The Tipa tree or Rosewood weeps small droplets which I always thought was condensation. But in fact it's caused by the Spittlebug and only occurs in the spring, apparently.
There are loads of these trees all over Palermo and also on Libertador in Martinez and Acassuso, so when you park your car under one, it will soon be covered by a film of the droplets.
I have no idea if it's toxic, either, but I now close my visor when riding under them.