The impact of last summer’s heatwaves has increased awareness of the need to adopt on-farm practices that can protect fruit from sunburn.
There are several climate ameliorating techniques with various modes of action available to Australian apple growers. Some are already in widespread use while others need further evaluation before uptake by industry.
In this series featuring ‘sunburn’, we consider those adaptation options that can be deployed to reduce sunburn in existing commercial production areas of South-Eastern Australia planted with current apple varieties.
Back to the beginning
This month we go back to the beginning to consider orchard design and tree structure.
Fruit exposed to direct solar radiation are more prone to sunburn (Figures 1 and 2).
Location and design of orchards, tree architecture and associated canopy management all have important influences on the potential to create protective shade that can keep fruit temperatures below critical levels that induce sunburn.
Aspect
Tree rows are best orientated to avoid exposing fruit to extensive periods of direct solar radiation.
New plantings should use a layout that maximises shade protection from the tree’s foliage at times when afternoon temperatures are highest.
During heatwaves, it is often observed that plants experience most sun damage on their north-west sides because direct rays of the sun reach fruit from this direction during the hottest time of the day.
Sunburn on apple fruit can be reduced significantly if more leafy growth is induced by pruning on the north to north-west side of trees to provide more shade protection.
In South-Eastern Australia, recent thinking suggests that ideal row orientation in hedgerows is likely to be north-west to south-east so that a larger proportion of the canopy and fruit is exposed in the morning and shaded in the afternoon—although this needs to be validated by modelling and field research.
Furthermore, trees trained on a cantilever (at 30 degrees from the vertical, slanting towards the west) in north-south rows may be effective at protecting fruit from sun exposure in the afternoon although this also needs to be tested.
It is common for damage to be high at the ends of rows and along boundary rows where trees are more exposed to direct solar radiation. Shelter belts can provide afternoon shading for outside rows; however, shelter belts will reduce wind that is beneficial in reducing fruit surface temperature.
Air circulation
Adequate air circulation in the orchard is important for expelling hot air surrounding the fruit.
On windless days, less heat is lost from fruit and rates of sunburn typically increase.
Air movement down rows of trees should be encouraged. Row directions located against the prevailing wind direction interfere with air circulation, resulting in higher fruit temperatures.
Another advantage of north-west to south-east row direction in the Goulburn Valley is that this is the prevailing wind direction during extreme heat events.
Shading with foliage
During recent Victorian heatwaves, fruit outside the protection of leaf canopies were often badly sunburned.
The shift to high tree density apple orchards in South-Eastern Australia over the past 25 years usually sees orchards pruned and trained in two-dimensions creating planar hedgerows. In combination with low vigour rootstocks producing smaller trees with reduced canopies, these new training systems leave fruit more exposed to solar radiation and susceptible to sunburn.
Improvement of light penetration to increase blush in red varieties is a key objective of current tree architecture but orchardists worldwide are experiencing increased negative impacts from rising temperatures when trees manipulated to increase light penetration overly expose fruit to the sun (Willaume et al. 2004).
Canopy leaves
Appropriately positioned canopy leaves can be as effective as over–tree shade-cloth in reducing heat build-up in apple fruit (Andrews & Johnson 1996).
Fruit exposed to the sun can be more than 10C hotter than shaded fruit (Thorpe 1974; Ferguson et al. 1998).
Nitrogen fertilisers
Summer applications of nitrogen fertilisers can encourage the growth of larger and denser leaves that do a better job of shading fruit.
In Western Australian trials, Granny Smith apples remained greener and were less prone to sunburn following high nitrogen applications that increased vegetative growth. However, increasing nitrogen availability to trees can have negative impacts on the development of red pigments (anthocyanins) in red varieties.
Summer pruning
Summer pruning needs to be adjusted to local conditions and coordinated with other cultural practices to avoid excessive sunburn.
Theoretically, summer pruning and leaf thinning should be performed on cool overcast days when the forecast is for cloudy weather in following days to allow time for antioxidants and beneficial heat-shock proteins to accumulate prior to exposure to full sun (Brown 2009).
Heat-shock proteins
Heat-shock proteins play an important role in protecting cellular biochemical processes during periods of stress, although the timing and duration of exposure required to encourage their production is not fully understood.
It is believed that slow, progressive exposure of apples to solar radiation may allow their build-up.
Future orchard management strategies might make better use of this knowledge and help prepare fruit for extreme sun exposure but for now, a switch from summer pruning to autumn pruning should help reduce the incidence of sunburn.
Sudden exposure
Branches that shift suddenly under increasing fruit loads can expose previously shaded apples to the sun and these are at greater risk of sun damage (Figure 2).
If late thinning is undertaken, growers should avoid removing the uppermost fruit from a cluster, as sunburn is more likely to develop on the remaining, newly sun-exposed apples, especially if hot days follow soon after thinning.
It is safer to remove the lower fruit in a cluster and leave the fruit which has already adjusted to the sun (Bergh et al. 1980).
Future canopy management
In an increasingly hot future, canopies may have to be better managed to maximise shading of fruit in the afternoon.
Row orientation, trellis and training system, pruning and even the choice of rootstock should all be considered as management options to reduce sun damage.
NEXT ISSUE: evaporative cooling, shade netting, sunscreens/sun-blockers/anti-sunburn products
For Literature cited and images, see Tree Fruit Dec 2014
