For behold, there is a curse upon all this land, that destruction shall come upon all those workers of darkness, according to the power of God, when they are fully ripe; therefore I desire that this people might not be destroyed.
What curse is Alma talking about? This one:
Behold, this is a choice land, and whatsoever nation shall possess it shall be free from bondage, and from captivity, and from all other nations under heaven, if they will but serve the God of the land, who is Jesus Christ, who hath been manifested by the things which we have written.
Although the “curse” isn’t really a curse. It’s just something that always has happened and always will happen: if people are being wicked, they will be destroyed, or put in bondage, or suffer some other kind of punishment.