Two of my 13 queen palms are rapidly browning. At first, I thought it was just normal browning of older fronds, but I then noticed that no new bright green growth was emerging from the center spear. The center spears turned brown within a matter of days to weeks.

1. What could be causing this?

2. Is this condition reversible, or is the palm likely beyond recovery? I’d like to avoid having to remove and replace more queen palms if possible.

Unfortunately it is most likely untreatable fusarium wilt. There is no cure. Hopefully you have not spread it to the remainder of your queen palms via pruning tools. It is critical to sterilize your pruning tools with a bleach and water solution between trimming each tree to avoid further infection. Fusarium is easily spread. You should dispose of the infected trees ASAP. Butia and Mules are also susceptible. Does anybody have any experience with Allagoptera arenaria and fusarium? I planted one in an infected area 10 years ago and no problems since.

@Primerose assuming that there haven't been any crazy weather swings (insanely hot/wet/cold/etc), I'd guess a few diseases could be at work. Something like a root rot would usually show up slowly, and not in days/weeks from healthy to dead-looking. The only exception might be if you are in clay soil and the palm is sitting in a "bowl" of water all of a sudden. Something like a badly leaking irrigation valve could drown a palm. Queens are known to love water, so this seems less likely...but worth checking to make sure your system isn't dumping a ton of water on those two palms.

The below 3 diseases are incurable but maybe you can prevent them from spreading. My guess is Fusarium, but I can't say from photos if it has the 1-sided frond death pattern:

• Thielaviopsis - fungal disease of the upper trunk. I had a couple of queens catch this, and they died literally within days. The fronds dessicated like your photos, and the upper trunk was splotchy brown inside. Thielaviopsis is floating around in the air all the time, and according to UFAS/IFAS it enters via open cuts (pruned off fronds) and easily spread with pruning tools. https://ask.ifas.ufl.edu/publication/PP143

• Fusarium - fungal disease with a distinctive "one-sided death" of fronds. It's incurable and easily spread with pruners. I lost one Queen to this ~7 years ago on the SE side of my house, and the W side ones were fine for several more years. It kills really fast too. https://ask.ifas.ufl.edu/publication/PP278 This one is easy to see visually...most of the time. You can see my photos here for reference: https://www.palmtalk.org/forum/topic/62561-fusarium-strikesaaaaaaaaaaaaaaaaaa/?do=getNewComment

• Ganoderma - fungal rot of the lower trunk. This is probably the worst case, because it can stay soil-resident for years (or decades). A common sign is a shelf-like fungal conk on the lower trunk, but it's not always there. https://idtools.org/palm_symptoms/index.cfm?packageID=1111&entityID=3320

Given that your queens don't have a lot of trunk height, it's probably impossible to tell between Thielaviopsis and Ganoderma just by looking at the dead trunk. A lab test would tell you the exact disease. In FL there's a lab in Gainesville that tests samples for common diseases. I'd bet there is a local AG Department lab if you want to have samples tested for confirmation.