The most useful thing a thermal camera has ever found for us was a drainage fault, not a disease.
A north-east block on a 4,100 acre farm had been treated as a fungicide problem for two seasons. The thermal pass in week three showed a cool band running diagonally across it, in a line no crop disease has any reason to follow.
It was a collapsed tile. The repair cost less than one of the fungicide passes that had not worked.