The national-desk copy chief of The Washington Post, Bill Walsh, disapproves of this style decision, though, despite what his paper's sports desk does. In his book Lapsing Into a Comma, Walsh wrote, "Three RBI? Is that like three POW? It's silly, if well intentioned, to try to apply this kind of internal logic once you've switched from a spelled-out term to an initialism. The plural of an initialism is the initialism plus s. Prisoner of war/prisoners of war, but POW/POWs. Run batted in/runs batted in, but RBI/RBIs."
That makes sense to me, so I thought I'd consult Walsh about WMD versus WMDs—and, while I was at it, FAQ versus FAQs, for "frequently asked questions." This last term is a bit different, because in the spelled-out version the s does come at the end. The evolving convention is nonetheless to call a list of questions an FAQ.
Walsh told me, "I try to avoid WMD outside quotations, but I think it's clear that the plural should be WMDs, following the POWs principle. FAQ strikes me as a singular noun, even though it stands for a plural concept. An FAQ (or a FAQ, for those who pronounce it 'fack') really means 'a list of frequently asked questions,' not 'a frequently asked questions.'" Very true. Walsh's thoughts about how to treat the plurals of initialisms amount to as sensible a set of principles as you'll find.
NITISH JHA, of Pretoria, South Africa, writes, "English is my first language, so you can imagine my shock and indignation when a French colleague told me that I was using the word orchard incorrectly. Apparently, the word orchard is used in conjunction with non-citrus fruit, whereas for citrus fruit one must use the word grove. Thus we have apple or guava or mango orchard but orange grove and lemon grove. None of the dictionaries I consulted gave a reason for this distinction. For that matter, not all of them even made this distinction explicit. I am looking for an explanation of why such a difference exists, if indeed it does.Can you help?"
I thought it was their own language the French were picky about! I checked fourteen English usage manuals (more than dictionaries, these specialize in drawing fine distinctions) and two dictionaries of synonyms for you. The only book to list either orchard or grove was Richard Soule's 1938 Dictionary of English Synonyms and Synonymous Expressions, with an entry for grove that reads, "Wood, woodland, thicket, copse. See forest." Hmmm.
I also checked databases of recent English-language newspaper articles in the United States and elsewhere. Throughout the English-speaking world citrus grove, orange grove, and lemon grove are indeed seen far more often than citrus orchard, etc. And apple orchard is seen far more often than apple grove. Evidently, newspapers seldom have occasion to mention guava groves or guava orchards, but when they do, the more common phrase is, as you say, guava orchard.