Why doesn't anything fit the map?

Every one of the 73 isolates carried the A2 mating type. P. palmivora reproduces sexually only when A1 and A2 types meet; without both partners, the population propagates asexually — clones of itself. Genetic data independently confirmed the clonal pattern.

If the diversity inside Colombia isn't generated locally, it must have arrived from outside — repeatedly, from different source populations — and been carried across the mountains. The most likely carrier is infected plant material.

"Cacao germplasm moves extensively across Colombia through breeding and distribution programs, and that movement creates a human-mediated dispersal network — one that can distribute unrelated pathogen lineages across regions that would otherwise be disconnected," Restrepo said.

"Understanding how a pathogen is structured across a landscape is not an academic exercise," she added. "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."

How dangerous are the strains?

The team also measured virulence by inoculating cacao leaves with each isolate. Results spanned more than a tenfold range. The most aggressive isolate drove disease progression more than ten times faster than a reference strain. Several others caused milder symptoms than that reference.

Critically, virulence did not align with genetic cluster. A DNA fingerprint says nothing reliable about how dangerous a strain will be in the field.

A subtler finding emerged too: isolates originally taken from leaves caused significantly more damage on leaves than pod-derived isolates did. The authors flag this as an early hint of tissue-specific adaptation, one that warrants follow-up.

What should growers and breeders do now?

The practical recommendations follow directly from the data. Because human movement of plant material appears to be the primary dispersal channel, phytosanitary rules around cacao germplasm exchange need strengthening. And because every Colombian isolate so far is A2, keeping the A1 mating type out of the country should be a national priority — its arrival would unlock sexual recombination and the rapid evolution of more aggressive strains.

The authors call for coordinated action across Colombia's cacao regions and urge growers to diversify the cacao genotypes they plant, so no single source of resistance bears all the selection pressure.

The paper appeared as an editor's choice selection in the journal Phytopathology. The takeaway: P. palmivora in Colombia behaves as a genetically diverse, geographically unconstrained, and phenotypically unpredictable population — one that arrived through human action and keeps spreading the same way.