Unknown cause of necrosis in Phals

[Drosha]

Hi Everyone.

l have been growing phalaenopsis for 6 years and they have been doing great. l used to grow them in my dining room and lounge until l got a hothouse 2.5 years ago. Since then l have got a huge problem. Come winter, cells are collapsing in the leaves and l can't find the reason behind it. l water them every 5-6 days, never on the leaves, temperature ranges between 16 and 28 degrees. Come spring the new leaves are fine and they all are pushing flower spikes now. They look very odd something like 1 leave, 1 spike. this is not happening to all my phallies. Some of my phals are just fine old and new. Can anybody help?