Strictly based on the image above too dry is my first thought...followed closely by fertilizer build up in the soil.

What does the entire plant look like, container size, etc. and what do the branch tip(s) look like?

Hi! One plant is in a very large maybe 15 gallon? Pot. The other is in a smaller 3 gallon. I think the tips look okay. There is some discoloration on stem if larger one. I have had problems with scales last summer. Treated with Hort Oil and been Oil several times last summer. I sprayed all my plumerias with Hort oil again in March and again in May . There are no yellow leaves this morning on the large Plumeria and very slight yellowing beginning on on 2 leaves if the smaller one. Don’t know what’s going on in the stem On the 3rd picture? Thanks for your suggestions!

It is possible the horticultural oil burned the older leaves if it was applied during full sun heat of the day. I still think it is too dry and possibly over fertilized. It might be a good idea to leach the salts out of the soil and/or do a soil refresh.

I would not do anything related to the 3rd image. I have a few of those on various plants and it does not appear to be a problem.

Thanks so much! I applied the hortoil at 8PM at dusk so I don't think its that. I will refresh the soil.