Chlorosis (leaf yellowing) on mounted Brassavola nodosa

Does any one know what causes this? I'm at a loss. It's being grown mounted in a empty clear plastic pot outside in full sun . It gets daily water and regular fertilizer and monthly Epsom salt.

 

Thanks Ray. It doesn't appear to be spreading and it's just on two leaves so I was considering just removing the affected leaves. Would that probably stop it

 

Ok. I actually asked a experienced grower from my society and he noted it was only on the very oldest two leaves and thought it was just done with those leaves and getting ready to drop them. But I'll be on the lookout for any spreading. I water the plant every day so I can keep a close eye on it.

 

I ended up just pulling the effected leaves off. Plant seem to be doing fine. No other leaves affected and it's putting out several new growths and roots.

 

I have been keeping it a bit drier, which it doesn't seem to mind especially as it's getting much cooler

 

That's what it ended up being for me. My plant is fine nos