As the temperatures in the orchard start to rise, AgFirst’s John Wilton looks at how to minimise sunburn, avoid nutrition-related problems and improve colour towards lifting quality and packouts at harvest.
By now, the first pass of hand thinning should have been completed. In order to produce high quality fruit, it is necessary to groom the crop by adjusting crop loads to suit growing conditions and culling out any obviously low-quality fruit. Carefully check crop loads to identify areas where there may be still too much fruit. Pay particular attention to lower vigour parts of the orchard. These areas tend to over set and it is difficult to get the hand thinners to take enough fruit off in the first hand-thinning pass.
Check fruit counts and monitor fruit growth. Around about the beginning of December, it is possible to forecast likely harvest fruit size and certainly identify fruit which will fail to reach minimum marketable size. At this time, it is possible to see the fruit size range within the crop so excess crop load can be thinned off by size and position in addition to taking out blemished fruit.
Approximately 90 days after full bloom (AFB) fruit on trees under stress or over cropped will begin to show a drop off in their rate of sizing. Regular fruit size measurements around this time will quickly detect areas in the orchard where fruit sizing has begun to stall. If detected early and these areas re-thinned by fruit size, it is possible to restore fruit growth to the remaining fruit.
Where irrigation water supply has become limiting, dropping crop loads down by taking off small fruit, as well as exposed fruit on sites with little leaf cover that is likely to sunburn, will enable the remaining crop to reach marketable size and quality.
In warm regions with daily ambient temperatures exceeding 28 to 30°C, there is sunburn risk to fruit. As fruit size increases, sunburn risk rises. At about 45mm fruit diameter, the fruit becomes vulnerable to injury. The level of sunburn injury is determined by fruit surface temperatures (FST). Mild sunburn injury referred to as skin browning usually occurs when FST is in the range of 45 to 49°C, with more severe necrosis injury occurring at FST above this level. FST on calm sunny days is usually in the range of 14 to 17°C above ambient temperatures. A slight breeze will reduce FST by about 3°C. This probably explains why there is often more severe sunburn injury in lower tree fruit than in the more exposed upper tree fruit.
In addition to these two levels of sunburn injury, there is a third form which can occur at lower temperatures called photo oxidative bleaching which occurs when there has been a sudden change in light exposure. This is the typical sunburn you see from late thinning or selectively picking fruit out of bunches during harvest. Sunburn bleaching will also occur as a result of excessive summer pruning or branch movement as crop weight comes onto branches. Many Australian orchards are under netting. There are usually lower levels of sunburn in netted orchards due to the effect of the netting reducing light intensity and therefore lowering FST.
Studies into sunburn incidence show that it is the mid-afternoon sun that causes most injury. Shade or protection from mid-afternoon sun will minimise sunburn injury. There has been a lot of research effort into trying to develop protective sprays for sunburn. As far as I can ascertain, products available for sunburn protection have only been partially successful or come with numerous undesirable side effects such as difficulty in removing their visible residue on harvested fruit. One of their more serious problems lies in the difficulty of actually obtaining satisfactory coverage to the vulnerable fruit surface. It really needs to be sprayed down onto the upward facing fruit, rather than blown upwards from the typical axial fan orchard air blast sprayer. Some form of tower sprayer which sprays down onto the upper tree should give better results.
Mild sunburn browning injury will often mask out as fruit colour intensifies towards harvest, enabling the fruit to be marketed because the sunburn is no longer visible. While such fruit may now appear unaffected, its internal quality has been changed and the fruit is less suited to long term storage. The main internal changes include more starch degradation, increased flesh firmness, higher sugar levels but depressed acidity and lower flesh moisture. Affected fruit therefore, has poorer flavour and drier fruit texture.
Factors associated with increased sunburn injury
Sunburn is made worse by:
- Over cropping – leads to higher fruit to leaf ratios and lower levels of annual extension growth necessary to give adequate leaf cover to protect the fruit.
- Water stress – has a major influence on sunburn incidence, particularly for varieties ripening in the first half of the harvest season when temperatures are high. Mulching has been shown to reduce sunburn by improving soil moisture availability and lowering soil temperatures.
- Nitrogen deficiency – results in weak growth and poor-quality leaf.
- Fruit size – larger fruit have lower surface area to volume so develop higher surface temperatures.
- Late thinning and selective picking of bunchy crops.
Nutrition, pit and blotch management
As fruit sizing kicks in, the developing crop has high requirement faor potassium so where levels are marginal, deficiency symptoms can appear during December. These show up as typical marginal leaf scorch symptoms if leaf potassium levels drop well into the deficiency range at around 0.65 to 0.70 per cent. Levels below 1 per cent can be considered marginal and in the zone where a response to potassium fertiliser could be expected. Where leaf potassium levels are around 1.2 to 1.4 per cent it is unlikely that there would be a positive response to potassium fertilisers. Potassium leaf levels much above this range should ring alarm bells because it could signal an enhanced pit and blotch risk or potassium induced magnesium deficiency. Contrary to popular belief, lifting potassium leaf levels above 1.2 to 1.4 per cent seldom results in fruit colour improvement, unless perhaps nitrogen levels are really high.
Magnesium deficiency is often a problem as harvest approaches for trees carrying excessive crop loads. This is because developing seed is a very strong sink for magnesium.
