I agree , and i’m hopeful that one of us gets the blessings to plant those seeds that matter. I felt like this about hybrid persimmons 20 years ago and now we can grow them. Give it time we are close to a breakthrough.
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Thank you for the kind words. I’m a big believer that as a community, we can push hardy citrus forward, and across a wide spread of regions. The more of us that are trying to breed and select improved varieties, the higher our collective odds are to find something exceptional.
There’s a lot of luck involved - take Carolina Lime for example. It was not bred on purpose and yet it is one of the best all around hardy citrus of the modern era. The US-942 was bred as a superior HLB-resistant rootstock and yet it produces fruit that are surprisingly decent. The Iva Swingle was likely an off-type seedling of Swingle Citrumelo which sprouted up from a commercially grafted citrus, meaning it bypassed the initial selection for off-type seedlings which are disposed of. The Bishop Citrandarin was supposed to be a US-852 but varied just enough to produce a sweeter fruit.
The point being that all of these new and improved hardy citrus were chance finds! I have yet to try many of the hybrids developed by hardy citrus breeders of today, but one I am excited for is Staruzu which has already proven very hardy for us and the fruit seem appealing as a lemon substitute.
The breeder who goes by Ilya on the Tropical Fruit Forum has made several promising hybrids like the Staruzu and seems to be making the most progress out of anyone today. Most of his crosses involve the 5star Citrumelo, which is improved over Citrumelos like Swingle, but still passes on some of the grapefruit bitterness. My focus will be utilizing Citrandarins like Bishop instead, to focus on more orange and mandarin-like fruit.
While we are only recently becoming more involved in breeding, we already have several promising hybrid selections. Three of which are from 50% trifoliate hybrids but they have a citrus-dominant appearance and qualities, indicating potential for better fruit.
I was actually amazed looking at one of them yesterday, our ‘Bi Tai Tri’, which looked so similar to the Taiwanica next to it, but with slightly smaller leaf size and some bifoliate and trifoliate leaves on occasion. It looks like the genes shuffled in a way where it is now closer to 75% Taiwanica and 25% Trifoliate Orange. Enough for it to be hardier than Taiwanica, but hopefully the fruit are more similar!
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All 10 rootstocks produced an acceptable number of seeds and good seedling emergence from those seeds. The rootstocks ‘Swingle’, ‘US-802’, ‘US-812’, ‘US-1283’, ‘US-1284’, and ‘US-1516’ had a high percentage of true-to-type seedlings and correspondingly good potential to be propagated by seeds.
This is from a study of trifoliate orange hybrid seedling rootstocks, because trifoliate orange is resistant to citrus greening (probably a soil borne fungus?) and may be the solution to citrus greening.
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It is interesting that despite its high % of nucellar seedlings, Swingle has several recognized variants now. When growing Swingle from seed, I have seen maybe <5% off types, and only found one worth keeping based on my selection criteria.
The Trifoliate Orange and Australian citrus like Finger Limes are especially valuable for breeding HLB-resistant varieties. Sun Dragon is a noteworthy one with navel-like fruit that have a tasty orange/pummelo-like flavor from what I have heard. It is 12.5% Trifoliate Orange since one of its parents was US-119.
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Do you know if there are different sacaton citrumellos, like is the one from Lyn Citrus the same as the one that Stan has?
I think Sacaton would be a very good candidate for using as a seed parent with backcrossing to poncirus plus with >40% zygotic seed.
We’ll breed the most HLB resistant citrus in existence here up north where the disease will never be a problem 
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Yes the trifoliate orange laughs at greening disease.
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Also the desert lime is reported to be immune to it and probably resistant to it with seperate genetics than trifoliate.
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It’s different genetics, yes.
Trifoliate I’m less familiar with but my understanding is it’s similar to how SugarBelle and some other true citrus are resistant, they do a much better job of containing the damage the bacteria does. They generally don’t clear the pathogen but do a good job of handling the infection.
Australian citrus, by contrast, produce a peptide that’s fatal to the bacteria. They straight up kill the pathogen before it can become established. Which is a weird phenomenon because that species of bacteria wasn’t present in Australia so there’s no reason for Australian citrus to have evolved that peptide. It’s likely that it’s actually targeting a different pathogen and happens to also work on the bacteria that causes HLB.
