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.
l'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.
I marcotted a few Durians twenty years ago. All the ones I know of are bearing well, are healthy and vigorous and have no problem with phytophthora. I can't say the same for many grafted Durian trees, of which there must be thousands.
Just recently I read an article from the 'Manila Bulletin' that in Malaya some farmers are mounding up the soil around the Durian trees to fight phytophthora in the soil. Perhaps this also gives the Durians a second chance to root above the graft.
I brought up all these examples because we tend to plant too close to the surface and have to support and demobilise the plant by staking and tying. This sometimes does a lot of damage to the bark of the plants.
Perhaps we should plant six inches deeper than the pot size or in heavy soil conditions, mound the soil up six inches. This also would help the rootbound plants to grow a new root system above the old. This should especially apply to marcots.
Reaction from people I have mentioned this to has been positive, and some are trying it out on existing plants. Many of the plants I produce in the future will follow this method and will be tagged as nurse root grafted with planting instructions. In the future, some of the trees we now have incompatibility problems with, should have a long and healthy life.
Reference: Hartmann and Kester, Plant Propagation - Principles and Practices.
DATE: September 1996
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