Thanks Sue for explanation. I'm not very good at observing (for now). I'm linking my old post where there are a couple of picture of affected leaves.
http://garden.org/thread/view_...
It seems more like spider mites if with this term we are referring to the same pest, that we call yellow spider, and it's a mite (Eotetranycus carpini) .
If I get it right spider mites love a hot and dry environment (this was the perfect case this summer) but I saw the same the past year and it was a cold and rainy summer; rust loves humidity, correct?
The link you posted shows there's should be no rust in Italy, but it's there in the closest country, so I would imagine it could travel.