I’ve read some accounts that say it’s so effective you can graft Australian citrus to susceptible varieties and they will be at least partially protected. No idea how true that is though, or under what circumstances.
It’s definitely the better of the two forms of genetic resistance when it comes to HLB. My understanding is there’s a new generation of citrus rootstocks with finger lime ancestry in the works.
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Does anyone have the Australian citrus? I’ve seen it mentioned quite a bit in discussions about HLB resistance but have never seen pictures of it.
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@Johnsgard has the desert lime and the finger limes.
I have gotten some finger lime seeds and now I have 50+ seedlings and some desert lime scions (with no successful grafts so far) from him.
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@KS_razerback Stan’s Sacaton appears different, it is perhaps a seedling of the original, but growing the side-by-side Stan’s has more pummelo-like qualities. But the one from Lyn Citrus does produce a fair amount of off-types. Stan’s seems almost entirely zygotic making it excellent for breeding - I have one seedling of it which seems especially promising!
@a_Vivaldi I had no idea that’s how it worked with the Australian citrus, you really learn something new every day…it is funny to think that in the pursuit of cold hardy citrus we may inadvertently breed for HLB resistance! Just wait until we can get some crosses with Eremocitrus and Poncirus in the mix…
Speaking of which, @Fusion_power, I have the Australian citrus hybrids Eremorange and Eremomandarin in my possession. Eremorange is at least hardy to 10°F with no damage and superior resistance to dessicating winds. I will be testing Eremomandarin this coming Winter, and I hear its hardiness is not far off from Eremorange.
I am rooting cuttings and grafting trees to have available in our nursery since I really want to get these out ASAP so we can all breed with them. As soon as I get some decent blooms I am going straight to work crossing these with our Citrandarins!
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Thanks, I’m more than a little interested in citrus but so far have only dabbled with poncirus and a few non-hardy lemons.
What I can guess and infer is that there is very high potential to breed a citrus that is 1) sweet 2) winter hardy to -20F 3) immune to huanglongbing 4) highly flavored. The problem is that it will take a lifetime or two given current breeding methods. Is anyone working with genetic tools to shorten that timeline? I could even see a GMO version of moving genes from one citrus variety to another or possibly using GMO methods to turn off the resin genes in poncirus. There is also potential for reducing seed count.
I’ve read that there are actually 2 different poncirus species. Has anyone to your knowledge worked with Poncirus polyandra?
By the way, I looked at that map of poncirus invasive locations. It is nowhere near complete. I know of 3 different poncirus locations that are definitely not on that map.
Also, any comments on this citrus posted by Loafnut? I got a seedling tree from him a few months ago.
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I think we already got our golden ticket for that known as poncirus+.
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I’m reading up on it now that it’s come up in the conversation, the peptide is significant but it isn’t the only factor at play, which is good since it means that tolerance or even resistance isn’t tied to expressing a particular gene that may or may not get inherited and fully expressed.
The first thing is just that they’re more resistant to the psyllid in the first place: https://www.sciencedirect.com/science/article/abs/pii/S0168945224001845
Mostly because of certain volatile compounds they have.
This study looks more correlationary, but it mentions that the chemical composition of the phloem sap is different in resistant cultivars: https://www.researchgate.net/publication/378504157_Comparative_metabolomic_analysis_of_the_phloem_sap_of_nine_citrus_relatives_with_different_degrees_of_susceptibility_to_Huanglongbing_disease and suggests certain phenolic acids are an important component of the sap suppressing the bacteria. This study https://www.researchgate.net/publication/378504157_Comparative_metabolomic_analysis_of_the_phloem_sap_of_nine_citrus_relatives_with_different_degrees_of_susceptibility_to_Huanglongbing_disease came to similar conclusions, providing evidence that a variety of phenylpropanoids provide resistance.
There’s some stuff about immune response mediation, but that’s outside my wheelhouse. Roughly speaking it sounds like the bacteria can trigger auto-immune like conditions in vulnerable citrus, whereas in resistant varieties the immune response is more measured and more effectively targets the bacteria without destroying the phloem.
