Help needed, please, to identify weird alien growths on young pear
tree leaves.

A number of leaves on my 6 ft tall tree (five lovely fruit this year)
have developed bright orange spots on the topside of the leaf and
bizarre tumour-type growths on the underside of the spot, with spikes
growing out of them. They're actually quite horrifying and
extraterrestial-looking. Websearches haven't turned up any likely
disease, but I'm sure someone here will recognise such striking
symptoms...?

So what do I do? Remove the leaves concerned - or just call in an
exorcist?

Many thanks
Katharine