Dendrobium spectabile leaves yellowing and dropping

[Luke]

Hello, I am a newby. I live in Ft Lauderdale and have a collection of about 80 orchids. I’ve been a hobbyist for about 12 yrs. I have this large dendrobium spectabile that started blooming with about 8 blooms, then many leaves started to yellow and fall off. It mostly happens with a pair of leaves on the same pseudobulb at the same time. The newest growth is less affected. There are a lot of marks on the underside of some of the older leaves, but they feel smooth(photo). These leaves are mottled on the top ( photo)New leaves are also dropping( photo of top and bottom of leaf pair). I’ve treated it with thiomyl and orthene. Not ideal, but I am being aggressive. Any help is greatly appreciated. Thanks!