I live in a west suburb of Denver, CO.

Cottonwoods are native out here, they were here before western
civilization.

Now they banned the ones that put out the cotton though, and only sell
cottonless types. But they're great for filling in areas and preventing
erosion, as they grow about 5 feet a year.

So a few years ago, I planted 5 of these, to hold an embankment
together. I planted them about 10 feet apart. They grew like crazy, as
you can see.

http://lakewoodcolorado.net/cottonwood/

But now, the 2nd and 4th one ( every other one, which is odd ) has
developed some kind of disease of the leaves and bark, and it's not
something that is affecting the other 3.

On the diseased ones, the bark looks smooth, on the healthy ones it's
thick and grainy like normal.

You can see the disease in the leaves, with the perfectly healthy one
right behind it.

Does anyone know what's causing this, and the cure?