Water testing for disease
I am not aware of anyplace that will do water testing for plant diseases. There is testing available for e coli and other human pathogens.
The safest water source for irrigation is from a well. Service sources do have the possibility of contamination with diseases such as Phytophthora. We do know Phytophthora travels in surface water. So what is happening in fields up stream can affect your plantings if you irrigate from that stream.