Hello Joan,

It is very difficult to diagnose a problem in the tree. The sun is shining on the tree causing a glare and making it difficult to identify the tree let alone see a problem with it. To identify a problem, we need to identify the tree. Please take a picture in which there is no sun and a close up of the area that is thinning.

Thank you for contacting Ask Extension.

The cedar tree thinning at the top. New pictures

Hello Joan,

Thank you for the pictures. There are many factors that cause an arborvitae to brown, including improper planting, lack of water, too much water, environmental changes that could affect drainage and lead to root damage, insects, diseases and winter burn.

Read more about Arborvitae diseases here: https://apps.extension.umn.edu/garden/diagnose/plant/evergreen/arborvitae/patchyyellowtobrowndiscoloration.html

And here: https://ag.umass.edu/landscape/fact-sheets/arborvitae-needle-blight#:~:text=The%20best%20course%20of%20action,overwinter%20in%20the%20tree%20canopy.

And here: https://pnwhandbooks.org/plantdisease/host-disease/arborvitae-thuja-occidentalis-root-rot

Insects such as spider mites may cause damage to arborvitae. Check for mites by placing a white paper under a branch and give it a shake. The moving black dots that fall to the paper are mites. Read more here: https://extension.psu.edu/spruce-spider-mite

Attached is another article about stress related evergreen dieback: https://www.extension.purdue.edu/extmedia/id/id-477-w.pdf

Finally, for a definite diagnosis, you can submit a picture or sample to MSU Extension’s Plant and Pest Diagnostic Lab: https://www.canr.msu.edu/pestid/ .

A certified arborist can properly assess your tree.

To find an arborist in your area, go to www.treesaregood.org, the website of the International Society of Arboriculture.

Thank you for contacting Ask Extension.