Lack of water? I don't know how much rain you have had over the past few months.

Certainly drought sounds like a good explanation, and if you lived in the South, that would definitely be the problem if you hadn't watered. Depends, though, how mature the trees and shrubs are - conifers go brown quickly during dry conditions, but if broadleaved trees and shrubs are doing it, and the plants have been in more than ten years, then something else is going on. Do you live near fields which are sprayed periodically? Weedkiller drift can cause these problems.

are they in the same area could it be something in the soil or has there been any spray drift from chemicals? sorry but more info will help

I was thinking honey fungus Bamboo?

Not impossible, Pamg, but usually that starts in an area and kills everything in it first before spreading - if it spreads, and it doesn't always. Its not as common as all that, either. Were it that, the honey coloured toadstools should appear next month.

I don't know anything about your locality, Hesperides, but could there be a gas main or fuel pipeline under or near your garden? Are nearby trees and shrubs outwith your garden being affected? Generally, a disease would be specific to one plant type and slower acting than in your description so I think lack of water or a chemical substance more likely.
Are you in a new build house and are these new trees and shrubs; if so could the ground be contaminated with something that was there before you moved in?

To the best of my knowledge, we don't have honey fungus, here in the States (thank goodness!).

The first thing that I would think is that someone has sprayed an area nearby with a "weed killer" with prometon, which is actually a soil sterilant, and has a tendency to spread underground with the rain. It was originally designed for industrial parking lots, but people keep mistakenly using it in home or park situations. First it kills the weeds, then it kills your landscaping, then it goes on to kill the neighbor's landscaping, then it goes on down the street. Without treatment, it can last 5 years in the soil. The only treatment used to be digging activated charcoal into the ground, but humates have been found to be much easier to apply, and more effective, too.