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Although the apple growing business is in the doldrums at the moment, orchard businesses, which intend to continue into the future, must continue to redevelop their production base.
There is considerable evidence around the world to show that production costs are lowered by intensive systems on precocious dwarf rootstocks. Further more, higher quality fruit, particularly in regard to colour; flesh firmness, brix levels and uniformity are produced from these intensive systems on dwarf rootstocks.
New apple plantings perform best on land, which either has not grown apples before, or has been out of apples for some years. While this is the ideal, it's probable that much of our future planting will be into land which has recently been taken out of apples.
When apples follow apples, or pears on quince rootstocks, a condition referred to as specific apple replant disease (SARD) is likely to occur. Similar replant disorders are reported for cherries when cherries follow cherries and for pears following pears. The severity of replant disease depends on soil type; density of the affected species root levels in the soil, and the amount of time the land was out of the crop. Length of time in the crop does not appear to be an important factor, even one or two years as is the case with nursery crops is sufficient to give a severe replant problem.
As a general rule lighter, sandy soils show greater sensitivity towards SARD than heavier soils.
Pot tests using apple seedlings to compare their growth in fumigated and un-fumigated soil have been developed to test for SARD in soil from areas to be replanted. These tests are not generally available in New Zealand. The validity of the test and its interpretation is also open to question because recent data on the affect of fumigation on replant tree performance from Hort+Research shows that even though there was little affect on tree vigor the trees growing in the fumigated treatments showed significant increase fruit numbers and particularly fruit size.
This increased yield, and particularly higher value fruit from the larger fruit size will easily recover the cost of fumigation so it is our opinion that where ever there is likely to be a SARD problem pre-plant soil fumigation is desirable.
The steps required for successful orchard re-development are:
- Remove the old planting as soon as possible.
- Root rake to remove as much of the old roots as possible.
- Subsoil to break up any machinery pans, which may be present.
- Attend to any drainage deficiencies, which may be present.
- Check subsoil fertility and pH levels - if nutrients or liming required plough these in.
- Cultivate and level - if the replacement orchard is not being planted immediately sow down pasture or a suitable cover crop. Certain break crops are believed to reduce SARD levels.
- If the orchard is to be replanted, immediately begin preparation for re-planting by:
- Marking out rows
- Ridging along the rows if the soil is of heavy texture to improve drainage flow away from the crown of the root system.
- Fumigate along the rows with chloropicrin or formalin for control of SARD. Chloropicrin fumigation needs to be done in the autumn while soil temperatures are 10 to 120C or above. Formalin works best on wet soils under low temperatures.
- Once fumigant has dissipated from the soil begin tree planting.
Note (1) Trees are best planted as early as possible in the winter but can be held for later planting if soil conditions are unsuitable provided their root systems are kept moist. Coolstore in an ethylene free environment if planting is to be delayed until after bud break time.
Application of mono-ammonium phosphate (MAP) at 10 to 15g per tree mixed into the planting soil around the tree will further improve tree growth. To prevent the possibility of root burn and tree death avoid direct contact between MAP and the roots.
Note (2) Fumigation for SARD will not control problems with phytophthora root rots and may even increase susceptibility. So if there is a risk of phytophthora specific precautions need to be carried out against this disease.
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