South-eastern Australia has experienced its hottest weather on record in the past five years. Heatwaves and days with a maximum over 38C often caused significant damage to pome fruit crops and demonstrated that the industry is vulnerable to high temperature during fruit development.
Risk of future losses is expected to increase in south-eastern Australia because the region's climate is projected to become increasingly hot with an associated increase in the number of damaging heatwave events.
Intervention to reduce crop loss seems to be essential for growers to remain economically viable and to compete in a global market with highest quality fruit.
Sunburn damage
Sunburn damage (Figures 1a,b,c) is more related to the maximum air temperature experienced during a production season than to average annual temperatures.
In the Goulburn Valley, daily maximum temperatures have increased by 0.02C/year since 1965, whereas the annual maximum has increased at nearly three times this rate (Thomson et al. 2014).
Sunburn can occur at any stage of a fruit's development but conditions are most suitable for sunburn in the three months prior to harvest.
Visible light, ultraviolet-B light, infra-red radiation, extreme heat, low relative humidity and low wind velocity can all interact to influence risk of sunburn.
However, exposure to direct beam solar radiation is the primary factor governing sunburn risk because it directly increases fruit temperature.
Normally fruit surfaces exposed to direct sunlight are at least 11C higher than surrounding air temperatures but can be up to 18C higher during hot weather with no wind (Schrader et al. 2003).
Wind reduces the differential between air and fruit surface temperatures by transferring heat to the air. Reduction in wind during heatwaves will increase the risk of fruit damage.
Three types of sunburn
Three types of sunburn have been identified in apples (Schrader et al. 2011).
'Sunburn necrosis' results from high fruit surface temperature alone and appears as a necrotic spot on the sun-exposed part of fruit on the tree.
Sunburn necrosis is associated with thermal death of epidermal and sub-epidermal cells when the fruit surface peel reaches 52C ± 1C.
A second type, 'sunburn browning', is caused by joint exposure to high fruit surface temperatures (46C to 49C, depending on cultivar) and ultraviolet-B but does not result in cell death.
Necrosis and browning are the two main sunburn types.
The third type, 'photo-oxidative sunburn', requires visible light and affects shaded apples that are suddenly exposed to solar radiation.
Initially affected areas of the skin appear bleached and later become necrotic.
Photo-oxidative sunburn can occur without ultraviolet-B radiation and at much lower fruit surface temperatures than sunburn necrosis and browning—typically at fruit surface temperatures below 31C (Felicetti & Schrader 2008).
Nonetheless, while experimentation may separate the various contributions of fruit surface temperature and radiation wavelengths, fruit in an orchard can be naturally and simultaneously exposed to all the environmental factors that potentially initiate sunburn. There is likely to be some combination of injury symptoms in field situations.
More browning & necrosis expected
Projected increases in temperature will likely increase incidence of temperature dependent types of sunburn (i.e. browning and necrosis), but most likely will not have significant direct effects on the occurrence of photo-oxidative sunburn (Racsko & Schrader 2012).
Rising atmospheric CO2 concentrations could promote stomatal closure resulting in decreased transpirational heat-loss during heatwaves, increased fruit surface temperature of apple fruitlets and increased risk of temperature dependent sunburn types at early stages of fruit growth (Racsko & Schrader 2012).
Modern orchard practices increase sunburn incidence
Incidence of sunburn on apple fruit has also increased in response to changes in orchard management practices.
In particular, widespread use of dwarfing rootstocks and training systems for modern high-density plantings have created smaller trees with fruit that are more exposed to solar radiation and increased risk of sunburn (Kammereck & Schrader 2000).
Water stress
Water stress has often been associated with increased susceptibility of apples to heat injury but results from a small number of deficit irrigation trials tend to show very little difference in sunburn between low and high volume irrigation regimes.
In an irrigation experiment conducted in a commercial orchard at Shepparton East in the Goulburn Valley, fruit temperatures of Cripps Pink apples (Pink Lady™) were monitored at one minute intervals in single plots irrigated at 162% and 38% of grower practice.
Rows were oriented north–south and trees were trained to Central Leader.
Thermocouples were inserted below the skin on sun-exposed cheeks of six fruit on the east and west side of the canopies.
Air temperature was monitored in an open area beside the orchard block.
There was no difference in mean fruit temperature in the 38% and 162% treatments but diurnal patterns differed between the east and west sides of canopies (Figure 2).
Fruit assessments at harvest indicated no significant difference in sunburn browning incidence between the two treatments with 22% and 19% of fruit affected in the 38% and 162% irrigation treatments, respectively.
However, sunburn necrosis increased from 1% to 4% of fruit affected in the 38% irrigation treatment. The impact of irrigation needs further consideration and research, however, until more information is available it seems prudent to ensure that trees are well-irrigated prior to, and during, heatwaves.
Premium growing conditions heading south
Continued global warming could push the premium growing conditions for SE Australian perennial crops into more southerly latitudes and possibly to higher altitudes.
However, moving production to cooler regions would require substantial investment and drawbacks could include restrictions on available new land, competition from other existing land uses, lack of water and current irrigation infrastructure and the relatively unknown long-term performance of new sites.
Moving location could be necessary if the impacts of climate change and heatwaves exceed growers' adaptive capacity at their current locations.
However, the good news is that several strategies are currently available to growers to help reduce losses due to sunburn.
Some adaptation options have already seen widespread use by the Australian fruit industry and others have been extensively developed in the pome fruit production areas of North America, Europe and South Africa.
NEXT ISSUE: Orchard design, tree training & pruning to reduce sunburn.
see Tree Fruit November 2014
