This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Black pod disease destroys up to a quarter of the world's cacao every year, roughly 700,000 metric tons worth more than $2 billion. The water mold responsible across the Americas is Phytophthora palmivora, and in Colombia it is the dominant Phytophthora on cacao.
How it travels among the country's three Andean regions, though, has remained an open question. A study published in Phytopathology and selected as an editor's choice now provides the first comprehensive portrait of the pathogen's genetic structure in Colombia. An international team of researchers genotyped and tested the virulence of 73 isolates gathered from 48 locations across the country's three main cacao-growing regions.
What they found confounded expectations. Colombia's Andes divide into three cordilleras—western, central and eastern—creating a topography that has historically constrained movement of people, crops and organisms across the country's highland terrain. The research team, led by Silvia Restrepo, president of the Boyce Thompson Institute, hypothesized that this mountain complex would similarly limit the spread of P. palmivora, producing populations that had evolved separately on either side of the ranges, much as the Andes have shaped population genetics in other cacao pathogens.
The team scanned the genomes of all 73 isolates, looking for differences scattered across hundreds of positions in the genetic code. The analysis sorted the samples into seven distinct genetic clusters. Not one of them mapped to geography.
In other words, the Andes were not a barrier. "Understanding how a pathogen is structured across a landscape is not an academic exercise," said Restrepo. "It determines whether a resistant variety bred in one valley will hold up in the next one, whether quarantine lines are drawn in useful places, and how quickly the pathogen can evolve around whatever defenses growers deploy." Every one of the 73 isolates was the A2 mating type.
P. palmivora needs two compatible mating types, A1 and A2, to reproduce sexually. With only A2 present, sexual recombination isn't happening—the population is reproducing clonally, a conclusion the genetic data independently confirmed. Clonal organisms don't generate this much diversity on their own.
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