The theme the Future Orchards® team will address over the next 12 months is ‘quality’. Quality fruit production requires a long-term plan; it doesn’t just happen at harvest and post-harvest. As this article will be read in the later part of the 2018 harvest, the time is right to start the quality discussion during the post-harvest period.
Setting up the orchard for the 2019 crop starts now. The post-harvest period is a key time for nutrient uptake. Nutrients stored in the post-harvest period by the tree power next season’s initial growth flush, which provides the leaf area to drive next year’s fruit set and fruit size.
Nitrogen is the most important nutrient and the one required in the largest amount. Balancing nitrogen levels in pome fruit is tricky. Too much and there is a fruit colour problem. Too little and there can be fruit set problems, insufficient growth, small leaves and small fruit.
In the ideal world, nitrogen levels around harvest need to be at the lower end of the optimum range or even slightly deficient – say, leaf nitrogen levels in the 1.8–2.0 per cent range for colour-sensitive varieties. This is about 2.2–2.4 per cent in the January/February leaf test. Levels vary among varieties. Fuji, for instance, always shows higher leaf nitrogen levels than other varieties and will show poor fruit set at a higher leaf nitrogen level than other varieties. For good fruit set in most varieties, we need spring leaf nitrogen levels in the 2.7–3.0 per cent range from a sample taken about four weeks after full bloom.
A couple of post-harvest foliar nitrogen sprays, such as urea at 2–3 per cent concentration, applied approximately between two and three weeks apart is a very efficient way to ensure there are good, readily available spring nitrogen levels without having excessive nitrogen, which can happen when large amounts of nitrogen fertiliser are applied to the soil. Where soil nitrogen levels are low, it will be necessary to apply some post-harvest fertiliser nitrogen to the soil as well.
Potassium is the element that is required in the second-highest quantities by the crop. The amount required depends on cropping level and soil type. We need to be careful in making sure there is a good balance between potassium and the other cations (particularly calcium and magnesium). Too much and the uptake of these nutrients is impaired, leading to problems with calcium disorders such as bitter pit, or magnesium deficiency.
Unlike with nitrogen, soil tests generally give an accurate picture of the potassium status and leaf analysis is very accurate for potassium too. In the developing crop, high demand for potassium occurs around the middle of the growing season as fruit demand escalates. We often see potassium-deficiency symptoms expressed between early and mid-December. Peak demand for this nutrient occurs much later in the growing season than for nitrogen. For this reason, we put less emphasis on post-harvest applications of potassium fertilisers.
In regard to magnesium, I have not come across much evidence to support post-harvest application. However, magnesium is very mobile in the tree so it is probable that post-harvest foliar magnesium sprays move readily out of the leaves into the buds and wood before leaf fall. Magnesium demand is high towards harvest when the seeds are maturing
Calcium is a critical nutrient for minimising storage problems such as bitter pit. Calcium is also the most plentiful nutrient in most soils so soils are generally not short of calcium. The problem is one of root uptake and distribution within the tree. Calcium is taken up only through actively growing root tips so most of the root uptake occurs during tree root growth flushes. There are two main root flushes a year. The larger of these occurs in the post-harvest period and the second one, which is probably more important for supplying fruit calcium, occurs from the late dormant period through until the crop load shuts down root growth in late spring. Calcium uptake is dependent on providing a good soil environment for root growth. Roots do not grow in dry soil and need well-aerated soil as well. In dry climates, it is necessary to apply post-harvest irrigation to ensure root growth. The spring root flush is also dependent on having good soil drainage, particularly on heavier soils in locations which have high winter rainfall.
Cation balance within the soil is important for maximising calcium uptake. Calcium base saturation needs to be 70 per cent or higher, magnesium 10–15 per cent and potassium in the 3–4 per cent range. To lift calcium base saturation levels, soils with calcium base saturations of less than 65–70 per cent need either liming where pH is low or gypsum where pH is high and excess levels of sodium are present.
Boron is mobile and readily translocated into buds as leaf fall approaches so can be applied as a post-harvest foliar spray. The other trace elements are less mobile and generally do not move out of the leaves into buds as leaf fall approaches so are better applied during the growing season.
