I could be wrong also but that does appear to be sunburn. A couple of weeks ago, the outside UV intensity was unbelievably high for the South. I am sure you guys experienced that same issue. These sensitive plants can't take direct exposure of these magnitudes.
The only other means is to send twig samples off to your State for testing.
Ornamental Insect Control
Bermuda and Zoysia sodding
Seasonal annual and perennial color enhancements
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