I think you have it nailed about the difference. Leaf Streak gets brown (on any part of the leaf) but does not show up on a cloth when wiped, and Rust gets those day-glow orange spores that will come off and show up on a soft cloth. There are some cultivars that have been documented as dying back completely from rust, so you have chosen your daylilies well, or kept the more resistant ones. Those more susceptible can be kept easier in the north where they may die back nearly completely, but get reprieve over winter freezing temps, or in the south, normally at seller's who prevent rust expression by spraying. I'm actually envious that you have rust, for its importance as a measure for how cultivars and seedlings are doing. I've been chastised for wanting a rusty fan or two ... but you are right that you cannot fight a disease, or improve stock, unless you truly have a way to test its performance and don't mask it. Perhaps that is a special role that garden hybridizers play - we don't need to prevent rust in our cultivars, so we get the chance to road test it from the get go and learn about it first-hand.
Good on you!
I think the difference between Leaf Streak and normal aging and browning is that regular dying back of older leaves is usually from the tip and/or a general yellowing that turns brown. Most Leaf Streak, I think, happens haphazardly anywhere on the leaf... but would enjoy hearing other input, too. I'll load up some pics of the difference I refer to, as usual, once I get this posted to the thread and come back to edit after finding the pics.
.... Okay, here is what I think is natural die-back of leaves. Generally, outer leaves that brown fully at the tip, start to turn yellow overall, and will soon separate pretty easily from the fan by a gentle sideways pull on the leaf.
On the other hand, I think this next pic is leaf streak. Its all over the place, as irregular patches or spots, in the middle section of a fan, anywhere. This crop came right after our 106-degree days so I ascribe it to heat stress that resulted in leaf streak. But, I suppose it could also just be called "sunburn" (?) O.o
