By Lexie McClymont, Ian Goodwin, Rebecca Darbyshire and Susanna Turpin
As we head into summer, the pear team from the Horticulture Centre of Excellence share their latest research on sun and temperature damage in red-blushed pears.
Pear fruit are subject to sun damage from high fruit surface temperature. The potential losses of new fresh market red-blushed varieties could be high without better knowledge of their susceptibility to sun damage.
Many studies have been undertaken in apples to determine fruit surface temperature thresholds for different types of sun damage and to develop relationships between weather conditions and fruit surface temperature. Similar information for pears will be useful for the design and management of orchards where growers are shifting from traditional growing practices (free standing vase-shaped trees planted at low density) to modern high density plantings using dwarfing rootstocks and training systems that create ‘fruiting walls’ with greater fruit exposure to solar radiation.
Our research focused on ANP-0131 pears that were bred as part of the Australian National Pear Breeding Program and will be marketed under the name Deliza®. We monitored the fruit surface temperature of ANP-0131 pears on BP1 rootstock continuously from late-December until harvest during the 2014-15 season at the Department of Economic Development, Jobs, Transport and Resources (DEDJTR) research farm, Tatura, Victoria.
Fine-wire thermocouples were installed just under the skin of 54 fruit from four trees to measure fruit surface temperature every minute. The trees, planted in winter 2009, were 0.75 metres apart in north-south oriented rows spaced at 4.0 metres and trained on Open Tatura trellis. Two of the trees were grown on the eastern arm of the trellis and two were grown on the western arm.
At harvest, all fruit on the four trees were assessed for level of sunburn damage on the following scale:
0 = no sunburn
1 = bleaching of colour or very slight sunburn browning
2 = slight browning
3 = moderate browning
4 = severe browning
5 = slight sunburn necrosis
6 = moderate sunburn necrosis
7 = severe sunburn necrosis
Overnight and during the early morning, fruit surface temperatures were similar to air temperature. As fruit became sun-exposed, fruit surface temperature rose above air temperature. The maximum fruit surface temperature recorded was 50.8°C on 2 January 2015 at an air temperature of 38.3°C.
Although fruit surface temperature correlates strongly with air temperature, it is the energy provided by direct solar radiation (i.e. sunshine) that predominantly drives fruit surface temperature during daylight hours. For example, air temperatures were similar on 2 and 3 January 2015; however, cloud cover lowered direct solar radiation in the afternoon on 3 January resulting in lower fruit surface temperature.
Other weather conditions, such as air temperature, wind and relative humidity, mediate fruit surface temperature by influencing the transfer of energy from the fruit to the atmosphere.
Studies published in a 2003 Acta Horticulturae article titled ‘Environmental stresses that cause sunburn of apple’ conducted in Washington State, United States of America, showed that apples are most at risk of sun damage in the afternoon, between 14:30 and 16:45. In our experiment, fruit grown on the western arm of the trellis reached maximum fruit surface temperature in the afternoon, most commonly between 17:00 and 18:00 daylight saving time.
However, maximum daily fruit surface temperatures were often reached earlier in the day for fruit on trees grown on the eastern arms of the trellis. For instance, fruit surface temperatures up to 44°C were recorded between 09:00 and 10:00 for fruit grown on the eastern side of the trellis, and one such fruit recorded a fruit surface temperature of 49°C at 11:00 in late January.
Overall, fruit grown on the western arm of the trellis were the hottest, but fruit grown on the eastern arm were still susceptible to sun damage.
|Sunburn browning (%)||Sunburn necrosis (%)||Average sunburn score|
|Table 1. Percentage of fruit with sunburn browning and sunburn necrosis, and average of sunburn assessment scores (0-7).|
This study was supported by funding from the Victorian Department of Economic Development, Jobs, Transport and Resources, and the Australian Government Department of Agriculture.
About the authors
Lexie McClymont, Ian Goodwin and Susanna Turpin all work with DEDJTR at the Horticulture Centre of Excellence and Rebecca Darbyshire is from the University of Melbourne. Contact Lexie for more information on Lexie.McClymont@ecodev.vic.gov.au or 03 5833 5260.