US-802 and other citrus rootstocks

[clarkinks]

A rootstock that may prove useful in our colder area. According to worldofcitrus it contains trifoliate genetics. We know it should have cold hardiness.

“ Notes

Released rootstock from the USHRL program that has been extensively evaluated in Florida by USHRL personnel as indicated in the source history. More information on these evaluations is available from Dr Kim Bowman of USHRL. It has also been included in some trials in California conducted by Prof Mikeal Roose of the University of California, Riverside.

According to the Release Notice, "In all field avaluations, US-802 rootstock yielded vigorous and productive trees that were tolerant to endemic disease and pest problems and exhibited excellent survival. The major positive attributes of this new rootstock are resistance or tolerance to citrus tristeza virus, citrus blight, Phytophthora palmivora, and Diaprepes root weevil, induction of high vigor and good fruit productivity on grafted scions, and ease of seed propagation. Notable negatives may be the tendency for trees grafted on US-802 rootstock to become very large and produce fruit with intermediate or low soluble solids concentration.”

Parent - 1 Siamese Acidless Pummelo

“An essentially acidless variety of pummelo. The tree is small in stature with drooping branches. The fruits are oblate with a slightly rough yellow rind. The flavor is insipid. Siamese Sweet pummelo has been used extensively in breeding programs and is the seed parent of Oroblanco and Melogold pummelo-grapefruit hybrids, Chandler pummelo, and Cocktail pummelo-mandarin hybrid. It is mature in the winter and holds well on the tree.

The clone known in the United States was introduced by the U.S. Department of Agriculture in 1930 and was received by the Citrus Research Center, Riverside, California, under the name Siamese Sweet (CES 2240).

The tree is typical of the Siamese group in all respects—dwarf and drooping, with round-pointed leaves and densely pubescent twigs and new shoot growth. The fruits are oblate to broadly obovoid, with large, crisp, easily separable juice sacs lacking in juice, and insipidly sweet with a trace of bitterness.”

Parent 2 - Gotha Road #6 Trifoliate

Official Selection of Poncirus Trifoliata - no notes

Origin

Florida, USA

1970·Kim Bowman - USDA

Submitted by

Brady Mitchell •@cascadiaadmin

Colwood, British Columbia, Canada

Submitted on: January 1, 2026

US-802 Citrumelo (Siamese Acidless Pummelo x Gotha Road #6 Trifoliate)

Here is a discussion of rootstocks

“

1999 - USDA released ‘US-852’ citrus rootstock, which supports highly productive, medium-sized trees.

2001 - USDA released ‘US-812’ citrus rootstock. This hybrid rootstock produces healthy medium-size trees with a wide range of scions, and consistently yields large quantities of good quality fruit. ‘US-812’ also exhibits resistance to citrus blight and has the CTV resistance gene ctvR.

2007 - USDA released ‘US-802’ citrus rootstock. This hybrid rootstock has field tolerance to the Phytophthora-Diaprepes disease complex, resistance to citrus tristeza virus, and good fruit productivity. The rootstock ‘US-802’ combines high vigor and large tree size with the other positive attributes.

2007 - USDA released ‘US-897’ citrus rootstock. This hybrid rootstock has field tolerance to the Phytophthora-Diaprepes disease complex, resistance to citrus tristeza virus, good fruit productivity, and good fruit quality. The rootstock ‘US-897’ fills a much-needed niche as a dwarfing rootstock well adapted to Florida conditions.

2010 - USDA released ‘US-942’ citrus rootstock. This semi-dwarfing hybrid rootstock produces healthy small to medium-size trees with a wide range of scions, and consistently yields large quantities of good quality fruit. ‘US-942’ also exhibits field tolerance to the Phytophthora-Diaprepes disease complex, resistance to citrus blight, and has the CTV resistance gene ctvR.

For additional information, contact kim.bowman@ars.usda.gov.”

Rootstocks Released by Dr. Kim Bowman : USDA ARS

[a_Vivaldi]

There’s debate as to if US-802 was bred from Siamese Sweet or just a regular Siamese pummelo. I’m growing it myself and hopefully will be and to check if the fruit are low acid or not soon, potentially next year.

US-802 itself is a BIG plant, and it grows fast. Hardiness has been good for me, it’s never suffered any damage, whereas some citrumelos like Dunstan have been badly damaged when small or with lots of new growth. I’d personally wager it’s hardy to low single digits or 0 F.

Been growing this for about three years. It’s 12+ ft tall.