Here are two articles about the peptide.
This paper seems to be the most recent and thorough, but it’s paywalled:
https://www.science.org/doi/10.1126/science.adq7203
One of the major sources is this paper, which is publicly available:
Related commentary:
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That’s quite the combination of traits. Citrus is diploid with a haploid count of 9. To me that sounds like a lot of traits are going to be genetically linked unfortunately.
Anecdotally, cold hardiness beyond zone 8 or so seems somewhat inversely correlated with fruit quality. But that’s mostly going by F2 trifoliate hybrids with citrus, so who knows. And there might be some sneaky tricks to achieve at least some results, for example even though sweet tasting fruit might prove difficult to breed into trifoliate without losing hardiness (trifoliate orange itself doesn’t have great brix numbers) there’s a recessive mutation of a gene that controls anthocyanin pigment and citric acid accumulation which effectively turns both completely off, at which point even lackluster brix can taste sweet. Depending on what chromosome that gene is on, it might be possible to breed a trifoliate orange hybrid (starting with poncirus plus to avoid the resin) that, with sufficient backcrossing, would have both copies of that recessive gene but would otherwise mostly retain all of trifoliate orange’s original chromosomes (don’t ask how many generations of backcrossing that would take and how much genetic screening it would involve, yikes!) and therefore, hopefully, its hardiness. Maybe figure out if Citrus glauca’s desiccation resistance is controlled by a single gene or just a small number of them and if so, repeat the same kind of darned hybridization and backcrossing woebegone ordeal to get a mostly-trifoliate orange hybrid with the desiccation resistance (and therefore enhanced winter tolerance, perhaps down to -20 F like you mentioned) and you’ll have an extraordinarily hardy citrus. Throw in the miracle peptide from finger lime, maybe figure out how to up-regulate its production during an infection if necessary, and the recessive acidless trait, and wham! you’ll have a citrus that meets those criteria.
Probably after like 200 years of breeding and a couple million in genetic tests. 
Embryo-rescue might be a useful tool for expediting things. Nucellar seeds start out as zygotic seeds. It’s only during fruit development that the polyembryonism shows up, so in theory you can make all the crosses you want between whichever varieties and ignore if they’re zygotic or not and just embro-rescue all the seeds. Doable, just a little bit expensive.
Or use somatic fusion, especially since a lot of desirable citrus don’t produce pollen and a few don’t produce seeds. Though tetraploid citrus are known to have very poor health most of the time, so that might not be a great route except for when you want to crank out some oddball hybrids to then breed seedless triploids; that’s how Gator Bites mandarin was bred.
I’ve admitted before that I’m much less ambitious than many other citrus breeders. I’m just trying to get some sweet citrus I don’t have to protect here in zone 8. Hence my enthusiasm–I’m confident of results within my lifetime 
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Im
I’m a little confused, wouldn’t it be possible to to breed any trait of the fruit into the trifoliates’ cold hardiness with enough crossing and backcrossing?
I wonder if citrus is like some stone fruit breeding I read before, where if you make f1 hybrids then cross between the the f1 hybrids and select among the f2, traits heavily for plant morphological and disease resistance that match one of the species that made up the original hybrid, the fruit will highly resemble it too. But then if you use that one that matches the its grandparent to backcross to the other grandparent, the next offspring will be a lot more cleanly mixed and mashed traits between the two species and not as likely to have the traits all unique to one species be be expressed together.
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Prague chimera obviously produces a lot of zygotic seedlings. What everyone seems to be missing so far is that formation of ovules and pollen involve different tissue layers.
- The fruit itself is always parent tissue. It is possible for parent tissue in a chimera to revert back to either of the original species with reversion to trifoliate being more likely if exposed to cold weather.
