Hi, I have about 20 arborvitae trees and 12 of those trees have completely dry out and died. I have 3 that are completely health and some that are dead and they a healthy one. Can these trees be dying from spider mites or some kind of fungus. Do you have any information that can help me understand why they are dying?
Marion County Oregon arborvitae
Sometimes, there are root problems due to poor drainage that can cause this type of damage. So, if you see evidence of poor drainage, cut back on the watering rate. That said, make sure your remaining trees/shrubs are getting enough water and are properly fertilized to minimize stresses. . Normally you should give Arborvitae a deep watering once a month late spring to early fall if there isn't sufficient rainfall.
Spider mites can cause damage by sucking the chlorophyll out of the plants. If you see evidence of tiny webbing or actual mites (tap branches over a white paper), you might try hosing the tree/shrub down with water daily for a few days. This might help reduce any mite population.
The damage you're seeing could also be from Arborvitae (thuja) Borer, a type of bark beetle that girdles trunks and branches causing death of the trunk & branches. This usually only happens when the trees/shrubs are stressed due to drought and/or other causes. Healthy trees/shrubs that are full of sap are normally able to repel these bark beetles. You should cut down a dead shrub and see if there are holes in the bark and girdling under the bark. You may even see larvae or beetles. Dead shrubs/trees and dead branches should be removed and disposed of. Here is a link to some information about these bark beetles. https://pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/arborvitae-thuja-borer
Here is link to another site with more information about bark beetles that may be helpful. http://ipm.ucanr.edu/PMG/PESTNOTES/pn7421.html?src=blog18800