David.
I couldn't agree more with the latter remark, but would like
to make an observation on the former!

Living in Plymouth where the air is exceptionally clean (except
for atomic radiation) roses have always been subject to savage
attacks of blackspot *except* where they were grown very close
to roads (for instance the 'islands' separating dual carriageways)

There they may well have grown somewhat stunted - but still
very florifulous - but I never saw 'Blind Pugh's Curse' on them.
I'd hazard a guess that it was the almost constant flow of carbon
monoxide across their leaves.

In my own gardens, and treated better than a mother in law, I have
always fought a non stop battle with the damn spot.

John

It's possible that sulphur monoxide which is one of the chemicals emitted from
vehicle exhausts acts as a mild fungicide. Sulphur powder is used as such and
has been for many years. The carbon monoxide is unlikely (as far as I can work
out) to have much effect on the fungi, but may do.

Interestingly though, sulphur deficiency in the soil affected many plants
before the industrial revolution. Sulphur can be added to make soil more acid
and treat lime induced chlorosis and is used by plants directly too, as far as
I remember. The industrial revolution pumped tons of sulphur into the
atmosphere which came down with the rain and to some extent diminished the
extent and severity of this problem. The interesting point is that with so
much effort being put into reducing sulphur dioxide emissions from cars, etc.
lime induced chlorosis is likely to become more of a problem again, though
probably not for many years.

Hoping that this is interesting, and not too controvertial!

Dave.