Hello,

Overwatering as well as overfertilizing are some very possible causes for which your plumeria leaves are turning yellow.

High amounts of the "middle number" phosphorus in your superb bloomer will cause the soil to get so saturated that the necessary nutrients such as iron and manganese not only will be washed out but won't be absorbable by the plant. Iron for example works as a catalyst for the production of chlorophyll in our plants (chlorophyll makes our plants green). This iron deficiency is termed "Chlorosis". I recommended feeding your plumerias a water soluable Iron supplement such as "Gardeners Choice Iron Booster" I found it at Armstrong Nursery. This will help to restore the essential materials back in your soil and help to bring back the acidity in your soil. As we are approaching the end of summer, I wouldn't recommend usage of Bloom Busters with the high middle numbers at this time as fall is approaching as well as hibernation/dormancy time.

Click on the link below to read more about Chlorosis.
You will notice that they say to refrain from using too much of phosphorus. So hopefully next year you can supplement the regular feeding with also Iron supplements as well to restore the balance within the soil.

Hope this helps.

Rodney aka Hawaiianbuilt

Here is a link that might be useful: Chlorosis