I’m sure few people will recognize that the title of this post was taken from Robyn Hitchcock’s second solo album, but it seemed appropriate nonetheless.
Several weeks ago, I collected some quaking aspen (Populus tremuloides) leaves to try to raise the agromyzid fly larvae that were mining within them. The larvae exited to pupate, and when the leaves had turned brown I removed them from the vial to reduce the likelihood that the puparia would become consumed in mold. To make sure no puparia or parasitoids were still in or on the leaves, I was holding the leaves up to my desk lamp one at a time, and I was surprised when this pattern appeared in one of the backlit leaves:
I have no idea what’s going on here. An internet search of “concentric leaf spot” produces this vaguely similar photo illustrating “tomato spotted wilt virus.” So maybe this is caused by a virus, but it might also be a fungus or bacterium. Whatever it is, what I’m really curious about is how it’s producing this crazy pattern.
Edit, 10/24/2019: I see that the above tomato virus link is now dead, and I’m not sure if this one will work if you don’t have a Twitter account, but it shows a similar pattern of concentric rings on a chestnut oak leaf, caused by Cristulariella leaf spot (a fungus).
BugTracks 
There is always something new to learn or observe. If you don’t see something new every day, you’re not looking.
There’s a photo of rhododendron necrotic leafspot with a similar pattern at
http://pnwhandbooks.org/plantdisease/node/3857/print
Look in particular at the paler pattern in the upper left of the left-hand leaf.
Here’s a hypothesis (and nothing more than a hypothesis) about how this pattern could arise: When cells in a leaf detect infection, sometimes cells immediately around the infection will undergo apoptosis (programmed cell death) in an effort to isolate the infection. This happens in any number of leaf-spot diseases, where we see an infected region surrounded by a circle of dead cells. What if the ring of dead cells is unsuccessful at containing the infection? We would then get an additional ring of infected tissue outside the dead tissue, the leaves outside *that* ring would undergo apoptosis, and so on, creating a pattern of concentric rings. However it happens, it makes for a great photo!
That makes sense. Thanks for the thought!
I found something similar, hiking up Mt Monadnock today. Here’s the photo: http://staticworldofcardgames.com/images/random/01-swirly-leaf-monadnock.png
Would love to know what caused it. I was guessing a virus. I think this is a sugar maple but I’m not sure.
Nice pattern! As to the cause, your guess is as good as mine.