Breeding for Late Blight Resistant Tomatoes

[reed]

I think every tomato disease know is ubiquitous where I live but I still grow plenty of tomatoes.  Sometimes they do croak completely in late season but our canning jars are already full by then so I don't care all that  that much.
 
I have never done much of anything except save seed if I have choice, from less infected plants. And I compost old vines in the spot they grew and put tomatoes in a different spot the next year. Plenty of diseased leaves drop off anyway and the diseases are everywhere anyway so there is no point in wasting organic material in the interest of garden cleanliness.

I have recently been de-hybridizing the varieties Iron Lady and Celebrity. I grow the decedents of these in close proximity to my favorite Mr. Stripey (not Tigerella) in hopes a cross might happen. The F1 of Iron Lady was terrible flavor wise but some offspring after two seasons are much better.

[Diane Whitehead]

I am also de-hybridizing various blight-resistant ones -Iron Lady, Ferline. Losetto, Crimson Crush, Cocktail Crush.

A fawn managed to squeeze itself into my deer-proof area and eat the tops off them all.  I still haven't found the place where she is getting in but I will dig up what remains of the plants and plant them in my greenhouse, hoping for tomatoes from them in the winter.

[Roland]

In 2021 i will start with some F2 plants from Galahad. Galahad is a F1 homozygous ph2 and ph3 beef tomato. Do u guys know the pedigree of Galahad?
Is it a cross from beef, normal or cherry tomato parents?

[William S.]

I am most excited about the dehybridizing Lizzano so far. Short season enough to be good breeding material. Iron lady and Skykomish are a bit long season here. Though no blight here for the most part, hit about ten or fifteen years ago. So it will be back sometime.

[Roland]

I think Mountain Magic is a good F1 to dehybridizing because one parent is a good producing beef tomato with ph2 and ph3 and some other disease resistant genes. The other parrent is a very sweet orange cherry tomato with the Rin gene. Dehybridizing can give a very good cherry tomato with good flavor, Rin gene for long hangtime and resistance for blight ph2 and ph3.

Some other selections from Mountain Magic can give very good beef tomato`s.

[William S.]

I don't think exsertion is much of a hedge against unintended PH2 and PH3 loss, inbreeding is inbreeding regardless of some percentage of uncontrolled crossing and if I select for the external traits I want; essentially reselecting for Big Hill, I stand to loose the PH2 and PH3 traits which I can't see. I think the best course of action is to save and perhaps freeze F1 and F2 seed and or send it to someone who regularly has bad late blight now. Though another route might be to maintain a large population size while minimizing selection.

I wonder how much it would cost to do marker assisted selection for PH2 and PH3 in terms of doing this Deppe style Iron Lady PH2 and PH3 introgression. I wrote one such company and asked. Will see if they respond.

[Joseph Lofthouse]

Following the example of Raoul Robinson in Return To Resistance, would lead us to eliminate all known resistance genes from the population. And then select among the inter-crossing survivors for whatever can eek out an existence. This selects against a few alleles with large effect -- that are subject to short-term defeat, and selects for a multitude of alleles with minor effects -- which add up to robust long-term resistance.

[William S.]

As for the reference to Raul Robinson--this refers to the idea that there is "horizontal" and "vertical" resistance, that these are actually different from each other, and that "horizontal" is slower to be overcome by the evolution of the pathogen. It's additionally assumed that horizontal resistance is dependent upon multiple genes with small affects, and vertical resistance is dependent upon one gene with major effects (the sort of gene university breeders transfer into their varieties). It's normally assumed that heirlooms have horizontal resistance, but this is only until someone actually does some crosses and looks.

Then, my impression is that the heirloom usually turns out to have no measurable resistance--or it actually has one of those major resistance genes, exactly the same ones involved in "vertical" resistance. Ph1 is a good case in point. It was present in a number of heirlooms, which would undoubtedly, in the absence of serious genetic investigation, been called "horizontal" resistance. It conferred serious resistance in its day. However, it was overridden by more modern lines of late blight, and now doesn't confer useful resistance.

Whether a variety's resistance is easily out-evolved by the pathogen is not any such simple thing as one-gene-bad, more-genes good, either. For example, Jim Baggett's pea varieties that carry one gene for resistance to pea enation virus and one for powdery mildew and one for wilt--which let's them be grown all the way from spring to fall--are still nicely resistant to the respective diseases, though they have been around for decades.

Generally, though, from first principles you can guess that if you have two different major genes for resistance to something, your variety is less vulnerable to evolution of the pathogen than just one.

Ph2 and Ph3 show a different repertoire of what lines of late blight they are sensitive too. However, both genes act as codominants and act quantitatively with respect to each other. So varieties that are homozygous for both generally show strong resistance to all strains of blight; varieties that are homozygous for just one, or heterozygous for both show less resistance, with levels more dependent upon specific strains.

Another way of looking at it. Let's suppose that after we've developed a new generation of heirlooms with Ph2 and Ph3 in them, whether someone who didn't know what we did would call them "horizontal" or "vertical" resistance. Well, before they did serious genetics, they would probably just assume they had horizontal resistance. If they then went and did the serious genetic analysis in the absence of testing specifically for Ph2 and Ph3, they would probably still think the resistance was horizontal. Because it would look quantitative under field conditions.

One thing that is very valuable about working with wild material is you might discover additional genes conferring serious resistance to late blight. To show up as a QTL in something as hard to evaluate as disease resistance, especially in variable material, you pretty much would be talking about genes with major effects, by the way. Various university breeders are still "mining" wild material looking for new genes. Additional new genes would be valuable. Could you test for them using marker assisted selection? No. Marker assisted selection is based upon someone having identified a distinctive DNA sequence in or near the gene for resistance. So you could test wild-derived late-blight resistant material to see if it contains Ph2 or Ph3. But not PhX, something new. Of course, if your material doesn't have either of those genes and is resistant, it presumably is something new. So then you'd just give some to one of the university groups which is spending day and night looking for new Ph genes, and they would undoubtedly be very happy to do the molecular biology and identify a marker for the gene.

Carol had this reply to a prior discussion of this. My response is simply to try both approaches. Belt and suspenders.

Garden space wise Carol's approach will probably never use more than space for a couple tomato plants in my garden. Couple Iron Lady's maybe a couple F1's to grow some F2 seed. Unless I get hit by blight and then it might expand. One packet of ten Iron Lady seeds is down by four seeds after three years. Also I do expect that if late blight does hit and stay sometime I'll still be able to grow tomatoes inside and in the greenhouse and work on this after the fact.

The general diversity approach implicit in the obligate outcrossing approach of Joseph's and the similar exserted stigma Kapuler approach tomatoes well those already have most of my garden. In theory if blight hits maybe a few of those hyper diverse interspecies hybrid resulting tomatoes will be resistant and get seed saved.