When the plants were installed, were the root balls loosened (if sold in a container) or the burlap removed (if balled and burlapped)? Is it possible the Arborvitae were planted too deeply, with the root flare buried (not uncommon among nursery-grown trees and evergreens)? Are the plants being checked for watering needs before being irrigated? We find that many Arborvitae are accidentally over- or under-watered, for example, even if soil moisture and drainage appears to be similar throughout a row planting.

Our plant pathologist is out of the office for a few days for a conference, but the black nodules pictured (which do look like spore-bearing structures) might be from a secondary fungus and not one responsible for dieback, given how extensive the browning sounds. We can have him give us feedback on the pictured symptoms when he returns. There is a leafminer (moth larva) that can kill branch tips by chewing into the foliage, but that would not cause an entire plant to die back, which sounds like the situation here.

Our primary suspect when dealing with Arborvitae die-off is usually root-related, either moisture level or drainage, poor root structure or health upon planting, or some other environmental factor. If the gardener removes one or more of the dead plants for inspection, they can look for indications of root rot (pulling apart roots that shed a mushy outer layer, leaving behind a stringy inner core), vole damage (stripped bark near the soil level), borers (holes or sawdust in the main trunk(s)), or even staking injury to the trunk (girdling or abrasion).

Can the gardener share with you a photo or two of the planting site or entire plant, showing the pattern of dieback?

Miri

Our plant pathologist doesn't see any indications of infectious disease that would cause widespread dieback; he says that the black dots that might be producing spores (as in the microscope image) tend to be secondary decay fungi and not any responsible for actively killing healthy growth. Therefore, we suspect it is a root issue at its core, either related to poor root quality at the outset, planting depth, drainage or watering practices, or something else interfering with good establishment.

Miri

Thanks so much! I appreciate you confirming. We should see soil test results soon but I suspect poor drainage is also part of the issue.

You're welcome!