It looks like a possible combination of 2 things, 2) You will see small segments, not much larger than a pin head get dark brown or black from lack of water. They are an indication of stress. If you had gone too long between watering, or the amount of water loss from the plant due to higher temps and/or lower humidity was not matched by an increase in water input, you will see these, It is an after the fact indicator (in economics they call it a trailing indicator) of some prior event or stress. This in not at all uncommon in August and September when temps are warmer and perhaps fluctuate more. Yellowing leaves can also be a response to the same stress as the plant tries to balance the leaf surface it is carrying with the water input it is receiving. 2) The yellowing could also be an indicator of insufficient nutrition. Make sure you water the tree, and l;t it soak up the water for a while, and then apply and water in the fertilizer.
Lastly, you will see much the same behavior at the end of the season when the tree goes dormant, and the plant withdraws water and nutrition from the leaf as part of the dormancy process, Here is an end of season example:
