Large patches on the back of new Phalaenopsis leaf
 

My Phalaenopsis finished flowering about a three months ago, started growing a new leaf a month ago, noticed this black spot on the back of the new leaf about a week ago, seems to be spreading, quickly, have no idea what this is, never seen anything like it on the plant, what is this and what do I do to make it go away! Pic attached.

Picture correction
 

Sorry, this was the pic I intended to attach!

looks like fungal or bacteria, could be pest as well any chance taking a close up?

If it's spreading it's probably best to remove the leaf. Use a sterile sharp knife/scissors and dab the edge with cinnamon.

I work on the principle of always removing something that's spreading fast and asking questions later... you don't want it to spread to the entire plant before you've realised it is killing the plant.

closeup
 

So here's a closeup picture, the spots are very dark, almost black and look like pitting. I'll probably go ahead and remove the leaf this evening.

Not sure if this helps, but as far as moisture and light go: I soak the roots and medium about once a week for watering, and mist some of the roots every day, the air is probably a bit dry in here as i have been running the A/C quite a bit. The plant gets a good amount of sun. Everything I do to the plant has been constant for about a year and a half.

In the research I've done I've come across Physan 20 and Phyton 27 as two products which might help, any input on those as possible solutions?

Id do as Rosie says....It seems to be spreading and you dont want it on the other leaves....you can grow another new leaf ....forget about saving this contaminated leaf by spraying it with chemicals.

Check it with a magnifying glass to see if it is pitted or if any pests there.because it maybe mite damage.

It looks to me like a standard manifestation of dual infection by viruses on phalaenopsis.
Cutting that leaf get rid of the ugly leaf out of sight, but viruses will still be present in the plant.
Test your plant or just keep this plant away from other plants.

Other than crown rot and minor brown leaf spotting (both by bacteria), phals do not suffer bacterial/fungal disease much.
They are many viruses that affect phals though.

I don't think test kits pick up viruses other than CymMV and ORSV, but there are many many more that affect phals. This is a bad case I'd say.

This is my personal view though, others may disagree.