Leaf Spot and Pollution
If I remember my mycology courses correctly (we are way back in the
early history of England here), the impact of the extreme coal-based
smogs in London up until about the early 1950's was to prevent the
growth of most trees in the area. The only tree that really survived
successfully was the plane tree [Platanus occidentalis and orientalis
and their hybrid acerifolia). In the US, these trees are usually called
sycamores, which is a different tree (Acer) in England. The reason the
plane tree survived was its habit of shedding its bark in sheets and,
thus, removing the soot that accumulated. At the same time, this tree
was host to a leaf spot, which was apparent because it cause large black
spots to appear on the leaves. That is, except in areas of heavy smog
where the leaf spot was absent. It was believed that, the high sulphur
content in the coal fumes killed the fungus. Of course, it killed lots
of Cockneys as well and those that could afford it moved out of London
and eventually arrived in places like Canada.
I have looked through my library to find the actual reference for this
information to no avail. I do have an account of the plane tree/smog
story which, by the way, also mentions that one of the herbs to survive
the air pollution were the irises because they had thick cuticles on the
leaves (London's Natural History by R.S.R. Fitter 1945).
Ian, a cockney in smog-free Ottawa.