An idea has been germinating in my head for some time now using this rose to create a line of healthy, cold hardy, thornless roses. Had I the time and place in which to do it, I'd be hot on this line of breeding.
Dr. Robert Basye wrote in his ARS annual articles that, had he the time to do it again, he would have used Basye's Legacy with R. Wichuraiana to include the fourth source of blackspot resistance in his breeding line. He would have self crossed promising seedlings of Wichuraiana X Legacy, or vice versa, to fix the thornless trait, then proceeded to breed with the results.
What I'd propose is to cross thornless Wichuraiana X Rum 10, the thornless form of Multiflora, select promising seedlings from the offspring, then, perhaps either self the best, or cross promising siblings to fix the thornless trait. Only one selfing should be necessary. Once thornless offspring are created, they can be tested for value in breeding as well as garden plants. These results should provide exceptional vigor, cold hardiness, disease resistance, and who knows what kind of fragrances? Both species have made successful root stocks as both are easy to propagate from cuttings and are fairly adaptable to many soil and climate types, as well as scion types. These are all traits worthy of engineering into a breeding line.
The original polyantha roses were crosses of Multiflora and China roses. Later, some were brought into the class through crosses of Wichuraiana ramblers with Chinas, Teas and Hybrid Teas. Once you have developed your Multiflora X Wichuraiana thornless hybrid, some of the more successful crosses could be replicated to see how the second species has modified the results.
Basye's Legacy is now being used for breeding, so there are and will be more, hybrids of it to use in future breeding. This line brings very good health, cold hardiness and thornless ness into the gene pool. Early Legacy hybrids, such as my Lynnie, or even Legacy itself, could be used with the multi-churiana seedlings to create a breeding plant containing all the major sources of blackspot resistance, good cold hardiness and several different sources of thornless ness. Since there would have been only one self cross, there should be virtually no threat of undesirable recessive traits coming out in later generations.
The basic combination of the breeder I propose and Legacy would yield a rose containing Multiflora, Wichuraiana, Moschata, Rugosa, Carolina and Hybrid Perpetual genes. Clustered flowering would be expected, and high levels of disease and cold resistance should be assured, depending upon the other roses used in the mix and seedling selection.
As for what to use with this breeding line, the sky's the limit! I'd strongly suggest considering miniatures after witnessing Ralph Moore's results between species and minis. His Torch of Liberty has produce some fine seedlings, again, my Lynnie being a good example. Torch of Liberty resulted from Orangeade X Golden Angel. Orangeade functions to increase the flower size and saturate colors. It can introduce proclivity to blackspot, so seedling selection should be done with that in mind. Golden Angel combines several sources of Wichuraiana from both parents with several Hybrid Teas and miniatures. It also contains Little Darling, which has Wichuraiana in it and is a source of thornless roses. Harvey Davidson's thornless breeding line rests heavily on Little Darling's shoulders. Golden Angel, itself, could be a good choice for use with the multi-churiana hybrid.
Softee, one of Mr. Moore's miniatures, is a combination of his two most prolific breeding roses. On one side, it contains Wichuraiana and the floribunda, Floradora. On the other, is the sister seedling to his Rise'n Shine, which is a cross of Little Darling and his Yellow Magic. Both of these roses contain much Wichuraiana blood, providing good health, vigor and cold hardiness potential. One of my seedlings, which I created in hopes of it being a decent breeder, is a cross of Softee and Basye's Legacy. This thornless, constant blooming miniature may contain the potential for breeding dwarf, thornless modern roses, with, hopefully, improved health and cold hardiness. It's hoped this will be commercially available in the near future.
Other Moore minis which have produced thornless roses include Golden Gardens, Cal Poly, Sequoia Ruby, Rise'n Shine and Joycie. While these may yield seedlings which lack prickles, what isn't known is how disease resistant nor cold hardy they may turn out to be.
By combining the species which were responsible for the original polyanthas with Basye's Legacy, or one of its close offspring, my hope would be a race of polyantha-like roses which exhibit the improved cold hardiness of the originals, combined with increased disease resistance and thornless ness. Once this line is developed, imagine what could be created by returning to the better Old Garden Roses and introducing their genes into the pool!
Some OGRs to consider might be Gloire des Rosomanes and the found rose, Grandmother's Hat (Barbara Worl). Both are lightly prickled, extremely fragrant, reliably repeat blooming and somewhat shade tolerant. All of which are worthwhile characteristics to bring into your recipe.
Ralph Moore created a small series of thornless roses nearly half a century ago. Luckily, they're still available. Using a self seedling of Gloire des Rosomanes crossed with the polyantha, Etoile Luisante, he created Renae, Carolyn Dean, Everbloom 1 and with further crosses, Climbing Yellow Sweetheart and Dancing Doll. All of these are either thornless themselves, or capable of producing thornless offspring. Though not really cold hardy in extreme situations, they do provide the removal of the prickles, repeat bloom and good disease resistance. One or more of these could be quite interesting for use with the multi-wichuria hybrid.
I hope these ideas and suggestions inspire someone to actually follow the thread and create some of these roses. I would enjoy hearing the results and thought behind the course you take. Good luck!