- Formation of a flower is a different animal. I’m going to describe this in very generic terms so keep in mind that some parts of this may not be correct for citrus. This is the general path for flower development. First, a cell goes through rapid duplication via mitosis (normal cell division with chromosome duplication). It forms a super cluster of cells usually 16 cells in each super cluster. Once the super cluster is formed, 8 of the cells diverge and become normal parent tissue. Eight more cells go through meiotic division forming a group of 8 haploid cells that basically disappear, and 8 more cells that become an ovule. Each of the 8 cells has a different function inside the ovule. Five of them are kind of traffic directors causing changes inside the ovule, guiding pollen tubes, etc. Two of them become the proto-endosperm as a single diploid cell. This is important because endosperm is triploid tissue. One haploid cell can absorb a pollen granule to become the embryo.
- Pollen goes through a different sequence typically forming clusters of 4 cells which go through double division then split via meiosis producing twin haploid pollen cells. This is important because a pollen granule is not a single haploid cell. It is always a pair of haploid twins.
- When a pollen tube forms in the style, the twin pollen cells migrate to the ovule where one merges with the cell set up to become an embryo and the other merges with the diploid cell set aside to become endosperm. The result is a diploid embryo and triploid endosperm. These cells have to replicate numerous times to form the seed.
So what is important about this wrt the Prague chimera? Pollen forms from a different cell layer than the embryonic super cluster. Therefore it is entirely likely that some of the pollen grains are derived from poncirus tissue. The result is that Prague chimera is literally designed to produce a high percentage of poncirus/satsuma hybrid seed.
One way to verify this would be to manually pollinate some sweet orange seed with pollen from Prague chimera and then score the resulting seedlings according to poncirus traits.
I took a look at the seedling I got from Loafnut. Leaves are large, single, but with extended petioles with wings. Thorns are soft, green, and only an inch or so long. Color is medium dark glossy green. While I have limited experience with citrus, the winged petioles suggest a complex hybrid of some sort.
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That is absolutely true about prague , and exactly what I just talked with McKenzie’s about when I ordered one about a month ago. I know this from things not related to citrus. I suspect the carolina lime is also on your list. They have small plants growing for me now of both. They are adding some cold hardy pomegranates to my order.
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This is valuable information guys! Thank you. I did not have the scientific explanation, but I suspected on a more simple level that the flowers of Prague could not be purely Satsuma, nor do they appear exactly so. The only other convincing explanation was gene transfer as seen in graft hybrids.
This seems to be a recurring phenomenon with these chimeras, I remember reading about a Lemon + Poncirus chimera which was much less stable and produced an array of seedlings with varying phenotypes.
In my own experience, we have witnessed several seedlings with hybrid appearances, resembling exactly what you would expect from a Satsuma x Poncirus cross. I am only intending to keep one, which is most citrus like in appearance and leaf taste. Its hardiness is probably on par with Thomasville, but certainly less than other Citsumas/Citrandarins.
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Based on the data we have collected so far, I do have my doubts about achieving all of this at once, however it is promising to see varieties like Poncirus+ which lacks resin, and Conestoga 010 which is lower resin, while both are still hardy to -11°F or so.
We may eventually have hybrids which are just as hardy yet with fruit quality similar to the lowest-quality 50% Poncirus hybrids. This doesn’t sound like much, but it would change the game for zone 6 growers, and give them more options for processing fruits.
Many Poncirus strains and hybrids which closely resemble it do measure high brix (12 and 11°Bx for the above), so all we really need is a meaningful reduction in acidity to make these fruit much more palatable.
We already have US-119 which carries an acidless gene or genes, and is low acid itself. It may be a good candidate for crossing with Poncirus+! I would take on this project if not for our zone being too warm to test for zone 6 cold hardiness.
Haha, I do think there is realistically much more hope for sweet citrus in zone 8, since we already have several promising examples (both acidic and sweet/tangy) despite limited resources going towards this effort.
Breeding for hardy, acidic citrus is of course much easier since resinous off-flavors are the main concern, something Carolina Lime does not have much of at all. I think some interesting “lemons” can be produced as well, ideally with that signature lemon flavor/aroma. Yuzu is already a great substitute, as can be Taiwanica or even Swingle, but they are missing something.
I am interested in making crosses with Citremon, Yuzu, and perhaps some others to try and re-engineer a hardy lemon. I crossed Ichang lemon with Thomasville this year which might produce an interesting result too.
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