Pit and blotch is the main nutritional problem over the summer period. For pit prone varieties, in particular Granny Smith, Kanzi™, Jazz™ and Braeburn, robust foliar calcium programs need to be applied over the summer period. Excessive tree vigour and high summer temperatures increase pit and blotch risk. Where trees are out of balance, varieties not normally associated with pit and blotch problems such as Fuji, Gala, Red Delicious and Golden Delicious can also give problems.
Water stress increase pit and blotch risk because during periods of water stress the tree sucks water out of the fruit to maintain leaf health and this also removes calcium because this element travels only in the xylem. Calcium lost in this way is generally not replenished because at this stage in the season, movement from leaf into the fruit is through the phloem as translocated photosynthates. A good foliar calcium spray program is required to maintain adequate fruit calcium status.
Varieties prone to magnesium deficiency, such as heavy crop Fuji, need magnesium foliar sprays too to maintain leaf health which is important at this stage in the season to protect fruit form exposure to the sun.
High nitrogen status towards harvest needs to be avoided because of the adverse effect nitrogen has on fruit colour development. Although adequate nitrogen levels are necessary for canopy development, flowering and fruitset, this nitrogen is sourced from mobilisation of stored reserves accumulated in the previous autumn. It is generally considered that for mature bearing trees, most of their nitrogen requirement should be applied after harvest in the previous season. Some can be applied in the spring but for red and partially red varieties, it is unwise to make nitrogen applications later than about four weeks after full bloom.
Excess nitrogen imbalances within the tree delay colour development leading to over maturity at harvest as well as a number of post-harvest storage disorders. For red and partially red varieties, there needs to be good nitrogen levels around bud break and bloom to drive the initial shoot growth and set the crop.
Then, as the season progresses, nitrogen levels should be run down to leaf levels, around 2.0 per cent at harvest for good early fruit colour development.
Granny Smith is the exception to this rule. It requires higher nitrogen levels to maintain its deep green colour into the harvest period. It is also very sensitive to calcium chloride leaf injury so requires a calcium nitrate based foliar calcium program for pit and blotch control.
Where canopies are dense and suffer from high shoot vigour, it may be necessary to summer prune to improve light levels within the canopy. In hot climates summer pruning needs to be done with care to avoid opening the tree up to too much light penetration which could lead to sunburn problems.
Summer pruning needs to be carefully supervised to make sure it is not excessively severe. The other problem I have often seen with summer pruning is the loss of next year’s crop due to the summer pruners removing too much two-year-old wood.
Summer pruning needs to target only the unwanted current seasons annual shoot growth.
In situations where a lot of summer pruning is necessary, there may be a fundamental pruning, cropping or excessive vigour problem. High levels of summer pruning represent loss of fruiting potential because the photosynthates driving the excessive shoot growth should have been going to the fruit instead.
For red and partially red varieties, reflective mulches are becoming standard practice. Their role is to bounce light falling in the inter row area back up through the canopy. This improves lower tree fruit colour and also strengthens flower bud development for next season.
Good fruit colour depends on fruiting sites receiving a minimum of 20 to 30 per cent of ambient light. Reflective mulch needs to be put down around three to four weeks before harvest.
To minimise damage to the mulch where multi use mulches are used, it is normal to remove it at commencement of harvest. Sometimes it is left down during harvest to improve later pick fruit colour. Where this is intended, the reflective mulch is usually pulled to the side ahead of the pickers, then spread out again once that pick is finished. With careful planning, reflective mulches can be used two or three times through harvest by moving it from early to mid-season then later harvested varieties.
Our data indicates that use of reflective mulches will lift average packouts by 10 to 15 per cent as well as reducing over maturity problems due to picking being delayed while waiting for colour. We have also found that generally higher colour strains have a better response to reflective mulch than poor colour strains. There is only one solution to a block that struggles for colour. Pull it out and replace it with a better colour strain or variety.
Harvesting needs to be well planned ahead of time so that it runs smoothly. Pre-harvest drop can be a significant problem with some varieties, notably Red Delicious and Scifresh.
The growth regulator, ReTain®, has very good stop drop activity, as well as being very useful for harvest management, particularly in early harvest varieties such as the Gala group. It needs to be applied four to five weeks before harvest commences in warmer districts to ensure it goes on before endogenous ethylene production begins. Under good conditions, harvest can be delayed by seven to 10 days if not longer. Sometimes a second application is applied 10 to 14 days after the first one to lengthen the harvest delay period.
Naphthalene acetic acid (NAA) is also very effective for pre-harvest drop control but will tend to advance rather than delay harvest, particularly when applied at rates of 10ppm or higher. Research in the USA has shown that multiple low rate NAA sprays at 5ppm give more effective stop drop effect and will have little, if any effect on fruit maturity.
If wooden, bins need to be tight with no nails or splinters present to damage fruit. Tracks should be smooth to minimise bumping the fruit about in the bin. Forklift drivers must be well trained to handle bins gently and trucks need good suspension to give the fruit a smooth ride to cool store or packhouse. Every time an apple moves, there is potential for bruising or stem punctures.
About the Author: John Wilton, Horticultural Consultant, AgFirst, New Zealand | t: +64 6872 7080