Apple trees are an iconic national symbol in America, but the future of this beloved fruit could be in doubt, reports Science.

Something is killing apple trees across the American countryside, and the epidemic is reaching plague-like levels. Worst yet, scientists are completely clueless as to what is causing the mysterious pestilence.

The puzzling affliction is being called RAD, or rapid apple decline, and it typically begins on a single tree limb. As the leaves begin to grow, they curl up and turn yellowish-red while they are still small. This then spreads to other limbs until the entire apple tree dies. Sometimes the disease seems to spread from tree to tree like a contagion, other times it manifests randomly across an orchard.

"Rows of trees collapse for what seems like no reason," said Kari Peter, plant pathologist from Pennsylvania State University.

This isn't the first time something like this has happened to apple trees. A similar unexplained phenomenon seemed to crop up back in the 1980s, but it pales in comparison to the latest epidemic, which began in 2013. Without being able to identify the underlying cause, scientists can't be sure if the two outbreaks are related.

What's causing this?

When it comes to plant pathology, there are the usual suspects: viruses, fungi, bacteria, parasites and insect infestations, etc. But so far, the problem doesn't seem linked to any of these. Scientists have tried a wide range of chemicals to combat each of these potential suspects, to no avail. It's possible that there is no pathogen, and the trees are withering due to a range of environmental stressors, but it's unclear what those might be.

While the disease is widespread, some areas are being hit particularly hard. Up to 80 percent of orchards in North Carolina may show symptoms of the deadly illness, for instance. Apples are one of the continent's most valuable fruit crops, worth some $4 billion last year in the United States alone, so the mystery illness threatens entire agricultural sectors.

Perhaps the two strongest leads regard the observation that RAD is most common in densely-packed orchards with fewer weeds. That might mean that concentrations of herbicides could be impacting the trees' health. Furthermore, modern apple farming methods pack trees into orchards at impressive densities. Instead of planting about 250 trees per hectare, high-density modern orchards can have 1,200 or more. Because tightly packed trees must compete for nutrition and moisture, this strategy could be what's harming the trees.

Still, the patterns seen during RAD outbreaks are difficult to parse and aren't always consistent.

As scientists scramble to identify the cause of the epidemic, farmers are bracing for another lost season while hoping for the best, with fingers crossed. Experts are concerned, however, that it could be a very bad year for the American apple.

"It wouldn't surprise me if we get more reports of apple decline," said Sara Villani, a plant pathologist at North Carolina State University.

Apple trees are mysteriously dying all across America and nobody knows why
In some regions, as many as 80 percent of trees could be in danger from RAD or rapid apple decline.