In the Orchard


There’s been an increase in damage caused by high temperatures at the fruit surface and ultraviolet light.


producing high-quality fruit. Changes are already having impacts that growers can manage to mitigate, including the increasing prevalence of sunburn on apples.


G


Production loss due to sunburn on apples is a case in fact. Production loss due to sunburn (Photo 1 showing sunburned apples in the market) has been particularly high this year. Shown here are fruit with a relatively low level of sunburn but severe enough to reduce the saleability. Photo 2 gives a better comparison between normal colouration and sunburn on Gala apples. Sunburn toughens the skin and reduces eating quality.


When dealing with sunburn on apples it is a case of inches – or perhaps better stated, a matter of degrees.


As a panel of experts indicated to a packed audience at the University of Victoria’s 2016 Ideafest: “… models showing a two-degree temperature rise are probably optimistic”. A climate change of 2oC in the Okanagan would definitely impact this production parameter.


Sunburn is environmental damage on apples that is caused by both high temperatures at the fruit surface and


rowers must consider the impacts of a changing climate in their strategies for


By Bill McPhee Apple sunburn and climate change


light (specifically Ultraviolet-B


radiation). This is a disorder that is ideal to show the direct impact climate change can have on fruit quality. The precise


conditions causing damage were outlined


by Dr. Larry Schrader, a researcher with the Washington State University lab in Wenatchee. The three levels of sunburn illustrated in photo 3, taken from Dr. Schrader’s research, show necrosis on the left, browning in the centre and photo-oxidation on the right.


All levels disfigure the fruit, making it unmarketable.


Necrosis occurs when surface temperature reaches 51oC and is the result of thermal death of the skin cells. Browning is the most prevalent and occurs when fruit surface temperature reaches 45.5oC for the most susceptible varieties, and can appear as various shades of brown from bronzing, as shown in photo 1 above, to dark brown. Both previous types require UV-B radiation while photo-oxidation, which occurs at a lower fruit surface temperature, is generally the first symptom seen in a block and does not require UV-B radiation.


Sunburn Temperatures (Fruit Surface Temperatures)


Symptom Temperature Celsius


Necrosis Bronzing


12 51-53 45.5-48.8


Fahrenheit Time 124-128 114-120


It is important to note that the surface temperature of a fruit exposed to afternoon sun can be much higher than the air temperature and, under extreme conditions, time of exposure necessary for severe damage is short. As average daily temperatures rise, the potential for the surface of the fruit to reach the critical temperatures to cause sunburn becomes more likely. One hypothesis to explain the high level of damage in the Okanagan in 2016 is the fact we had a


Exposure


10 min. 60 min.


Various levels of browning and burning (necrosis) occur as temperatures and time combinations increase.


relatively cool spring followed by sudden hot weather. That


3


condition is often associated with Type 3 sunburn, which is a photo-oxidative process that leaves a bleached-like symptom.


Sunburn can be controlled with sprays that filter UV-B radiation and/or prevent surface temperatures on the skin from reaching the threshold temperature. Growers are advised to watch weather reports and apply preventive sprays prior to heat events.


An increase in the incidence of sunburn is inevitable as average temperatures rise. Sunburn and its direct relation to high air


temperatures is an excellent example of the relationship between disorder and temperature.


Most diseases that occur in the Okanagan will have some correlation to weather pattern shifts caused by climate change. They may be related to moisture changes, temperature changes or combinations of the two. The climate change project, supported by the Investment Agriculture Foundation, gives us an opportunity to examine other major disease problems in relation to climate change and an opportunity to be proactive to shifts in weather patterns rather than reactive.


Prevention is always more effective than crisis management.


British Columbia FRUIT GROWER • Winter 2016-17 1


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