My tomato plant's leaves are turning brown in some places (photo).
It seems to affect mostly older leaves, but not all of them, as you can see in the image. It starts with a few small brown spots that grow larger over the space of a week or two and eventually most of the leaf turns brown, as shown in the upper left. The plant is otherwise fine, blooming, yielding fruit and producing new shoots.
This particular plant is kept indoors on a windowsill, and therefore not exposed to rain and shielded from many parasites and diseases (this particular window doesn't even open).
What could this be?