I’m not confident the fruit will be any good though. The leaves are quite harsh tasting, and while the fruit are very large they’re also noted to be quite seedy and the seeds are likewise large.

For me it’s excellent breeding material. I don’t know yet if the tree is worth much for it’s own merits though. If it ends up being low acid then perhaps yes.

I will report on the fruit once I get some.

[clarkinks]

@a_Vivaldi

Beautiful tree! It’s huge! Thanks for letting us know more about it! This description covers a lot of ground they gave on the usda website.

“This hybrid rootstock has field tolerance to the Phytophthora-Diaprepes disease complex, resistance to citrus tristeza virus, and good fruit productivity. The rootstock ‘US-802’ combines high vigor and large tree size with the other positive attributes.”

[KS_razerback]

Have you ever tried bending the top branches down to induce flowering?

[a_Vivaldi]

Funny enough, US-802 until this week was the only citrus I have, and I have many, that didn’t flush new growth this year. An Ichang papeda grafted to it flushed twice but the main stem which is US-802 didn’t grow at all. It’s flushing now though, and I’m expecting it’ll put on another foot or two if past growth is any indicator.

My guess is that it’s a bit more drought sensitive than most of my other varieties. The spring and early summer were extremely dry, but the last few weeks have been good and rainy.

[a_Vivaldi]

Nah. It’s only been a few years and the first two years it was a waterspout which I wouldn’t expect to flower. I have enough other stuff to work with that it’s ok if it doesn’t bloom for another year or two.

I actually bought a few grafted citrumelos from Madison Citrus the other year specifically to get some plants that I’d be mostly sure would bloom and give me pollen this spring. Ironically, I ended up using almost exclusively Ichang papeda pollen though.

[clarkinks]

This is interesting

[KS_razerback]

Sounds like the us 852 might be a good candidate for mass planting seeds out from just like was done with Conestoga ones, or using as a seed parent, because of its high zygotic seed percentage.
Another highly appealing one for me would be the Sacaton citrumello, because it has huge fruit.
Here’s a excerpt from a research paper I had found before.
“The percentage of zygotic seedlings was <10 in C-32 citrange, Gomeri and Indio rough lemon, 10–30 in C.P.B. 4475 citrumelo, Cuban shaddock, Volkamer lemon and Yuma Ponderosa lemon, and >30 in Sacaton and Terra Bella citrumelos, Taiwanica sour orange and Yuma citrange.”
I personally think best route to take to get the fully cold hardy citrus for here would be to backcross the 50% hybrids to trifoliate (such as the Poncirus plus), rather than growing seeds out of a self pollination, because some of the dominant traits that important for a fully cold hardy one could be not inherited in the 50% hybrid. And a backcross to trifoliate would gaurunteed all traits that positively impact cold hardiness would be available.

[clarkinks]

@KS_razerback

I agree that’s a good approach. A person could plant all seeds from the greenhouse out every year until some lived. Keep a trifoliata in the greenhouse for a backcross. There is no guarantee that would work. The other thing is citrus don’t like our dirt. I grew trifoliata outside 3 years here at my farm. It is a tough orange but it hates wet feet which clay soil gives them. They also like lose soil which we don’t naturally have. They also don’t like that alkalinity.

[KS_razerback]

Interesting you say they dislike wet feet, because my row of trifoliate orange seedlings in our flood garden sat a week underwater enough that the rooted gooseberry cuttings all died, the peppers died and the apple seedlings look horrible, but the trifoliates looked just fine and are putting on good growth.
I have read before that trifoliates can be highly variable on what soil they can be grown in vs other citrus that are pretty consistent within each species.

[clarkinks]

@KS_razerback

I grew them beside my pond in that micro climate but mine never thrived there. Citrus in general don’t like wet feet. The trifoliate are tough and will tolerate it for a time. US942 is a great looking rootstock. It is fast growing and reportedly delicious. Woodlanders said this “ US-942 began life as a rootstock, bred by the USDA and released in 2010 after years of Florida field trials, a careful cross of the Sunki mandarin and the curious Flying Dragon form of trifoliate orange, Poncirus trifoliata. As a rootstock the record is stellar: compact, productive trees, strong resistance to Phytophthora and tristeza virus, and better tolerance of citrus greening than most, which is why growers across the citrus belt have come to trust the number.

