Yellowing Leaf of Mottled Leaf Paph

[Roberta]

A question that I would ask the OP is, how long have you had this plant. Nutrient deficiencies only show up over an extended period of time (and Paphs are particularly light feeders so can go for months with no fertilizer at all before showing any problem and even then, it would just be slower growth) If it is a relatively recent (less than a year) acquisition, I'd look at temperature. This type of Paph does like to be on the warmer side, so if it got a chill (or cold water in a leaf axil) this could be a result.

They are also very forgiving of water quality so pH problem (especially on the high side) is unlikely... The Huntington Botanic Garden in southern California gives their very large, world-class Paph collection well water of 300-800 parts per million TDS (it varies), pH around 8 or a bit higher - they found that the Paphs grow better with the "liquid rocks" than with pure water! (Phrags are different - they require fairly pure water, they get RO)

[AnitadeWaard]

Thank you so much for your detailed response, Brian! I had our water tested and it is very hard: 212 mg/L of CaCO3. Unfortunately there was no pH measurement with the test: a garden soil test lists the pH as 'neutral', but that might not mean very much. I've tried to water the orchids with either melted snow or boiled tap water, and add 1/4 tsp of MSU Tap Water orchid fertilizer once a week.
The bloom is now fading so I will repot the plant and of course check the roots as well. None of the other leaves are yellow so it might have been water in the leaf node: will be careful about that, moving forward.
Thanks again for all your excellent advice!

[aliceinwl]

I’ve seen this on Paphs I’ve acquired where the new growth did not have a good connection to the parent plant and its own roots were in poor shape. In my case, the roots of the parent plants weren’t in good shape either (make sure you know what to expect with Paph roots as they’re different looking and tend to be brittle: leave anything firm alone).

As long as the Paph has some healthy roots, the plants rebounded quickly when repotted into bark. For small fans with no roots or plants with no healthy roots, sphagnum moss has worked much better for me. In cases where the parent has some decent roots but the small fan doesn’t (and isn’t well connected to the parent), I pot the two separately: one in bark and one in sphagnum.