What's happened to my Den Phal?

[mexicowpants]

Walking past my Den Phals this morning, I happened to notice this:


What is it? It had a pretty fast onset, just a couple of days ago it was fine. Now whatever this is is affecting one of this year's new growths and one of last year's.
I did kind of forget to water itthis past week (we've had a dry spell in the middle of our rainy season, so humidity had decreased somewhat), but I can't see how a few dry days would cause that.
The crowns haven't gotten wet, there's been good air circulation, no new plants have been introduced to their growing area... Basically business as normal.
It hasn't gotten any worse since this morning, but it's definitely got me concerned. What is it, and how should I proceed? I REALLY don't want to lose this one!

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