Plants drop lower leaves to excess when they are too wet, too dry or have a soil mineral deficiency. In your picture I see very little yellowing, maybe it is just normal aging and dying of oldest leaves.

Mildew can create patches like that on leaves and those leaves are more likely to drop. Scrape around and see if the patches are powdery.

I grew some for awhile.
The slugs loved them....and won.
Mike