Zinc is an exception here. As it can be phytotoxic in the growing season, if a zinc deficiency exists, zinc is best applied as a dormant spray well before spring bud break. Zinc uptake occurs through bark and dormant buds. Coverage of buds is better after leaf fall than during the post-harvest period so it is best applied as a dormant spray.
In regard to orchard nutrition, we need to recognise that with intensive plantings and the higher yields now being achieved, optimising orchard fertiliser programs to meet higher demands in the future will become more and more critical.
In many intensive orchards with precise targeted irrigation systems, tree roots get to explore only the limited wetted soil area rather than the whole soil that would be available to roots in less intensive systems. Fertigation may be necessary to address this situation.
Pest and disease management
Experience has shown that the benefits of applying pesticides for controlling insect pests such as mites and woolly apple aphid in the post-harvest period are doubtful. Sprays for scale insects and mealy bugs are best left until the late dormant period when it is easier to obtain good spray coverage and these pests are hatching, which makes their control much more effective.
Phosphorous acid sprays for protection against phytophthora root diseases need to be applied during the post-harvest period. Where apple scab (Venturia inaequalis; also often referred to as black spot) has become established over the growing season, leaf fall urea sprays at five per cent (100kg per 2,000L tank) should be used to hasten leaf decay and minimise the carryover of inoculum into spring to infect the following year’s crop. These sprays also work quite well when applied directly to fallen leaves once leaf fall is complete.
The post-harvest period is a good time to apply herbicides for control of persistent weeds. Translocated herbicides such as amitrole are much more effective when applied in the autumn, when many difficult weeds are beginning to die down for their dormant period, because as the sap flow withdraws from the foliage into stems and roots it takes the herbicide along with it.
In difficult soils which tend to form hard pans that impede root growth and water movement away from the upper soil layers, autumn is the ideal time to run a pan buster through the orchard. For best results, the soil needs to be dry so it will shatter when the pan buster is pulled through it. Incidentally, for pan busting to be an effective drainage tool, there needs to be free-draining material below the hard pan so that any perched water can get away easily.
Where there are drainage problems these are best addressed over the post-harvest period when the soil is in good condition to work with. Fruit trees need good drainage and will perform better when the water table is 60 or more centimetres deep and does not rise much above this level for any length of time.
In heavier clay soils, tile drains running across the rows at 1m or more depth and in the middle of the rows back-filled with a free-draining material such as gravel to allow surface water to easily enter the drains can be used. Pulling mole drains along the middle of the area between the rows in the late spring as the soil is drying out will make for a very cost-effective drainage system. With time, tile drains tend to become blocked with soil and roots so regular cleaning in autumn will ensure they work effectively.
Pruning and training
While it is difficult to do detailed pruning when leaves are present, the post-harvest period is a good time to take out any heavily shading vigorous branches. Where tree vigour is excessive, removing these branches early in the post-harvest period reduces vigour because it reduces the supply of translocated photosynthates into the rest of the tree, particularly the roots. Secondly, it increases light penetration into the remainder of the tree. This will improve bud development in the rest of the tree, leading to better fruit the following year.
In non-bearing trees and younger trees where cropping has failed to bring the branches down, the period between harvest and early leaf fall is a good time to do branch training. As a rule, I am not keen on doing a lot of branch training because for most varieties selecting good branches and pruning out any steep branches is an effective low-cost alternative to tying down or propping out steeper branches and a smarter way to manage tree structure.
There are some varieties, notably Cripps Pink (marketed as Pink Lady®) and Scifresh (marketed as Jazz™), which tend to produce rather stiff, upright branches that need to be trained before cropping begins. While leaves are still present and with diameters of less than about 2cm, branches are flexible enough to be trained. Branch thickening happens quite rapidly at this time of the year so branches will set in their new position within two or three weeks. Use biodegradable rather than polypropylene twine to ensure you do not have the problem of disposing of the latter when it has done its job.
Be careful not to pull the branch off the leader at its union. The trick is to wind the branch sideways first, then pull it down.
About the author
John Wilton, Horticultural Consultant, AgFirst, New Zealand, +64 6872 7080
This article has been produced as part of the Future Orchards® program. Future Orchards is a strategic levy investment under the Hort Innovation Apple and Pear Fund. It is funded by Hort Innovation using the apple and pear levy and funds from the Australian Government and is delivered by APAL and AgFirst.