There are a few pictures of leaf streak in the AHS Daylily Dictionary, the top two were taken by a plant pathologist:
http://www.daylilies.org/ahs_d...
Interesting, I see that the link chalyse gives above has "borrowed" an image of daylily rust from the front page of my daylily rust info pages, which are at:
http://web.ncf.ca/ah748/rust.h...
Anyway, disregarding that, if you go to my daylily rust info pages link above, click on FAQ in the left navigation bar, and then scroll down, the third question on the FAQ page illustrates differences between fungal invasion of aphid and spider mite damage (presumably the leaf streak fungus) and various manifestations of daylily rust. A daylily with some resistance to rust may just get brown spots without producing spores but that's not going to happen in a garden without rust (except maybe on a new plant).
Another possible cause of browning daylily leaves is leaf scorch, see this link:
http://hyg.ipm.illinois.edu/pa...
When it's only the bottom leaves turning brown I would agree it's likely normal aging although it could suggest low nitrogen. When plants are low on nitrogen they will move it from the older leaves to the newer growing ones, and in monocots the yellowing may start with a V shape at the tip of the older leaves.
