I'd keep them isolated until you can see if they have anything that looks to be spreading. Once you've had them a few months you can re-assess.
In my case the dark areas on my Phals have not spread and appear to now be blemishes rather than active infections: they're dry and the tissue involved is dead and surrounding tissue looks healthy. I have these plants growing with all my other Phals and give them the same care. But, I am extra careful when watering to not get the dark areas wet and I inspect them every time I water to make sure nothing has changed. This might be an abundance of caution on my part.
I think some of these rots set in when they're in the stores and wrapped in those plastic sleeves and/or watered from above. These conditions seem like they would be good for decreasing airflow, allowing water to sit in the crown and leaf axils, and letting opportunistic bacteria and fungi take hold. Once this is no longer the case and the orchid is getting proper care, the bacteria and fungi can't persist, and the infection stops progressing. It's symptomatic of bad husbandry so there isn't really much risk of spread once conditions are corrected.
I can't handle upkeep for mounted orchids so all of mine are potted :-) I think you're pretty much in the clear on the Phals with the good root systems if your husbandry is good. The dark spots might even be more cosmetic than rot in their case; the old roots at the base of the stem naturally die off leaving some discoloration or dead material.
---------- Post added at 10:40 PM ---------- Previous post was at 10:21 PM ----------
I just want to add: the dark areas are likely dead so they'll be there forever. If nothing changes after a few months and the surrounding tissue looks healthy: your orchid has a dead spot (likely from a past infection), but the plant is fine. Time is the best way to tell what you're dealing with.
Last edited by aliceinwl; 12-19-2018 at 02:34 AM..
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