In the course of about 2 weeks, this die off went from a small patch at the bottom of the tree on the right, spread upward and then to the adjoining tree. I see no evidence of pasts, no webbing, etc. Perhaps scale? But it's not sticky. The photo is taken facing east -- they are in shade after about 3 p.m. At first, I thought draught bec this began just after our heatwave. So I watered a lot over the following week. But it only got more widespread.These are in a line of 5, and I am panicked that it will spread to the others. Any suggestion would be so appreciated. These are about 16 yrs old & have had no past issues. Thank you!
We looked at your photos and it looks like the trees are browning due to type of root issue and/or a possible insect pest called the japanese cedar long honed beetle.
This new invasive insect is attracted to trees under stress. It causes branch dieback and browning. Look carefully for oval exit holes about 4mm x 2mm on the damaged limbs. Peel back bark and look for tunnels. The larvae feed in the cambium and prevents food and water for the tree. You will have to look for this. If you see it, all you can do is prune damaged limbs back to healthy tissue.
If you do not see evidence of insect damage, then most likely you are dealing with a root issue. This can be difficult to diagnose as this includes site conditions, poor planting techniques, environmental issues, etc. This may be planting too deeply, excessive mulch, poor drainage, soil compaction, etc. Here is some information on these types of problems
Check the drainage in the area and make sure the soil drains well and there are no drainage spouts dumping water. Mulch should be no thicker than two inches and keep away from the base of the stems. Water during dry periods. Check the soil several inches deep and water when dry. Prune dead wood back to healthy tissue. Do not overwater. Here is some information on irrigation