If delayed incompatability becomes an issue Mike Fabians' experience may be worth noting.There are quite a few options.
DELAYED INCOMPATIBILITY IN GRAFTING TROPICAL FRUIT TREES
And What Can Be Done About It
I have been working at Limberlost Nursery for 24 years and through the years I have noticed (or it has been brought to my attention) a lot of problems regarding fruit trees.
If a graft is not compatible, it will not graft together and will not grow. However, if the rootstock and scion are partially compatible, all sorts of problems can develop years down the track.
So here are some examples of delayed incompatibility:
takes a long time to grow together (Longan, Macadamia);
lots of suckering (Grumichama, Longan, Mangosteen);
slow-growing (Longan, Pulasan);
new shoots keep dying back years later (Durian, Pulasan);
short stumpy growth (Durian, Longan);
lack of vigour (Durian, Mamea, Purple Mangosteen, Avocado, Pulasan, Longan);
canker on trunks (Duku, Pulasan);
lots of fungus problems (Pulasan, Durian, Avocado);
premature death (Durian, Avocado);
marked differences in growth rate or vigour of scion and stock (Mangosteen);
smaller fruit or leaves compared with seedlings or marcotted (or cutting) trees (Star Apple, Longan).
Delayed incompatibility can develop 10-15-20 years later and trees may have a lot of fruit during that time. However, grafted trees may take 4-5 years to come into bearing, and they should have more than 5-10 years of fruit-producing life.
After all, seedling trees can live and produce fruit for many years (10-50+). So it stands to reason that all of the problems come because of the graft union not being perfect and unable to translocate all the nutrients that the trees need.
So if we can somehow bypass the graft union, some of the problems will be solved. Some fruit trees can be propagated from cuttings or marcots but not all. Those that will not take this way can be propagated by nurse root graft. This means that the grafting is done very low on the seedling and once the graft has united, it can be put at least 4-5 inches or more below ground level so that in time (a few months to a few years) the scion wood will grow a new partial or complete root system above the graft union. Nurse root grafting is well documented in some grafting books.
So I am not recommending anything new, only suggesting the nurse root graft, which has been used for many years in the propagation of apples, pears, grapes and rhododendrons, be used on tropical fruiting trees.
I'd like to point out that trees planted much deeper than the original pot soil level grow very well. Here are a few examples which have come to my attention.
About twenty years ago, I read The World Was My Garden by Fairchild. He was a plant collector for the USA Department of Agriculture around 1910-20. He wrote that when the Purple Mangosteen was grafted onto Platonia insignis and Garcinia mestonii, the Mangosteen made roots. What he failed to mention was the type of graft used. As the Mangosteen does not make aerial roots, the graft had to be a nurse root graft.
Shifting sand dunes at Cape Flattery and Fraser Island (and everywhere else on the earth) continually cover plants, but if the plants are not covered completely they do not die. In some cases they grow roots high up from the original sand level, then when the sand shifts again back to the previous level, these roots can be seen hanging from the trunks.
Twenty years ago, a friend planted some fifty jackfruit trees. They were seven to eight feet tall in pots. He dug three- to four-feet-deep holes with a post hole digger and planted the Jackfruit seedlings at that depth. Not only are they still doing very well, but have provided a lot of fruit for the Sydney market.
Ten years ago, I grafted a Grumichama. It kept suckering, so I mounted up the soil about a foot high around the plant. This stopped the suckering. Since then, it has been dug up twice and replanted (there were plenty of new roots above the graft) and it is bearing and doing well.
We planted a grafted Soursop tree a few years ago at the nursery. A year later, the soil in this area was built up by two feet. This new soil covered the Soursop half-way up, but it was not adversely affected and is still growing strongly.
UPDATE ON DELAYED INCOMPATIBILITY WITH GRAFTED FRUIT TREES
I wrote an article on this subject which appeared in September/October 1996 Rare Fruit magazine 'The Exotics' No 100. At the time, I thought some people in the nursery industry may be offended but they were not. Just recently Peter Young from Birdwood Nursery talked about the same problem affecting the citrus (worst of all the dreadful Trifoliata suckering). I am referring to 'Australian Horticulture' of August/September 2001. I am very happy that someone else is noticing the problem and is doing something about it.
One may ask why, if grafting is so troublesome, do we do it. First of all, we can guarantee that the grafted tree will produce fruit, and if the selection is good, the fruit may be the best of thousands of seedling trees. All types of fruiting trees go through this selection process, thus providing us with such good stone fruit, pears, apples, citrus etc.
Since 1966, I have found that a few more fruit trees prefer the nurse root graft. (Editor's note - a nurse root graft is where the tree is planted with the graft below ground level as per illustration.) I will list these and give you the reason for each.
First of all I would like to bring to your attention some of the plants which have latex in them which will benefit from the nurse graft - lucmo, ross sapote, star apple and mundu (yellow mangosteen).
With normal grafting, lucmo, ross sapote and star apple have much smaller fruit than the parent tree but with the nurse grafting, larger fruit will be produced. Although mundu grafts well by normal grafting methods, it may produce larger fruit by the nurse root graft because it is in the latex category.
For the last one hundred years or so, many people have tried to graft mangosteen. I started 25 years ago and unsuccessfully tried all sorts of rootstock in the mangosteen family. (Looking back on it now, I am sure the further one moves from the grafting species, the less chance there is of success. That seems to be the golden rule. However, there are a few exceptions, as with all golden rules - citrus, okari nut, stone fruits and custard apple).
At present I have twelve fruit trees on the Limberlost fruit list on which I use nurse root graft - avocado, custard apple, governor's plum, white sapote, duku, kaffir lime, yellow grumichama, durian, giant lucmo, mundu, ross sapote, star apple.
Growing avocado on the tropical coast is a real problem, with only a few varieties producing fruit. I selected a few seedling trees which have good fruit and bear heavily. I use nurse root grafts on them to improve the health and life span of the trees.
Governor's plum root stock can have very nasty thorns, so it is preferable to place the graft below ground level. White sapote and duku are very slow to grow when grafted, but with the root graft, theyseem to be much faster and healthier.
I also use root graft on some citrus, mainly kaffir lime which produce good healthy growth. Kaffir lime will also grow from cuttings, but with difficulty.