The racetrack turf symposium was able to happen because of city government sponsorship.
The master who sponsors you is the one who provides your sponsorship. Without this person, you wouldn't have sponsorship at all, which is why you call them "master" (plus, that was one of the rules with the sponsorship).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.