Above is a photo of what I believe is a Cleistocactus growing in my garden. It is planted under the eaves of the house along the north facing side of the house, driveway side. For the last few years it has had some minor dieback on a branch or two but this year (2016) this has been quite significant. I have cut back all the dead branches well beyond where the damage is and there does not seem to be any visible disease or other problem. Can anyone identify or suggest a cause for this problem?