This used to be popular rootstock, anyone here using this?

Swingle Citrumelo

Still by far #1 in Florida commercial citrus, and rapidly moving toward #1 in Brazil. Excellent cold-hardiness if properly hardened before a freeze, excellent fruit quality, moderate tree size, bud compatibility with most scions, are among its strong points. It's also tolerant of most strains of exocortis viroid, which deleteriously affects most other Poncirus hybrids and pure Poncirus.

Disadvantages: Its root system is not as deep as some other stocks, so in Brazil, where they have always grown 'Rangpur' unirrigated, they are now having to install vast acreages of irrigation. Also, it is not tolerant of high-calcium soils. It is sometimes said that it needs an acidic soil, and that's generally true, but it seems to be calcium content, rather than pH per se, which is the problem.

I have a bunch of varieties I have budded to Swingle here in my yard that are all doing very well. PH between 5.5 - 6.0. (BC1, Browns Select, Mr Mac, Silverhill, Cocktail Grapefruit, Navel orange, Etc...) All are doing excellent with much better vigor than pure Trifoliata.

Most of Brazil's citrus industry is on highly acidic, low-calcium soils. Swingle will do very well indeed, there.

Swingle seems to be a true calcifuge, a plant to which the Ca2+ ion seems to be toxic, and/or the Ca ion interferes with uptake of other elements, independent of the pH. I don't know the full physiological explanation of this, unfortunately.

My suspicion (without proof) is that the trait comes from Poncirus, since it is the acid-loving of the two parents. On the other hand, I am not aware of Poncirus having an aversion to calcium per se. But i don't find it particularly surprising that a hybrid of two such different parents would differ dramatically from the parents in its phenotype.

Dear Malcom,from my experience Poncirus trifoliata has no 'aversion' against Calcium.
Because my four trees grow in plain soil, which was measured as pH7.2 up to 7.5
I explore no sign of deficiency or grow problems, they do very well. But in free ground, everything is diferent. But all trees were grown beside a law, which get's all two years a good potion of dolomic limestone, and in front of the house one "Hiryu" is still benet my small alpinum, which is build completely from dolomic Limestones, and the area get's planty of Calcium, to ensure a pH of about 8.
so even here, the lower soil levels contain high amounts of Ca, but get more acid in reaction, because here a sandy loamy soil with high organic matter is present, and I give the tree a cover of highly acid shredded pine bark, to ensure low soil reaction.... but I guess the Ca from the alpinum is still present in the soil.... but it's just guessing.

Problem is: It's difficult to ensure neutral soil reaction and adding high amounts of Calcium.
So a homemade test for that....

I remain considerin Poncirus trifoliata not as clcifuge, I remain considering it as intolerant about high soil pH....

Citrus Fertilizer Management on Calcareous Soils

http://edis.ifas.ufl.edu/CH086

How to manage soil for citrus | NSW Department of Primary Industries

http://www.dpi.nsw.gov.au/agriculture/horticulture/citrus/management/other-information/soil

In the case of using known Calcium
intolerant rootstocks does lead to
a problem in areas with calcareous
soils but we also have the ability
to force a correction to the Calcium
problem that leads to an overall effect
that we can see in the leaves of the plant
which can later lead to better overall
stability of the plant.

It is unusual to see the effects of Calcium
toxicity as much of the time what we see
is an effect that causes another symptom
that we equate as being an Iron chlorosis
mostly. Foliar sprays can help for the
short term but doing that does not make
the Calcium in the soil problem go away.
We can mask plants with high volumes of
Nitrogen to green them up but when the
plant shows signs of a nutrient imbalance
later it generally is more severe the next
time. Sure, give a tree Nitrogen to cover
up a Manganese deficiency or perhaps
clean up a mottle leaf Zinc deficiency
but later on we see the smaller sized
leaves be green from applying Nitrogen
but those leaves are somewhat stunted
as well. A rather common event to see
with several of the container grown
Mandarins, that we feel we are helping
the tree by using foliar sprays and by
applications of Nitrogen but we get a
symptom later that may be less to our
liking than dealing with the Calcium
problem to start with. It makes it tough
not to use Gypsum (add in Calcium to
get rid of or break down bound Calcium)
along with lots of water to help us. The
other issue is most people are not set
up to bubble in sulfuric acid into their
furrow or field irrigation water to help
things along either. Using sulfuric for
drip and micro-jet irrigation systems is
not advised here.

In either case we try to flush the Calcium
out of the soil profile with lots of neutral
pH water and/or soil conditioner and even
till in lots of organic matter or we use a
combination of all of them along with a
low pH water to help break down the
Calcium carbonate for us. What we do not
want is for the Calcium to become a bound
form that does not dissolve in water as then
we might not be able to get rid of it. We
can learn to live with excess Calcium in
the soil perhaps but over time we can impact
how much of the residual Lime is sill found
and retained in the soil after a series of
enhanced soil leachings along with applied
organic matter that can come from cover
crops, green manure crops or from forest
humus type soil additives. Just adding in
lots of organic matter in the soil alone will
make a difference for calcareous soils.

Jim

I think with 'Swingle', as with any other rootstock, it is important to remember the old citrus growers' saying "You choose your rootstock based on how you want your trees to die." A bit pessimistic, perhaps, but profoundly true -- no rootstock is perfect; all of them have one or more significant disadvantages. So you pick the one that is least bad for your own situation. And while 'Swingle' does have some significant disadvantages, it has been and remains our primary stock in Florida, and is rapidly moving toward that position in Brazil. That's because its advantages outweigh its disadvantages, for commercial grove/orchard trees. Of course, that does not mean it is necessarily best for a home garden tree, nor for a potted tree, for for all climates.

For production purposes I think this rootstock
has panned out pretty well for Florida over time.
It is not the rootstocks fault for disease issues
that have been re-introduced which has caused
some to trees to falter. Then again, the problems
we see may not be due to the rootstock either but
are more due to infected scion wood used for
budding. We blame the rootstock for the trees
decline but we seldom talk in terms as to what
we did to weaken the trees at the same time.
I think we have enough of an idea that this
rootstock can be used for calcareous soils.
We used to measure everything by how well
the commercial/production trees did in the
ground when there weren't any people growing
these plants in containers, in their homes or in
a greenhouse. There is a reason why we do not
try to grow too many standard sized trees in a
container and why those plants do so much better
in the ground. So, if the rootstock is the whole
issue then Swingle has passed the test of time
very well is how I look at it from a distance
away.

If Brazil wants to use this rootstock then they
can spend the time and some effort to work on
the soil limitations. I have to here when someone
wants to grow salt sensitive crops in an alkaline
soil, what's the problem? It is not the plant that
will falter on us, it is more of a factor of what
we did not do to help the plant along before we
put it into the ground, knowing we may have an
issue to live with later and were not prepared
enough in advance to deal with that limitation.

Jim