This is not a devastating disease of arborvitae, but it is a condition we see often enough to be bothersome. Leaves and stems on the inside of plants are often affected, but we see it most often on branches that overlap from one arborvitae to the next. We have tried to isolate and identify pathogens associated with this tissue to no avail. It is not any of the commonly found pathogens of arborvitae, and it is not the common internal browning of arborvitae that occurs in late winter. The condition we are seeing is not browning and drying of leaves but a blackening of leaves, often with leaves still firm, not rotted. We have not found any insects, mites, or related creatures associated with this blackening.
We cannot tell you with certainty what is causing this condition. We surmise that heat, poor air circulation, and possibly humidity are to blame.The condition does not spread but appears more or less all at once. It is most likely weather and site related, so chemicals are no help. Remove affected tissue in dry weather, water the plants in periods of extended drought, and fertilize in the fall to help plant vitality. Prune surrounding plants to enhance air flow in the area.