HortFACT - Superficial Scald in Pipfruit
D.M. Burmeister - HortResearch, Mt Albert Research Centre
What is it ?
Superficial scald is a postharvest disorder of apples and pears which results in browning of the fruit skin. Many New Zealand pipfruit varieties can develop superficial scald in coolstore, but it is most common in the cultivars Granny Smith and Red Delicious.
What does it look like ?
Scald appears as light or dark brown, and in severe cases black, patches on the apples skin. Such patches can be less than 1 cm, or cover virtually the whole apple. The skin can be smooth or wrinkly in appearance.
The scald referred to here is superficial scald - that is the scald which develops during long-term storage on the surface of the skin. Other types of scald or scald-like injury have been called, "sun scald", "storage scald" and "senescent scald". A type of scald has been recorded in Empire apples in the United States as a result of carbon dioxide induced injury.
What conditions are needed ?
Scald is more likely to develop in early-harvest fruit. In New Zealand varieties, scald development varies between growing districts and years. Cooler growing regions tend to produce fruit with less scald than fruit from warmer districts.
The disorder develops in coolstore and becomes visible when the fruit are removed from refrigerated conditions. Low levels of oxygen (less than 2% oxygen) can suppress the development of scald.
Scald is believed to be caused by low temperature damage early in storage. However, the symptoms do not become visible until after long-term storage (4 months or more).
A natural chemical within the fruit, alpha farnesene, has been linked to scald development. The oxidation of alpha farnesene is thought to be related to scald injury however, the exact cause of superficial scald remains obscure. Surface coatings on the fruit (eg. waxing) and some packaging lead to the retention of alpha farnesene which is thought to cause higher levels of scald.
Where is it found ?
Scald is found in pipfruit around the world. Some varieties are more susceptible to this disorder than others.
Why is it a problem ?
Superficial scald does not show up until the fruit has been in coolstore for several months. Once the fruit is removed from store, the scald gets progressively worse resulting in an unsightly apple. Although scald is a cosmetic disorder and does not affect the taste or internal quality of the fruit, severe cases will mean that the fruit is unsuitable for fresh fruit use. However, fruit with scald are still suitable for processing.
What are the current controls ?
In the past, some varieties of apples were individually wrapped in oiled paper to prevent scald development.
Today, postharvest drenching with the antioxidant DPA (diphenylamine) is commonly used to control scald. However, the future use of DPA is uncertain due to increasing unacceptability of the postharvest use of such chemicals. Research is being conducted on alternative methods of control, including controlled atmosphere, prestorage heat, ethanol vapours and calcium treatments.
What is being done ?
HortResearch chemists are attempting to determine the chemical origins of superficial scald as the basis for developing alternative control strategies. They have synthesised farnesyl hydroperoxide which is a precursor of alpha farnesene. The next step is to determine whether synthetically produced farnesyl hydroperoxide can induce scald in apples.
HortResearch scientists have also trialed warming treatments for scald control. Warming the fruit at 20ºC before, or for a period during, coolstorage appears to slow down scald development.
Atmosphere modification in the coolstore is another potential method of scald control which has proven successful in overseas trials.
Alternative natural chemical treatments for the prevention of superficial scald are being developed and tested in commercial storage environments. These treatments include dips or coatings made from natural products which inhibit scald formation.
Most treatments that are used to control scald become less effective if not applied soon after harvest.
See also:
Science Publication - Non-chemical Treatments for the Prevention of Superficial Scald in Apples
Written and submitted by Helen Percy for HortNET
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