We have been compiling the fungi in John Dean Park for years, and have listed hundreds.  But the other day I found a new one - a leaf-curl fungus that attacks Poplar leaves.  The pictures tell the story:
the third is a photomicrograph I took of some of the asci, which cover the surface of the yellow areas of the leaves. Asci contain lots of yellow oil, which explains the colour. This is a very unusual ascomycete, in that it does not make any fruit bodies. Members of the same genus, Taphrina, cause peach leaf curl, a fairly serious disease.