Answer: It's really difficult to diagnose a plant problem without seeing it, but I can offer some suggestions for you to follow up on. The damage could be caused by drought, but 30' tall trees should be able to find ground water and survive a temporary drought. The most likely causes are insect infestation or disease. A heavy population of spider mites can make the foliage die back. You'll need to look closely at the damaged areas for webbing, stippling, or even presence of tiny little insects (use a magnifying glass). Or, you may find fungal growth. Either of these problems can be controlled with the proper chemical sprays. Sadly, conifers generally do not resprout because they do not have dormant buds. So anything you prune off will leave a bare spot in the tree. Since these are valuable specimens I'd recommend hiring an arborist to assess (and hopefully treat) the problem.
Best wishes with your Italian cypresses!
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