Even though the harvest period is the busiest time of the year, there are a number of critical jobs that need to be done now to set the trees up for next season.
Critical post harvest crop husbandry
Relative to a few years ago, we have lifted orchard yield and packout performance almost beyond what we thought could be achieved several decades ago. The trees are now working really hard as we push yield and quality towards the upper limit so any impediment to root health or nutrition readily shows up.
In the last decade from 2007 to 2016, average gross production per hectare has gone from 50.4 tonnes to 59.6 tonnes, an increase of 18%. There are now many blocks across a number of varieties close to or exceeding 100 tonnes per hectare so we are starting to put real pressure on the tree’s ability to support the crop. To sustain and further improve orchard performance, all limiting factors need to be identified and rectified as far as possible.
A healthy root system is essential for top tree performance and I am sure many of the tree nutrient deficiencies and fruitset problems we are seeing are due to a poor root environment. Healthy roots need access to adequate soil moisture as well as good aeration to keep them alive and healthy.
The best way of achieving these two opposing objectives is to make sure that the trees develop of deep root system with a lot of soil volume to explore. This means having good drainage. In the ideal world, winter water tables need to be at least 60 to 70cm below the surface, preferably over a metre. Most orchards are on soils that have already been drained. The questions we need to ask then are:
(a) Are the drains deep enough as sufficiently close to provide
effective drainage for a permanent tree crop?
(b) Do they still work?
Over time drains deteriorate. Plugging with shelterbelt roots is a common cause of blocked drains. Most of the shelterbelt species we use have roots which are attracted to drains, particularly Casuarina, pines, willows and poplars. Where drain gradients vary, silting up can be a problem. Accidents to drains also happen, for instance, it is not unknown for posts to be driven through drains blocking them.
Immediately post harvest is a good time to check the condition of the drains, carry out maintenance on them and put in additional drains where required.
The post harvest root flush period is an ideal time to address nutrient problems. We are seeing increasing levels of nitrogen deficiency emerging in high performing blocks. This is not surprising because in order to obtain excellent fruit colour it is necessary to run tissue nitrogen levels down into the deficiency range as harvest approaches.
Around harvest, leaf nitrogen levels should be in the range of 1.8 to 2.2%, the leaves a pale green rather than deep emerald green and fruit background colour also a pale green to lemon colour, with brilliant red over colour. These low nitrogen levels, necessary for good fruit colour, are inadequate to ensure good fruitset next season and drive sufficient spring growth flush to support next year’s crop. Generally speaking, the nitrogen requirement for fruitset and spring growth flush comes from stored nitrogen reserves in the tree, not from spring fertiliser application. Trees short on nitrogen tend to give brilliant autumn leaf colour and earlier leaf drop than those with adequate or surplus nitrogen levels. Magnesium is the other essential nutrient that appears to be linked to poor root health. Magnesium deficiency symptoms are also very common on trees that are over cropped. Ripening seeds have high magnesium requirement so trees with excessive crop have problems with magnesium. Excessive cropping also adversely affects the autumn root growth flush due to the crop soaking up most of the available photosynthate supply and leaving little surplus available to drive root growth. Young, actively growing roots are necessary for good nutrient uptake from the soil as many cations, particularly calcium, are only taken up through actively growing root tips.
Many of our orchard soils need regular application of magnesium fertiliser sometime over the autumn and early winter period to maintain a satisfactory magnesium supply. Established fruit trees usually have mycorrhizal relationships so are very good at sourcing soil phosphorus therefore, trees are relatively unresponsive to fertiliser phosphorus except when newly planted.
Potassium is a major nutrient with high demand for fruit sizing and colour development. This demand comes on once the crop begins to size in December. Leaf analysis gives a very accurate indication of the tree’s potassium status. Potassium levels will also move inversely proportional to crop load. The heavier the crop, the lower leaf potassium levels will be.
Many New Zealand soils have huge reserves of potassium so fertiliser requirement depends on soil type rather than cropping level. Because of the adverse effect potassium has on calcium and also magnesium uptake, it is important not to over fertilise with potassium.
As crop increases, nutrient demand lifts in proportion to yield but tree’s ability to source it may decline.
*Adapted from data published in “Fruit mineral removal rates from New Zealand apple (Malus domestica) orchards in the Nelson Region” NZ Journal of Crop and Horticultural Science, Vol 34 pp 27-32, JW Palmer, G Dryden.
Most trace elements are relatively immobile in the leaf and there is little movement of them back into the tree as leaf fall approaches. Boron is the exception. Boron is very mobile and readily transported back into the tree so post harvest is a very safe time to apply foliar baron sprays.
The other trace elements are best left to spring/early summer or in the case of zinc, perhaps a fully dormant spray of zinc sulphate.
Increasing crop loads are putting tree support structures under severe pressure so repairs and maintenance of support structures should take high priority once the crop is harvested. Every year we see trellis failure and this year had trellis failures too, largely due to the very strong winds we experienced during January. We were very fortunate the February and early March rains fell under relatively calm conditions, otherwise there could have been a lot more rows down.
