The Nacional tree is a rare variety of cacao. Nacionals were thought to have gone extinct by the twentieth century due to a fungus. This fungus can spread from tree to nearby tree through the air and causes disease. But around 2013, cacao expert Servio Pachardo located some of these Nacional trees. The trees were in the Piedra de Plata coastal forest, within a hard-to-reach valley in Ecuador. Conservationists inferred that the Nacional trees in Piedra de Plata might have avoided the diseases that wiped out the other Nacional trees because ______.

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