Hetty,
I'm with Al on this one. Looks like a burn. To be safe, keep it away from all the other collection. See what develops. I would not cut out the damage, you may make more in the process. Also I would maybe keep a tent or umbrella over it during the hottest parts of the day. Water plus heat is ok. It's those cold spells with water that rot sets in. Also remember a little cinnamon sprinkled over it to keep the rot/fungus in hand. I am no expert, it is only my best guess. Keep us informed on this. I would like to know what comes of this. I too love this agave. Good luck.
Stush
