Brown Spots on Tips of Dendrobium Nobile Leaves

[KristenJ]

A couple of days ago I noticed a leaf on my Nobile Dendrobium had a brown tip. I thought it was turning yellow to drop off, so I wasn't concerned. However, now I have several of the leaves with these brown tips. It is only happening on the new growth. The older canes are okay.

I searched through the link from the top of this forum & think I might have salt toxicity. I've attached pictures for second opinions.

This nobile I have been treating for mealybugs, so it has not been receiving fertilizer, as I have been trying to get the pests under control first. I water it with filtered PUR water once a week. It is in a bark-like mixture. The roots sticking out of the top & sides of the pot appear to be as healthy as they have always been. I also do not see any crusty/salt buildup around the pot. None of the other dendrobiums are experiencing this problem.

If anyone knows the cause & how I can go about fixing this, I would really appreciate some advice! Thanks