Furr C-57 Citrandarin (Citrus reticulata x trifoliata)
Introduction
Furr C-57 citrandarin is is a hybrid between a Sunki mandarin and trifoliata. It was named ‘Furr’ in honour of Dr Joe Furr and his many contributions to Citrus research. C-57 was originally developed by the USDA breeding program at Indio, then later selected and tested by the University of California, Riverside before finally being released in August 2009 [1].
We imported Furr C-57 seeds into New Zealand in early 2025 (along with Alemow) and sold them to the public, making this the first release of Furr C-57 in New Zealand. This article includes all the latest information about C-57 and its behaviour that we can find. Eventually we will update this article to include results of its performance in New Zealand as well once that information is available.
Botany
C-57 seeds are highly polyembryonic, and so the vast majority (estimate about 95%) of seedlings will be exactly C-57 clones, and thus we can be sure that C-57 now exists in New Zealand. Some of the seedlings may by zygotic (hybrid) seedlings, and one easy way to find these is to look for monofoliate (single leaves). We have found a few monofoliate C-35 seedlings previously. These zygotic seedlings will be different to C-57 with possibly worse rootstock performance, but maybe the fruit will taste better. We would recommend not grafting these zygotic seedlings and instead growing them out instead.
C-57 is evergreen and so should have a somewhat longer growing season than trifoliata, which is deciduous and tends to start growing quite late in Spring, and stop growing early in Autumn. We hope that a longer growing season for Citrus will be useful in certain areas of the country with mild Summers and little risk of frost, such as the West Coast. Here we often have bananas still pushing out leaves for a few months after the Citrus have stopped growing, which was one of the inspirations for looking for a potentially better suited Citrus rootstock for our local climate.
C-57 when fully grown is a medium to large tree. Young trees grafted onto C-57 showed good tolerance to freezing in trials [2].
Furr C-57 citrandarin growing at Lyn Citrus, showing relatively upright growth habit and a medium crop load. Image from [8]
Regarding cold tolerance of C-57 on its own roots, we don’t have a great source for this as almost all C-57 trees are grafted (so the cold tolerance of the scion becomes the weak point). Few people are growing it on its own roots. Tough Citrus states that is is cold hardy to 0 degrees Fahrenheit, which would be equivalent to -17.8 Celsius [3]. We estimate that C-57’s cold tolerance would be closer to -15 Celsius. Trifoliata itself can handle approximately -23 Celsius, and cold hardy mandarins usually can handle up to about -8 Celsius.
Genetics
Thankfully there’s no mystery as to the genetics of C-57 as it was created by breeding program. The seed parent was a Sunki mandarin (Citrus sunki syn. Citrus reticulata) which is a wild mandarin species, and the pollen parent was Citrus trifoliata (syn. Poncirus trifoliata) variety ‘Swingle’ (not to be confused with Swingle citrumelo).
Also another confusion to avoid, is many websites list C-57 as a citrange (orange x trifoliata hybrid) – that is completely incorrect, it’s a citrandarin (mandarin x trifoliata hybrid).
Sunki mandarin, also known as Sunkat or Suenkat, is a sour mandarin that is native to South China. Sunki has a long history as rootstock in its own right. It is tolerant of tristeza virus, somewhat resistant to Phytophthora, moderately cold tolerant and highly salt tolerant. Its fruits are small-medium sized and extremely sour. Sunki seeds are also highly polyembryonic [4].
Rootstock
According to the Growing Citrus in New Zealand, the most important characteristics in selecting a rootstock are:
Essential factors for New Zealand include: genetic compatibility between the scionand rootstock, tolerance of the stock and scion combination to citrus tristeza virus, and tolerance to Phytophthora, nematodes and cold, wet soils. [5]
C-57 was released with another two rootstocks at the same time, C-22 and C-54. When we were deciding which of the three Californian rootstocks to import, we ranked their disease resistances against the three main Citrus diseases:
Citrus Tristeza virus
Phytophthora
Citrus nematode
Bitters C-22
Tolerant
Moderately tolerant
Not very tolerant
Carpenter C-54
Tolerant
Moderately tolerant
Very tolerant
Furr C-57
Tolerant
Very tolerant
Very tolerant
From this we are able to quickly see that C-57 has the highest disease resistance of the three, and this is the main reason we chose to import C-57. C-54 doesn’t seem to have any advantages over C-57 and so we don’t think it’s necessary to source. C-22 is semi-dwarfing, has much better bud unions than C-54 and C-57 and is a lot more salt/alkaline tolerant and so we think that would be the second best choice.
In one of the early trials, C-57 was found to have the best Citrus nematode resistance of all tested rootstocks (RUBT-R in the middle is a pure trifoliata selection and performed significantly worse) [6]:
C-57 had the lowest combined nematode levels of all rootstocks in the trial [6].