Trellis failure is not only confined to New Zealand orchards. A recent Good Fruit Grower reports they have done some survey work on trellis failure to identify the factors involved in trellis failures. They have identified two types of trellis failure:
1 Soil failure in which the structure holds together but the soil is too weak to hold the posts up. I have not seen much of this problem here, apart from end strainer posts lifting out of the ground. We also see some leaning posts which is an indication of soil failure.
2 Material failure in which the structure itself fails. This
is where most of our trellis problems come from.
For weak soils, eg sands, they recommend around 30% of the trellis height above ground be driven into the ground. In the case of stronger soils only about 20% of the trellis height be driven into the soil.
There are numerous examples of material failure. The most common one is posts snapping off. Broken wired and staples pulling out also contribute to failure. For us planting trees off line is often the beginning of trellis failure because of the extra crop weight the structure has to support. Most of our support post failure has occurred where quarter round posts have been used. The knot area is their weak point and this is where they break. Sometimes quarter and half round posts may split longitudinally. Full round posts are much stronger and less likely to break under load. Post diameter determines strength, eg a 125mm diameter round post is 50% stronger than one only 100mm diameter.
Spacing between posts is also important. Studies in Washington into trellis failure has found that 60% of failures
had post spacing greater than 12m. In weak soils, posting wider than 6m apart markedly increased failure risk.
There are lots of trellised intensive orchards out there where the trees have developed an ominous lean indicating that trellis failure maybe imminent. Allowing tree height to extend too far beyond the height of the trellis puts huge stress on the structure and with the weight of the crop we are setting in the upper tree, it does not take much wind with rain to bring the whole row crashing down. You only have two options in this situation, either bring tree height down to the level that the trellis can support or beef up the trellis structure to enable it to support the additional trees top weight.
Where posts are too short and too far apart, driving longer intermediate posts between existing posts is a good starting point. Extending trellis height to support a greater proportion of the unsupported tree above trellis height can be considered provided the existing posts are well enough anchored to take additional loading.
Overseas in windy climates or where tall support trellis is required without the expense of going to extra long driven posts, it is common practise to brace posts across the block with a cable running along the top of the trellis posts, secured to a deadman either side of the block. Perhaps this is a technique we should investigate for high value orchard plantings in danger of row collapse.
Fix up loose or broken wires and make sure wires are securely attached to trees and support posts.
Immediately post harvest is the best time to prune summerfruit because disease risk, particularly silver leaf, is lowest at this time of the season. Where apple trees have excess vigour or shading branches, pruning those out now will help devigorate the tree because this will reduce the amount of photosynthate being translocated back into the tree to support future root and shoot growth. Taking out shading branches now will also improve bud strength on
the remaining fruiting sites.
Where trees have become too high and are putting trellis structure at risk, they can be topped immediately after harvest. This will improve light penetration into the lower canopy and improve bud strength there. Topping trees at this time of the season may reduce the growth response compared to dormant topping. However, it will not be as effective for reducing the vigour response as topping trees in late spring immediately after fruitset when the
crop load is there to take the vigour out of the tree.
As we have had a couple of prolonged wet periods this
autumn, there will now be high risk canker infection so good fungicidal wound protectants need to be applied to pruning cuts. Obvious canker infection needs to be pruned out before any general pruning commences.
POST HARVEST FUNGICIDES
As leaf fall approaches, good fungicide cover is necessary to minimise canker infection. This is an insidious disease which can be very expensive to control if it gets away. It can be kept at bay with effective post harvest fungicide programme.
Harvesting in periods of wet weather can lead to significant fruiting spur canker infection so there are benefits in applying an effective protectant fungicide immediately after harvest.
From left: Fig 1. These trees died when the drainage system became overloaded and water backed up into the root zone. Fig 2. Low spring nitrogen levels often lead to poor fruitset.
From top: Fig 3. Structure failure is an expensive problem. This row fell over because its quarter round interval support posts snapped at the knot. Fig 4. This is the same block as in Fig 3 two years later. The support structure has been strengthened by driving full round posts between every second existing quarter round post and tree height was dropped to within about 30 to 50cm of post height when the tops began to break out in mid growing season. Note the original post in the foreground and new round post about 3 trees down the row from it.
From left: Fig 5. The trees in the foreground are weaker growing so do not need extra upper tree support. The tall ones further down the row are too high for the structure and have had to be propped up to prevent them bringing the row over. Fig 6. These Cripps Pink trees are too tall for the structure. Fortunately, this variety has strong wood so holds itself up better than may other varieties. Even so, a higher trellis is required or the trees should be topped to stiffen their leaders so they can continue to hold themselves up.
Fig 7. This young Jazz™ planting in France shows the trellis post secured to a concrete pad, then held in position with a cable running across the top of the posts.
Fig 8. This trellis supports both trees and hail net. Every second post is cross tied by a cable.