Hello, I asked another agent and their response: 

From the images, I’m intuiting a fungal pathogen causing leaf discoloration.

I agree to allow the leaves to drop. My recommendation to the client is to gather its fallen/affected leaves as they drop during winter and dispose in the trash; best practice regarding reducing pathogen population. An additional maintenance practice that may assist in reducing incidence of fungal pathogen exposure is to prune the interior branching structures, allowing adequate air circulation through the canopy. Pruning should be committed around mid-March to avoid any late season freezing temps affecting newer growth.

Unfortunately, annual increased rainfall and higher humidity will be a potential issue for fungal infection affecting vegetation next year.

Vitex is ‘thought to be a native Southern Europe and western Asia’; from Clemosn Cooperative Extension online: https://hgic.clemson.edu/factsheet/chastetree/. I could not find information of habit (growth) related to native distribution and location (i.e. regions similar to a less humid and arid climate from its natural range). What I did find is the plant performs best in well-drained and variable soil and is moderately salt tolerant. Without specific pathogenic identification, I would not recommend general application of fungicides appropriate for ornamental plants; may not be practical for the client regarding attaining complete and adequate canopy coverage of a product. Clemson does offer information of a few varieties that are resistant to leaf spot (Blue Puffball and Shoal Creek). Varieties like this may be an option of consideration for future landscape planting.