When the fungus Moniliophthora roreri (M. roreri) infiltrates cacao plants, the subsequent effects can be catastrophic for plantations. Dark lesions and white spots like frost spread across the green cacao pod. It doesn’t help that the fungus can thrive in a variety of conditions.
Until the 1950s, M. roreri affected only northwest South America. Eventually it spread its reach across the continent, infecting at least 11 countries. Reports of pod loss averaged around 30 percent, but in some areas exceeded 90 percent, according to a study from The American Phytopathological Society.
From there, it expanded into Central America and Mexico.
“By 2000, Peru had 16,000 ha (about 50 percent of its cacao cultivated area) abandoned mostly due to M. roreri, which is still an issue based on personal communications with farmers,” Jorge Díaz-Valderrama, a native of Peru and a mycology doctoral student at Purdue University, told R&D Magazine. “In Costa Rica, there are about 7,000 ha abandoned due to M. roreri. Current losses can reach 100 percent in most of the countries in Latin America.”
Díaz-Valderrama and Purdue mycologist Mary Catherine Aime have found that M. roreri reproduces clonally, rather than sexually, providing scientists a new lens through which to view the damaging fungus. The study was published in Heredity.
Unlike other fungi in its group, M. roreri doesn’t produce a fruiting body, a mushroom.
According to Purdue University, fungi can have a variety of mating types, which are sexually compatible with one another.
M. roreri “belongs to the family of mushrooms Marasmiaceae, which all have multiple mating types,” said Díaz-Valderrama.
“However, we think that during evolution there must have been an event that disrupted the compatibility between M. roreri mating types,” he added. “Even though we show that the mating genes have all the characteristics or ‘requirements’ for compatibility, mating types do not mate.”
According to Díaz-Valderrama, the first reported outbreak of the fungal disease occurred in Ecuador in the early 1900s. Molecular evidence, however, points to Colombia as the fungus’ likely site of origin. The spread was likely accidental. Infected pods may have been unknowingly transported through South America into Central America and Mexico.
“Fungi usually start reproducing via cloning when they’re very well suited for their environment,” said Aime in a statement. “In terms of resources, sex is expensive while cloning is a cheap and easy way to produce through clonal reproduction.”
Armed with the new knowledge, researchers can now explore whether a certain mating type influences the virulent properties of the fungus.
Additionally, now that clonal reproduction has been confirmed, cacao researchers can work on developing resistant cacao cultivars, added Díaz-Valderrama. Cacao cultivars with some resistance have been developed, but full resistance remains elusive.
Going forward, the team hopes to understand what barriers keep the fungus from mating.
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