Thank you for your question. To better assist you can you tell me How old the plant is? When were the symptoms first observed? What was the Progression (i.e. younger leaves first or older leaves first) of the disease? How often do you water? How often do you fertilize? And are there any chemical or supplemental substances applied in the last 4 weeks?

Vitex is usually pretty free of disease. Considering the unending 100-degree temperatures and drought and the browning edges. The best possibility is the effect from heat and drought on it. Most trees show evidence, whether they are being watered or not.  

It's unclear if this is in the ground or in the pot, but adding fertilizer during this heat and drought only stresses plants more. It is possible that the browning could be attributed to that. 

Even with watering, inconsistent watering along with high night temperatures can impact a plant's ability to transpire from it's leaves. 

My recommendation is to check the soil with your fingers. If it's dry 2" down, it's time to water. And water at a low flow and slow so that all the soil absorbs it. If it's in a pot, being potbound can cause even more trouble and may require daily watering. Do not add any more chemicals, and wait for a break in the weather. 

Best of luck!