The following are some observations, in no way scientifically rigorous, but I give them as food for thought, as to what it is going to take to clean up the rust buckets. (The status of my two F2 seedlings is clearly subject to downgrading at any time, and to get a better evaluation, the F2 seedlings need to be moved into the rustiest part of the garden.)
I have an old seedling (my avatar shows its bloom) which I believe (label issues) came from a cross of ONE FINE DAY and COYOTE MOON. (Don't get excited by the polymerous bloom... it only did that one season, at about 20-25%, rarely since then, and has produced no polymerous offspring.)
ONE FINE DAY has an ATP rust score of 1.0 (as good as it gets for resistance), and has been outstandingly resistant here. (It dwells in a lot of shade, and has been exposed to heavy rust pressure last winter (2014/15) and this winter (2015/16).)
COYOTE MOON has a rust score of 2.9, and I have battled rust on it last winter and currently. (It lives in the same side yard as ONE FINE DAY.)
The seedling is also somewhat rusty, but seems somewhat less so than COYOTE MOON. (Ditto same side yard, lots of shade, yadda yadda. Note that this is a subjective and not objective evaluation, in that I am not counting rust spores or rust patches. Moreover, it took some years for rust conditions to evolve in the garden to the point where the seedling could get infected. The lesson here is that it may take several years and the right conditions to accurately evaluate any seedling. Since we all (presumably) have limited space in our seedling patches, that effectively means that we must cull out all but the least susceptible seedlings (to have room for the next year's crop), while leaving space to monitor those remaining seedlings for some years (or deliberately expose them at a young age to heavy rust under the right environmental conditions) , to get an accurate assessment.)
Getting back to the (presumed ONE FINE DAY x COYOTE MOON) seedling, I crossed it back to ONE FINE DAY and currently have two seedlings from it. (I'm not sure if I ever planted out any others... I do have issues with % germination and keeping young seedlings alive.
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Those two seedlings have both, thus far, shown good rust resistance (as in, I haven't seen any rust on them, yet, and they get watered with a hose sprayer, which would be conducive to rust, and there have been some rusty plants nearby). Those two seedlings do NOT dwell in the side yard; they currently reside on my patio, which has more sun than the side yard areas. While there has been some rust exposure there, it is not as much as in the side yard. If I were to be more rigorous about my evaluation, I should move the seedlings to the side yard, preferably right next to COYOTE MOON, and let it drip rust all over them. I may yet try that (one plant would have to be potted up from a tree pot first), but I'd rather enjoy the flowers on the patio. (The seedlings have small yellow flowers; I like yellow daylilies. YMMV.)
The point of all this is that one generation of crossing, even with a very resistant cultivar, isn't going to get you where you want to go. It is going to take you at least two generations (using 1.0 resistant plants in both generations of crosses), and probably more. (Almost certainly more if you do sibling crosses on the F1, rather than crossing to a 1.0 cultivar. More generations if the susceptible plant is worse than 2.9-3.0. More generations if the resistant plant has a higher score than 1.0.)
AND you have to give the candidate seedlings sufficient time and/or sufficiently rusty garden conditions (the worst rust micro-environment in your garden) for evaluation.
For what it's worth, using my half-baked rust prediction method...
ONE FINE DAY = 1.0
COYOTE MOON = 2.9
F1 sdlg = 1.95 (which should be "resistant" on the ATP 1.0-5.0 spectrum, but the seedling isn't
my idea of resistant (no, I don't evaluate degree of susceptibility rigorously, as in counting rust patches per inch))
F2 = (F1 x ONE FINE DAY) = 1.48 the seedling should have some good rust resistance, but it could be better
If I were to take it a step further, and cross the F2 back to ONE FINE DAY (again):
F3 = (F2 x ONE FINE DAY) = (1.475 + 1.0)/2 = 1.24 slightly improved resistance (In practice I would not make this cross; ONE FINE DAY has a low bud count here, and the bud count on these two F2 seedlings already leaves something to be desired. If I wished to continue playing with these two plants, I would either outcross to something resistant that would jack the bud count back up (while retaining the early bloom season and bloom opening traits), or I might do a sibling cross, hoping to recover a higher bud count from the F1 sdlg genes.)
If I were to sibling cross the F2s, I would guess that the F3 seedlings would have on the average the same resistance as their parents, but there might be some that are rustier, and some that are more resistant. (However, I might also recover some of the bud count and branching from the F1 sdlg, which got it from COYOTE MOON.)
So from just these few seedlings, you can see that we have got our work cut out for us, cleaning up the rust buckets. Since it's going to be a lot of work, we need to pick our battles wisely. Be certain that whatever plant you are trying to rescue (for whatever features you deem desirable), that X generations downstream it is going to be worth all the time and effort: you will have retained or enhanced the things that you loved about the original plant, retained or enhanced the bud count, branching, flower substance, flower opening, season of bloom,
and have gotten rid of the propensity to rust (to the extent that any of us can, with multiple strains circulating).
