Phal + Semi-Hydro + Black Spots Underside of Leaves + Problem

[empiref]

Hello All,

Hope everyone is doing great.

This is my only phal among a couple of Oncidiums and 13 Cymbidiums; they are all potted in hydroton, semi-hydro medium.

When I water every week, I flush the pot and do a check of plants.

This phal, has been in semi-hydro for the past 2 years, no problems at all. Last week, I lost a leaf of the opposite side of the leaf in question now. The leaf was turning yellow from the top towards the bottom.

This week I saw some black spots on the underside of a leaf, second from the bottom, the black spots have somewhat a hallow around them. the black spots are about 30% visible from the other side of the leaf.

I sprayed the entire leaf underside and top with an undiluted 2% Hydrogen Peroxide, before snapping the pics and posting this thread.

Phal is outside on a covered balcony, that is closed up with a thick plastic transparent curtain, winter time now, high temp is 18 c and at night it drops to around 7 c.

I checked the other leaves, no sign of any spots either on the underside or topside.

the pics are attached below, any help would be nice.

thank you all in advance.