Trees support a great deal of our wildlife, and many of them have species of beetle, fly, wasp, moth, butterfly, sawfly or mite that are uniquely dependent on them.
This is the Willow Tar Spot fungus - Rhytisma salicinum which is present on virtually every Willow specimen on the patch.
With so many species fighting for a bit of leaf, it's no surprise that conflict sometimes takes place: the following leaf has been attacked by no fewer than three organisms.
Firstly, there is a gall to the left (the pale green pea-like object). This is caused by a Pontania sp. wasp that affects the growth pattern of the leaf to make a hard structure that it can live - and feed - inside.
Next, we can see the brown marks which are the feeding signs of the larva of the Willow Leaf Beetle - Plagiodera versicolora. Again, virtually every Willow specimen shows signs of feeding, which is carried out by both larvae and adult beetles.
Finally, the entire leaf has been rolled into a tube by the larva of a Tortrix micromoth which, again, lives and feeds inside the tube.
This shows a more conventional roll in the leaf:
Now, does this look like the work of a leaf-miner? I think it does.
But the culprit is the larva of the Willow Leaf Beetle mentioned above.
The larvae crawl over the leaf surface, scraping away the green layers and leaving just a transparent layer that turns brown shortly afterwards. This is really just an external mine, but the larvae are far too large to fit inside the leaf. Large leaf miners come from large leaves, or very thick ones: most leaf-mining species are tiny.