Is there an easy way to test for resistance to late blight, or do we just have to grow out a lot of plants and wait for some spores to blow into the neighborhood so we can see which ones remain healthy?
Depends on your funding! Marker assisted selection is available. Jason Cavatorta posted this recently:
https://www.facebook.com/groups/479332809623391/posts/911998313023503 to the "Tomato Breeders" group on Facebook.
There I asked him a question about how he does it and he uses agbiotech in California.
https://agbiotech.net/dna-markers/?fbclid=IwAR2tgIaL-QuhNzxn_q2z_IaLTJYB9gFrzlZLK4TzlbhITOVUXxxQ2XydEDA you can click on the tomatoes there and see that they offer PH2 and PH3 testing a long with a lot of other tomato genes.
They can just test for PH2 and PH3 for us. Which probably wouldn't be too expensive for one set of tests. It would be pretty expensive though for a whole segregating F2 I am guessing.
I'm a little afraid to email them and ask what it costs because so far, my tomato varieties are worth about $50 a year if I grow them out myself and sell the seed which I have decided not to do for 2022. (I just sent them a question about the cost of the testing).
So let's say I wanted to make Exserted Orange into a PH2 PH3 homozygous line. I would cross it to Iron Lady and get a PH2 PH3 Heterozygous F1 every time. Then I could grow out the F2 and have every plant tested for PH2 and PH3. Any that tested as homozygous (if that is possible) could then be continued with.
Alternatively, we rely on those group members who have bad and fairly regular late blight to narrow things down a bit. If we had a really good performing say F6 we could just test it. Or just rely on its performance.
Another strategy to minimize testing costs might be to cut down on the likelihood to need as many repeat tests by back crossing to Iron Lady or by making an initial cross to a variety that would cut down on the chances of the offspring not being homozygous. For instance, if we wanted to cross Iron Lady F1 which is PH2/PH2 and PH3/PH3 with Galahad F1 which we know is at least PH3/PH3 we would only need to test the offspring for PH2 which means half the testing. Or if we wanted to cross Purple Zebra F1 which we currently think is heterozygous for both PH2 and PH3 with Iron Lady F1 it would be like backcrossing once to Iron Lady F1. Significantly more likely to have homozygous outcomes and should cut down the number of generations you would need to test.
Could also just do a lot of stabilizing and back crossing to Iron Lady F1 until you got a plant that was very likely to prove homozygous PH2 and PH3 before testing- basically introgression of whatever important traits you wanted into Iron Lady.
My thought to avoid testing costs is to just never personally create a finished variety for Late Blight Resistance but instead make crosses and then freeze some F2 seed and send some off to collaborators.
I started a thread for this, but I am curious basically about how much variation there is in Iron Lady. There has to be some to get hybrid vigor. It is possible that we could dehybridize multiple lines from Iron Lady for different trait variations. Like how short season could we get a dehybridized Iron Lady if we selected strongly for that desired outcome? I suspect the answer lies mostly in how much difference there is in days to maturity on the two breeding lines they use to make Iron Lady each year. Same with fruit size. I would expect both parents to be boring red tomatoes so it would be relatively minor differences.
