HI, 
I think the first step would be to have them checked by a certified arborist or a consulting arborist (a higher level of certification).  I'll include links to sites for both of those below.  They should be able to recognize a cultural/abiotic problem (like not enough water), disease, insect infestation/damage or herbicide damage.  They might need to order further lab testing to identify a virus from a specialized lab.  We used to have a Plant and Pest Diagnostic Clinic here at OSU but I just checked and at the moment, they're not accepting samples.  However, some labs do accept plant tissues for analysis.  They are listed in this fact sheet:
https://ohioline.osu.edu/factsheet/hyg-1132
But I would start with an arborist.
For a certified arborist:
https://www.isa-arbor.com/Credentials/Types-of-Credentials/ISA-Certified-Arborist
For a consulting arborist:
https://www.asca-consultants.org/
Let me know if you have any more questions and good luck,
Barb