Hello,
I have yellowing leaves on my plumerias (one plant worse than the other). The plants are in year 3 of growth. They are planted in a cactus mix and watered once a week and dry out in between waterings - it is now regularly over 100 degrees in Central CA. Full sun but not enough to burn the leaves. I fertilized in the spring with a balanced tropical mix, added Epsom salt a month later, and recently added a bloom fertilizer recommended for plumerias. There were some glassy wing sharpshooters for a few weeks but I trapped them and haven't seen any since. Could be some scale but it's tough to tell. What do the photos tell you?
Thanks for help

deadhead
I wish I had an answer for you. All I can think of it's the heat. Thanks for posting such great pictures to illustrate your point but could you maybe also take a shot of the underside of the leaves?

The leaves look healthy, other than the yellowness. At least I see no evidence of pest or disease. I have a suspicion it is a combination of heat and lack or enough watering to keep the media (at least) slightly moist. Without adequate water, the roots can't utilize fertilizer properly. Perhaps you can supply some shade between 11:00 AM and 3:00 PM and increase the watering. Spraying the leaves in the early AM and possibly again in the late PM might help. At least, that's what I do now that we have such high heat indices every day.

when we hat consistent 100°,I had to move my trees. If it is 100° and it 1) has reflection from the pool,and 2) is surrounded by cement, you can add almost 15° more to the ambient temperature. Try a little more water and but mostly a break from the sun, perhaps under a tree or awning for bright light?