Looks like you figured it out... by: Jacki Cammidge, Certified Horticulturist
Thanks for the picture - that helps a lot. I did some research regarding this, and found a post on Daves Garden Website that shows almost identical damage.
The responses suggest that it could be spider mites. Although the usual culprits form very fine webbing which is a dead giveaway, there are some types of spider mite that do the exact same damage, but don't have the webs.
The solution is pretty simple, especially if you don't want to use any kind of chemicals; spray the leaves, upper and lower sides, with plain water, every day, a couple of times. Within a week, this should eliminate the problem.
Other than that, a systemic insecticide will work too, but keep in mind that whatever you use, it won't cure the existing damage. New leaves should be unaffected.
Aug 01, 2020
What’s wrong with my Plumarria plant? by: Bernie Gleason
Oh that would be wonderful if it’s that easy! Thank you I will try your suggestion.