Agriculture Reference
In-Depth Information
water stress reduces fruit size, increases fruit dry matter,
increases the percentage of total soluble solids and leads to
earlier ethylene production. It has also been reported that
the rate that fruit lose water and shrivel in storage is greater
following a wet summer than after a dry one (Sharples
1975). Fruit grown with more water have a more porous
skin and are more prone to shrivel in storage. These
examples show how pre-harvest factors influence growth
and production that in turn set the post-harvest potential
storage life.
The underlying metabolic events that lead to the
production of symptoms in most storage disorders are
not understood. However, applied research has improved
understanding of the nature and underlying biochemical
and physiological mechanisms of some disorders,
enabling successful control strategies. Three common
potentially devastating but manageable physiological
disorders are described here: superficial scald, bitter pit
and flesh browning. Each example illustrates the
extensive and complex pre- and post-harvest interactions
of physiological disorders and also how an understand-
ing of their underlying mechanisms can assist in their
management.
Scald is thought to be induced by the oxidation products
of the naturally occurring fruit volatile, α-farnesene
(Huelin & Coggiola 1970), when its chemical breakdown
products kill the cells in the peel and cause the symptoms
associated with scald. A more general oxidative stress
within the peel has also been proposed to induce scald
development (Whitaker
et al
. 2000).
Control of scald
The simplest solution to the scald problem would be to
only grow apple varieties that do not scald. However, plant
breeding is a long-term solution, and many of the scald
susceptible varieties have desirable commercial attributes
that warrant the development of measures for controlling
scald.
It was first reported in 1919 that wrapping apples in
paper impregnated with mineral oil could control scald,
and that ventilation of the storage room could also reduce
the incidence of scald (Smock 1961). In the mid-1950s, the
chemical antioxidants diphenylamine (DPA) and ethox-
yquin were found to be extremely effective for controlling
scald, and DPA is now routinely used in many countries to
control scald as a post-harvest dip or drench, which should
be applied as soon as possible after harvest.
Delaying treatment by two weeks or more greatly
reduces the effectiveness of DPA. The rates of DPA
application depend on variety, district and the composition
of the storage atmosphere. Therefore, application rates
need to be adjusted according to the variety being treated,
not only to control scald but avoid DPA damage to the skin
(Ingle & D'Souza 1989).
1-Methylcyclopropene (1-MCP) has also been success-
fully used to control scald and is registered in many
countries for this purpose and to maintain fruit quality.
However, as with the DPA treatment, the timing of 1-MCP
treatment as soon as possible after harvest is important
(Watkins & Nock 2005).
There are a growing number of potential other post-harvest
treatments to control scald which are undergoing further
commercial research and development. These include low
oxygen storage, initial low oxygen stress and CA, applica-
tion of vegetable oils, initial ethanol vapour treatment and
pre-storage heat treatment (Ingle & D'Souza 1989).
Superficial scald
Scald is a major physiological disorder that occurs during
cold storage of some important apple varieties, such as
'Red Delicious' and 'Granny Smith'. Scald is character-
ized by brown, irregular patches that appear on the skin
during cold storage (Plate 5.1). The damage is only
confined to the peel and can be removed with shallow
peeling (Plate 5.2). However, this amount of damage
greatly downgrades fruit quality and grower returns.
The incidence of scald is affected by a number of inter-
related varietal, orchard and management factors (Ingle &
D'Souza 1989). Some of these include tree vigor, tree
nutrition, pre-harvest temperatures, sunlight, rainfall,
fruit size, and mineral content. However, the major factors
influencing scald susceptibility are fruit variety (the over-
riding factor in the development of scald), fruit maturity
(immature apples tending to scald more than more mature
fruit) and seasonal conditions (fruit grown in warm, dry
areas are more susceptible to scald than fruit grown in
cool, moist climates). In addition, for some varieties and
regions there exists an inverse relationship between the
number of days below 10°C in the month before harvest
and the incidence of scald. The severity of scald is also
affected by storage conditions such as composition of the
storage atmosphere, storage temperature, ventilation of
the storage atmosphere and length of time in storage.
Bitter pit
Bitter pit is a nutritional disorder of apples that is caused by
calcium deficiency in the fruit as it matures (Ferguson &
Watkins 1989). Bitter pit can occur on fruit in trees where
the tree may be adequately supplied with calcium but the
fruit may be deficient. It results from a complex relationship
