It's possible that it's a disease, but I'd look to over-watering as the prime suspect, especially if you've never noticed any significant wilting between waterings. The soil likes to be damp - not wet. Ideal is about as damp as a wrung-out sponge, so make sure the soil is getting fairly dry deep in the pot before you water. Use a wood skewer or a dowel rod, stuck deep in the pot, as your 'tell'. If it comes out obviously moist/cool/dark with moisture, withhold water until the soil is nearly dry, then, water so at least 10-20% of the total volume of water applied exits the drain hole. If you cannot presently water as described without the soil remaining wet so long it causes root problems, you might wish to ask about some tips that will help you water correctly.

I would snip the offending leaves off close to the stem & destroy them, just in case there is a disease issue.

Al