What surprised everyone is the fruit on the rootstock itself. Where most trifoliate hybrids carry the bitter trifoliate aftertaste as a family curse, US-942 mostly sheds it. The little fruits ripen bright orange inside and out, not much larger than a ping-pong ball, thick and soft in the skin like a small mandarin and easy to peel, with juice that runs somewhere between the sweet citrus of a calamondin and the tart brightness of a kumquat, and few seeds. Herschel of Madison Citrus, who knows the tribe as well as anyone, calls US-942 “one of the best-tasting trifoliate hybrids there is,” and we would not argue.

In the garden, grow US-942 in full sun and sharp, well-drained soil, in the ground through the warmer zones or in a large container to overwinter farther north. Hardy, disease-resistant, compact, and productive, the tree suits the collector of unusual citrus and the grower who wants a tough, good-tasting fruit off a plant that asks little. A rootstock that turned out to be worth growing on its own merits. “

https://woodlanders.net/products/citrus-942

Madison said this “ US-942 Cutting Grown Citrus Tree was grown from mature wood which is capable of blooming in year 1. US942 is a cross between Sunki Mandarin and Flying Dragon and is HLB tolerant and cold hardy. When ripe US 942 has hints of sweet with an orange flavor, with very little to no trifoliate flavor or after taste. The fruits skin is soft and thick similar to a mandarin. The flesh is juicy and taste comparable to a calamondin or kumquat but more rich and complex flavors. US942 has few seeds and has a bright orange color inside and out when ripe. Mature fruit is the size of a ping pong ball and is one of the best tasting trifoliate hybrids.

“ 

https://madisoncitrusnursery.com/products/us942-citrus-tree-for-sale?_pos=1&_sid=e14c7316b&_ss=r&variant=45382265340074

[KS_razerback]

Here is the page of the book talking about trifoliates. The second paragraph talks about the soils.

[clarkinks]

@KS_razerback

I think we could use us119 for breeding as well. I do grow this one.

[clarkinks]

@KS_razerback

Maybe it depends on the trifoliata on how well they like the wet feet. Us119 is a hybrid I was going to back cross.

These are 942 and 119 . You can see the heavy trifoliate influence in 942 in the 3rd picture.

[KS_razerback]

US 119 seems like a stellar trifoliate hybrid, with how it is said to be sweeter than regular mandarin(?) My concern with it, is that its only 1/4 trifoliate, so even with a backcross to trifoliate it’s uncertain what is highest levels of cold hardiness it can achieve.
I’ve also wondered about making my own f1 hybrids using the the parent of US 119 (Succari), because it is early ripening (not sure if it is just considered early ripening because it never has to lose much of any acidity), super sweet, and low acid/acidless. And use Poncirus plus as the other parent. And then backcross the best offspring of that cross to Poncirus plus to have some that can grow here.

[themulberries]

I think US-119 would be great to utilize in hardy citrus breeding since it carries the acidless gene(s) from the Succari sweet orange. It has already been used to produce the US SunDragon which is supposedly good in flavor and HLB resistant. Being only 1/8 Trifoliate Orange however, it is not very hardy.
Interestingly, I do have a Clementine x Thomasville growing now which is also 1/8 Trifoliate Orange and it is fully hardy down to 10°F and likely lower. It is an unusual case though because it has more trifoliate leaves than one would expect.

I’m no expert, but I believe crossing anything with US-119 could result in several possibilities such as: acidic fruit, acidless fruit, and low acid fruit (the latter being preferred for our purposes). Crossing it with Conestoga 010 or 026 might produce something good for zone 7, as well as Poncirus Plus. Breeding for here in North Carolina, I will try to cross it with the Bishop Citrandarin or Benton Citrange to obtain orange or clementine-like fruit.

If the US-802 can function similarly for breeding purposes, it would be wonderful to experiment with. I have two trees growing thanks to @a_Vivaldi, and they have been very vigorous and hardy for us.

[a_Vivaldi]

@themulberries

Interestingly, I do have a Clementine x Thomasville growing now which is also 1/8 Trifoliate Orange and it is fully hardy down to 10°F and likely lower. It is an unusual case though because it has more trifoliate leaves than one would expect.

Oh that’s strange. Between the few dozen Clementine x Thomasville seedlings I’ve had, zero had trifoliate leaves.

[themulberries]

I believe that I may have the same strain as Tom McClendon, who posted these photos. Between the hardiness seeming better than Thomasville, and the appearance of the plant and fruit, I must admit that I am suspicious as to whether or not it is truly Clementine x Thomasville, but rather CiClem x Thomasville. The latter would make more sense but I cannot confirm since it is an old hybrid.