo them. Instead, they monitor the crop carefully, removing the pods and burying them if they show the telltale signs of frosty pod rot.
A Waorani indigenous woman eats a cacao fruit in Gareno  Ecuador
A Waorani indigenous woman eats a cacao fruit in Gareno, Ecuador
Pablo Cozzaglio, AFP
Frosty pod rot has spread all through Latin America in the past 50 to 60 years, unwittingly carried by transporting of infected pods, the wind and especially the heavy rains. The disease has devastated some plantations, causing growers to abandon their lands.
Brazil is the only cocoa-producing country in the Americas that has avoided the disease. This is also why much of the world's cocoa production has moved to West Africa, although these regions are also susceptible to the disease.
The research team realized that fungal reproduction can be complicated because instead of male and female sexes, they can also have a variety of mating types, and this leads to a wide range of potential mates, up to 20,000 in some species. Others, under favorable conditions, reproduce clonally, just by copying their genome and creating billions of offspring.
When Catherine Aime, an associate professor of mycology, and Díaz-Valderrama dug into the genomics and population genetics of M. roreri, they discovered the fungus could reproduce sexually. The only problem was that there was no evidence that M. roreri had ever reproduced in the field or in a laboratory setting. An M. roreri mushroom has never been found, indicating the fungus had ditched reproducing sexually in favor of cloning.
"Fungi usually start reproducing via cloning when they're very well suited for their environment," said Aime, "In terms of resources, sex is expensive while cloning is a cheap and easy way to produce a lot of offspring."
Because of the prevalence of frosty pod rot in Central America  cacao production has moved to West A...
Because of the prevalence of frosty pod rot in Central America, cacao production has moved to West Africa.
YouTube
Interestingly, the researchers found both types of mating in the fungus in South America and only the clonal mating in Central America. This supports the belief that the fungus originated in South America, spreading to Central America where it spread rapidly.
The research also showed that what was believed to be two varieties of fungi were actually two genetic variations of the two mating types, the clonal and sexual. This leads to the possibility of conducting breeding programs to see which type is the more virulent and then develop a resistant cacao cultivar.
Díaz-Valderrama says chocolate lovers must not lose hope because "We're working on identifying biochemical components that could be useful for controlling frosty pod rot and protecting vulnerable cacao-growing regions."
The study, "The cacao pathogen Moniliophthora roreri (Marasmiaceae) possesses biallelic A and B mating loci but reproduces clonally," was published in the journal Heredity on March 2, 2016.
More about cacao trees, frosty pod rot, Chocolate, sexual reproduction, Cloning
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