After admiring my mother's pieris japonica shrub, I was kindly donated one which appeared to be suffering from an unidentified pest.
I know several of the shrubs from a neighbouring garden had to be removed with a similar problem. The leaves turn black and blotchy underneath and eventually the whole plant is affected.
Trawling the internet, I found a number of sites saying this type of pieris is susceptible to andromeda lace bugs.
The bugs aren't visible to the naked eye so I took a close up picture of some dead bugs on my shrub and magnified it in Photoshop (left). It more or less confirmed my suspicions.
I found a really useful piece on the Conneticut University website about the bugs which explains aggravating causes and treatment.
I am, however, wondering whether to look a gift horse in the mouth on this occasion and turn down the free plant for my garden in case the bugs could spread to other things.