It looks like you might be able to reach up to where you took the picture. If so, wipe it all clean with paper towels, and without starting the tractor just let it be for a while to see if diesel shows up. Try to isolate exactly where the fuel is coming from.
If you can't come to some determination, you will have to remove the tank to inspect. It may be a crack in the plastic from weight of fuel, age, and vibration.