C-57 is also moderately tolerant to calcareous soils. In New Zealand calcareous soils are very rare and mostly restricted to drier parts of the country with limestone based soils. In these conditions C-57 could be a useful rootstock or home garden crop.
C-57 also tends to produce a medium number of suckers, so remember to prune them.
Lastly, C-57 may have limited resistance to HLB:
Although thus far no rootstock has shown to induce the desired levels of tolerance to HLB, trees grown on some rootstock cultivars produce good yields under high HLB pressure and exhibit lower-than-average rates of fruit drop. These rootstocks include US-942, US-812, UFR-4, UFR-5, and the Californian cultivars C-54 (‘Carpenter’) and C-57 (‘Furr’) [7].
Fruit
We primarily chose C-57 due to its disease resistance and rootstock performance, so taste was not a big consideration. But still, what does it taste like?
There are not very many taste reports of C-57. Thankfully user Skandiberg from tropicalfruitforum asked Lyn Citrus for a taste report, which Brad kindly provided. I’ve reproduced the taste report here with permission from Skandiberg [8].
Brad writes:
C-57 12-14 seeds per fruit. Fruit larger, 7-8cm in diameter Somewhat sweet More acidic than C-54 and C-22 Not terribly bitter, less bitter than C-54 and C-22. Acidity is the main flavor profile, like a lemon. Might reduce acidity as it hangs on the tree? Easier to peel and separate into sections (liths) Juicy without much texture Yield is lower
Outside of a C-57 fruit
Inside of a C-57 fruitC-57 fruit showing easy separation of segments.
Brad added some additional comments:
None of this is very scientific.. I just sampled two fruit per tree at one time of year, so the information shouldn’t be taken as definitive.
The fruit on the trees were left unthinned, and the trees are watered, fertilized, and otherwise cared for as seed trees, not for fruit production. I don’t think this would have a dramatic impact but just want to add that disclaimer. The trees were all grafted onto carrizo rootstock and are about 8 years old in the photos.
I feel like C-57 is the closest to a decent fruit. The flavor is quite sour/acidic, but some people like to eat very sour things. The slightly larger size and ability to remove the peel make it much more attractive than the other two. There are more seeds per fruit, but because the fruit volume is larger it doesn’t feel as seedy when you’re eating it. And being able to divide it into sections means you can just deal with one or two seeds at a time. Much more manageable.
The C-22 and C-54 are basically mush inside, by the way. The peel can be carefully and slowly picked off, but it’s not easy or fun.
All these varieties are quite thorny.
Lastly Brad asked his wife for comments on C-57:
“-Sour but not as sour as a lemon. No puckering when eating it.
-Not a strong bitter taste, very slight bitterness but it’s not immediately noticeable
-Flavor somewhat similar to kumquat, but not the texture
-Adding sugar to the juice would probably taste good, or it could be used to add citrus flavor to dishes”
She ate the whole fruit pretty quickly and asked if there was any more. Her overall verdict was “not bad at all”. Just keep in mind that she likes sour flavors more than the average person, although I agree with her that it isn’t as sour as a lemon.”
So all in all, it’s a very good taste report for an F1 trifoliata hybrid. I have only ever tasted C-35 before (orange x trifoliata), and C-35 fruit definitely have a very strong ‘trifoliata taste’. It’s kind of hard to describe ‘trifoliata taste’, it’s a sort of musky, resinous and bitter taste. I had to cook that C-35 marmalade for twice as long before the smells dissipated, and even then it was still extremely bitter. For some reason, citrandarins (mandarins x trifoliata) seem to be relatively low in trifoliata taste.
Anyway we look forward to hearing the results of everyone who is now growing Furr C-57 in New Zealand. 🙂
References
Claire T. Federici, Ricarda S. Kupper, and Mikeal L. Roose. ‘Bitters’, ‘Carpenter’ and ‘Furr’ Trifoliate Hybrids: Three New Citrus Rootstocks. https://citrusvariety.ucr.edu/sites/default/files/2022-08/new-citrus-rootstocks-2009.pdf
Siebert T, Krueger R, Kahn T, Bash J, Vidalakis G. Descriptions of new varieties recently distributed from the Citrus Clonal Protection Program. Citrograph. 2010 Mar 20;1(2):20-6.
Niles, R.K.; Freckman, D.W.; Roose, M.L. Use of trifoliate orange as a comparative standard for assessing the resistance of citrus rootstocks to citrus nematode. Plant Dis. 1995, 79, 813–818.
Albrecht U, Alferez F, Chater JM, Zekri M. 2024–2025 Florida Citrus Production Guide: Rootstock and Scion Selection. https://www.researchgate.net/publication/347833845_Citrus_Production_Guide_Rootstock_and_Scion_Selection
