A load of black circles about the size of a penny have appeared all over
my concrete garden path, obviously some kind of mould or fungus or
something. NOTHING seems to be able to get rid of them. Ordinary
potassium hydroxide patio cleaner, applied with a good stiff brush, has
no effect. Bleach, even applied neat with a stiff brush, has no effect.
I now have a path that looks cleaner than it did when it was new, but
the black spots are still there. Anyone able to suggest anything?

Incidentally, I've been looking around the neighbourhood and nobody else
has these, in fact I've never seen them before anywhere. Everyone else's
concrete paths have, to varying degrees, a uniform layer of black grime
that doesn't look too bad because its even, whereas the contrast I have
between clean concrete and black circles is rather unsightly.

Any suggestions?