Hello Jeffrey

This could be one or more of several issues. Based on the picture this looks like either the shrub died from drought stress, or something has killed the roots or girdled the trunk.

This year we have had many reports of arborvitae dying, and the cause is thought to be this past winter’s effect, known as ‘winter kill’ and/or the drought stress this summer( and possibly past year’s droughts which affect trees for years). Did you water the shrubs deeply this summer during the hot, dry times?( a lawn sprinkler is not enough water for the shrubs)

Other issues to check- examine the trunk near the soil line and about 6-10 inches up. Are there any signs of gnawing, missing bark or other damage? Woodchucks, voles, rabbits can cause this kind of damage, which cuts off the tree’s ability to move water and nutrients. Are there any large holes near the root zone, indicating a large burrow has been dug into the roots?

The issue may be poor drainage or overwatering, which causes the roots to rot. Sometimes there is a disease at play as well, but that cannot be determined without a root sample. Overcrowding can also be a problem - if you planted them densely at first to create an instant screen and they have since grown a lot, eventually the weaker shrub can dieback.

Arborvitaes need watering, but make sure that there is not water puddling around them. Check the area for a low spot where water puddles. Dig down near the roots -check if they are dry and brittle( indicates drought stress); or soft, mushy and rotted( indicates flooded or diseased).

Certain diseases can cause blackened areas on the leaves. A sample would need to be diagnosed by a lab, or a certified arborist. MSU Plant and Pest Diagnostic lab provides this service; their fee schedule sample submission form and directions for submission are on heir website: https://www.canr.msu.edu/pestid/

You may ask the lab’s opinion by submitting your pictures to them via email- <personal data hidden>

References—

https://apps.extension.umn.edu/garden/diagnose/plant/evergreen/arborvitae/foliageturnsbrown.html

https://ag.umass.edu/landscape/fact-sheets/arborvitae-needle-blight

https://hyg.ipm.illinois.edu/article.php?id=15

Arborist search by zip code—. http://www.treesaregood.org