Q: I have nine arborvitae that have been in the ground three years. About a month ago I noticed blackening of the leaves out near the tips on several branches. The affected branches are just above ground level and isolated to a small area that faces East. So far, this is only on 2 of the nine trees. .
A: The black foliage is curious to me but seems to have no disease cause. I found this description online.
You subsequently sent me the results from the UGA plant disease lab, which said:
Abiotic disorder suspected
No pathogen was found. This type of dieback is most likely due to some type of stress. This could be a reaction to some heat/drought stress last summer or shading of the foliage (shade tends to thin foliage, the lower parts may not be getting enough sun). I suspect these newly planted trees (less than 2 years) are not completely established and were stressed this past summer. In addition, they may be dropping excess foliage acquired while in the nursery. Do not over water but do water during dry periods. Water deeply but not frequently. Sprinkler irrigation on the foliage could also cause problems.
The black needles still look odd to me…but I guess we have to take the word of experts!