At this time of year a walk through the meadows around Cambridge can be a messy affair, because there is so much frothy spit coating the wild flowers. Hawkweed and knapweed seem to be particularly prone to infestations.
The ‘spittle’ is a protective cover produced by small sap sucking bugs called spittlebugs or froghoppers. Unfortunately these insects are one of the most common vectors for the devastating plant disease Xylella fastidiosa (currently causing huge problems in the olive groves of southern Italy).
The UK is clear of the disease for now, but in an attempt to understand the ecology and distribution of these xylem-feeding bugs, volunteers (i.e. interested individuals) are being asked to report sightings now.
Frogend dweller's Blog 
I’ll come across spittlebugs from time to time, but not frequently. I don’t know if they’re anything more than a curiosity here. I’ve never heard of them being vectors of disease. I’ll have to explore a bit, and see what I can find.
Sounds like the bacterium started out in the Americas and caused problems in some crops: like coffee, citrus and grapevines. It may well have different insect vectors there, but with our abundance of sap sucking spittlebugs here in the UK they are modelling disease spread using them as a major factor.
I used to take a grass stem and persuade them out from their bubble-home when I was a child, I loved them! Hadn’t realised they were so destructive, though.
Ha, I did that too! The spittlebugs aren’t really a problem now, just if the disease is imported into the UK.