Phal. seedling leaves yellowing/dropping

[OrchidThief79]

I have a few Phal. seedlings in 2-3 in. pots that have been growing well, but recently I've had some of the newer leaves gradually yellow and fall off. The fact that they are new leaves worries me.

The plants appear healthy in all other respects. I know there are many possible causes for this, namely root loss and crown rot, but I havent overwatered them and I've used preventative fungicides.

Any ideas on what I can do? Thanks.