My Indian hawthorns are on a drip system and being watered for 25 min. 3 times a week starting at 2 am. Temperatures here in El Paso, Texas range from 72 to 97 degrees. My plants are over 20 years old. I don't fertilize them or spray with anything other than water them.
My Indian Hawthorn Is Dying
Indian Hawthorns, especially the older varieties, are prone to leaf spot disease, which spreads plant to plant by either rain splashing or irrigation systems. It is a virus. It looks like the virus from your pic. I have these plants also. Mine have the virus from time to time. The best thing you can do is keep up very good hygiene. Remove the leaves from underneath them and renew a thin layer of mulch often. Bag the leaves that have the virus on them and discard them.
Hope they improve!