A leaf from a lime tree (Tilia sp) with pale spots. The above photo shows that upper surface of the leaf, below is the lower surface:
Under the microscope the pale parts are revealed to be masses of fungal hyphae:
I have not (yet) been able to identify the fungus.
Specimen in first 2 photos taken in Whiteknights Park, Reading, UK, on 2014-05-03, that in the 3rd photo taken in the same place on 2014-05-12.