It starts on a single limb. The leaves grow small, and begin to curl, turning a pale yellow or reddish colour. Within weeks, another apple tree is dead.

But that's not the end of it. Adjacent trees in the orchard often begin to show the same thing. Again, the symptoms may start on a single limb. They don't end there.

This strange affliction is called RAD: rapid apple decline, or sometimes SAD, sudden apple decline.

While similar phenomena plaguing apple trees have been reported in the US and elsewhere since the 1980s, the most recent and most serious outbreak of the infection – if that's what it is – was first observed in 2013.

That's when plant pathologist Kari Peter from Pennsylvania State University encountered something "unusual" in one of her research orchards, inexplicably causing a massive die-off of apple trees.

The scientists used a range of chemicals in an attempt to stop the spread of whatever was killing the trees, but nothing worked – and the symptoms didn't match any known tree-affecting pathogens.

Before long, other orchards in Pennsylvania were reporting the same thing; since then, RAD has turned up in apple-producing areas across central, northeastern, and northwestern parts of the US, and also in Canada.

Reportedly, up to 80 percent of orchards in North Carolina may show symptoms of the deadly illness.

"Rows of trees collapse for what seems like no reason," Peter told Science.

What makes the problem so confounding is it doesn't seem to be tied to any one factor. Sometimes the condition spreads from one tree to its neighbours, but not always. Sometimes trees look like they have the lethal symptoms, but they turn out to be fine.

"In order to prove pathogenicity, you need to have trees with only the virus and see if it produces the symptoms," Peter told Good Fruit Grower.

"We still don't know if (the virus) is endemic and the disorder is caused by other tree stressors."

In a paper published this month that may be the most comprehensive study to date examining the potential factors involved, plant pathologist Awais Khan from Cornell University led a team looking at soil nutrition and weather conditions in orchards affected by RAD.

The research spanned five years of data, and sought to ascertain how viruses, fungi, and bacterial communities might be related to the disorder. Ultimately, the study was only able to suggest that weather stress and limited access to water in orchards might hypothetically be making apple trees more susceptible to… something.

"We did not find any statistically significant differences in soil and weather profiles of healthy-looking and declined trees," the authors conclude.

"Similarly, no particular fungi and viruses were associated with the symptomatic trees."

Khan says there's a need for broader and more in-depth studies on RAD and what might be causing it.

A separate study last year by researchers from the USDA identified four viruses, including a new kind of luteovirus, that are associated with the syndrome, although Khan and his team at this point state that these viruses are not causative factors.

Others have suggested beetles might be introducing a sickness to the trees, but again, it's not clear if the insects are making the trees sick, or are simply drawn to the sick trees.

With apples being the most consumed fruit in the US – a US$4 billion crop that's grown commercially in 32 states – scientists are racing against time to solve this deadly mystery.

"It might be more complicated than just a pathogen or an insect," Peter told Good Fruit Grower.

"I hope I'm proven wrong, but I think we are going to see another wave."

The Cornell University findings are reported in PLOS One, and you can find Kari Peter's FAQ on RAD here.