it may be bacterial and I would spray with a fungicide.
If it is a bacterial infection fungicide won't help. Fungicide only works on fungus.
That said I don't think it is either of the above. If you are concerned about it get a fine point Sharpie and trace around the outside edges of the spot. If it doesn't grow beyond the Sharpie marks you have nothing to worry about.
Phyton 27 is both bactericide and fungicide so I think it should work. By the way, my phal has 11 leaves and the 2 bottom leaves are turning yellow (very slowly, for months).
Yes I know. I just don't want anyone to think they can kill bacteria with fungicide.
If the bottom leaves are slowly turning yellow your plant is getting ready to shed them. It's perfectly normal for this time of year. Leave them on (even thought they look ugly). The plant will extract any remaining nutrients from those leaves and then they will drop naturally.