Feedback on an ailing Ficus

[W3rk]

My Ficus definitely limp through winter, I don't have a great set up (they are in a West facing window that only briefly gets them actual direct sunlight, though I do also have a supplemental light). They all suffer a good deal of leaf drop when they come in and over winter.

This particular Ficus started getting some brown spots and yellowing in a number of leaves. which have now dried up and curled. This is the most I've seen any of my tree leaves dry out as or before they died. A couple of pictures for reference below. This is a new problem for me and I'm not sure of a cause. There's another of the same ficus just to the side of it which is not having the same problem.

 

[Shibui]

Hard to tell at this distance. Note that most damage is to older leaves. Cold and sun would normally affect the top leaves first. When older leaves die off it is usually the tree trying to reallocate resources so that at least some parts can survive to keep it alive. I assume the others have been treated the same so nutrients should be similar and the other one is not showing the same symptoms. Maybe it did not get enough water at some stage? Ficus are really tough so the damage may take weeks to show up.
The other possibility is a fungal infection which would probably be worse for trees inside in sub-optimal conditions.