Brassalova cucullata-Die-back on leaves

[GotHookedonOrchids]

Purchased this Brassalova orchid at our local orchid show on 4/20/13. It is mounted and in an East-facing window. The tip on two leaves are slowing dying back, little by little each day. As you can see, I have some new healthy looking roots growing. I have been misting daily. Have not fertilized yet. What is causing this die-back on the leaves? I don't want to lose this little guy. Your help is so appreciated.

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