How does this work? And I'm talking like accessible to home gardeners and not professional labs or something which use molecular and genetic engineering.
So how does this go?

1: Do you breed with parent plants that are perhaps not resistant themselves, but not infected either and hope the following generation(s) are by testing them for that disease. If some of the progeny are, you select those specimens to continue breeding and improve the line.

2. If none of the progeny are, do you keep trying with the same parents, or ditch them as possible parents and look for new parent plants?

3. Can you also use parent plants that are infected with the disease to breed a resistant line (and bet on dumb luck) or ditch those too and only breed with non-infected plants?

Hi Lee-Roy-
Legend has it that the breeder of the original Knock-Out Rose did it by spraying his breeding roses with ground up diseased rose leaves. He used no poisons. Then he only kept the offspring that seemed most disease resistant. I also read an article somewhere about a Hellebore breeder, who said the key is to get rid of any plant that is not really vigorous and healthy, and only then look at the flowers. I think we have all had the unpleasant experience of buying a plant with a beautiful flower, but the plant is not vigorous and gets sick and dies. Or is not sturdy so it flops. In rose breeding, for decades they sprayed the h... out of them all, so they all survived in the nursery, but we all know how crappy those plants looked in our gardens, sick and spindly!
I have become interested in this topic too.
I think you could reasonably try using one parent that is disease resistant, crossing with one that has your desired flower form. I would avoid all use of pesticides, so you can see which ones are not resistant and "Rogue out" the yucky plants.
I think breeding with infected parents would be fine, then select the healthiest offspring. "Rogue out" the sick ones, do not use them for the next generation.
I did a 30 second internet search about Knock-Out Rose, here is a link to a little article on it, I know there are more extensive explanations.
I have not done this, but I think you should know in two generations if your program is working.
For example, in my gloomy chilly damp climate, zinnias are "iffy". But some years ago a new one came on the market, called Profusion Orange, that actually does OK here. If I were to try zinnia breeding, this would be one of my parents (if it is fertile I don't know if it is).
This kind of plant breeding does not require any fancy stuff, we can try breeding in our own gardens(well, maybe a blender to mix up a pathogenic spray). If I were to try and breed healthier Hollyhocks (Alcea rosea), I would not need a spray, every plant here gets rust, so it would quickly be obvious if a plant was resistant. I might try and cross with Alcea rugosa, the Russian Hollyhock, which are supposedly much more resistant to rust.

http://www.walterreeves.com/la...

Thanks for the explanation and link Mary (I'll read it later), but trouble is that I'm not aware of any other cultivars or resistant strains of the plant I'm trying to breed with. I already have an F1 generation (sprouted this spring) and are growing fine. But I don't know how to go from here really...

(It's Hymenocallis harrisiana)

Oh that's a neat flower!
I spent a few minutes looking it up. I think I have to get some.

Brent and Becky's Bulbs (bulb experts who sell online here) say it will grow in dry, average or wet soil, and has survived their Zone 7 garden.
Plant Delights Nursery (total plant nut Horticulturists) say it is from low swampy areas around Mexico City, which is a high elevation so often plants from their will survive here and in the U.K. and I would suppose Belgium.
The Pacific Bulb Society (super duper uber Plant nut geeks) mentions in one place these grow in arid areas, so I don't really know.
Anyway, there are many species of Hymenocallis, from varying habitats, some quite marshy, some periodically flooded, some from rocky slopes, so you could try to introduce other species into your program. Or just pick the best plants each generation, for your location. I saw one thread in the PBS website where someone said this genera is susceptible to Cercospora Leaf Spot, but that the species H. glauca is resistant.
Breeding plants could easily become your new hobby. I sort of got interested too, but haven't done much about it, mostly learning how to pollinate various things and saving seeds. I did get interested in iris- I know there are some really healthy really old cultivars that thrive in horrid dry shade around here, but when I bought a lot of fancy expensive new cultivars of Tall Bearded Iris they seem quite finicky, needing to be coddled, and are just not good plants for the home garden, like the old ones I see around old houses here, happily flowering for decades under trees. I bought an old out of print book from Schreiner's Iris on sale called The World of Irises, by The American Iris Society, which goes into detail about the species, genetics, and history of breeding. One could become immersed in this hobby for a long time...

What happened to your Hymenocallis that you are trying to fix with your breeding program?

For the past few hours I've been indulging myself in new Iris cultivars and maybe breeding 'm (sibirica that is), but that's a maybe haha.

Well my Hymenocallis are suffering from what I think is red blotch (Stagonospora curtisii), a fungal infection that attacks - what I noticed - leaves, roots and the bulb. Hippeastrum and Amaryllis suffer from it alot too.
So far there's no real cure; I've searched the net hundreds of times.

Here is a link I found for you.
Well you might try getting seeds of other Hymenocallis species, and seeing if any do better, or just keep it simple and grow out lots every year, keeping the best, like the Knock-Out Rose guy.

I have an Intermediate Bearded Iris that does well called 'Midsummer Night's Dream' (unlike all the fancy cultivars of TBI I bought. It reblooms every late-summer and fall, is blooming now. Here is a photoof it last October 18. Also the white rebloomer 'Immortality'. Also I have been messing around a little with the little Irises, really just getting seeds to start of oddballs. Have not yet intentionally bred any,maybe when I retire...

http://www.aces.uiuc.edu/vista...

Stagonospora curtisii is a difficult one. I'm not speaking from experience, but a lot of people on the SRGC forum have problems. Unfortunately, you will need to register (free) as you did here, so that you can use the search function. When I did a search for Stagonospora curtisii, 21 results showed.
http://www.srgc.net/forum/inde...