I thought it was dehydrating from growth of lichen which is spreading no matter what!   I’ve sprayed and sprayed.  Small patches grow to larger patches.  It started on the trunk and has spread upwards into branches and twigs Lichen … pronounced ‘ liken’ (American English)  or ‘ kitchen’ (British English)  On investigation, I discovered lichen isn’t a parasite, so it’s not sucking the juices out …