The original manuscript reads “the cures of God”, which Oliver Cowdery interpreted as “the curse of God” when he later copied the text from 𝓞 into 𝓟—that is, he interpreted the es as a metathesis in letter order, and so he switched the order of the e and s to give curse. But another possibility is that the original reading was curses and that in the original manuscript Oliver accidentally dropped the s after the r.
When we look at the rest of the Book of Mormon text, we find 29 other occurrences of the singular curse, but none of the plural curses. This fact strongly argues that Oliver Cowdery correctly interpreted cures as a mix-up in letter ordering and not as the omission of an s. Also note that there is one other occurrence of “the curse of God”, nearby in Alma 17:15: “and the curse of God had fell upon them”.
Summary: Accept in Alma 23:18 Oliver Cowdery’s interpretation of cures as curse since the singular curse occurs many times elsewhere in the text but the plural curses is never found.