Agriculture Reference
In-Depth Information
reported another symptom as alteration of internal metabo-
lism, which leads to an increase in anaerobic respiration
and, as a consequence, of abnormal metabolites, resulting
in the development of foul taste and odour. Anthracnose
(caused by
Colletotrichum gloeosporioides
) is the most
important disease in avocado that reduces fruit quality after
harvest. Friction damage, which is characterized by an oxi-
dation of the tissue that later inclines downward and
becomes necrotic, is one of the most frequent problems
during fruit harvest and handling. Mechanical damage
accelerates water loss and disrupts the superficial arrange-
ment of the tissue allowing a faster gas exchange. Cuts
break completely the protective layer of the fruit and
expose the tissue directly to the environment. The damage
is more serious due to inadequate packaging processes.
tolerated during the post-climacteric phase. Internal CI is
manifested as a greyish-brown discolouration of the flesh,
particularly at the base of the fruit around the seed. This
can be associated with vascular browning which starts at
the base of the fruit (rather than at the stem end, which is
often associated with stem end rots). In 'Hass' avocado
fruit, internal CI tends to occur after about 4 or more weeks
storage at about 6°C, depending on maturity and growing
conditions. Calcium content in the fruit might be a possi-
ble reason for differences in internal CI (Chaplin & Scott
1980). Internal CI is the key limiting factor to long-term
storage of avocados, generally associated with softening of
fruit during storage, and is increased by the presence of
ethylene (Chaplin
et al
. 1983). External CI occurs as
irregular patches of blackening on the skin and can be
observed during storage, but generally increases slightly in
intensity after removal from cold storage. The damage is
first seen in inner cell layers of the exocarp and then the
outer layers of the skin (Woolf 1997). In cultivars that
naturally darken during ripening, such as 'Hass', the
damage will be less apparent after ripening, but may be
discriminated as brown, corky skin tissue in ripe fruit.
External CI is generally induced by temperatures lower
than 3°C, but fruit become less sensitive with increasing
maturity, and ripe fruit are less affected. Fruit exposed to
low temperatures may be of poor internal quality when
ripe with a high incidence of rots and softening disor-
ders (Woolf
et al
. 1995), but will have lower incidence of
internal CI (greying). For 'Hass' fruit stored for long
periods at standard storage temperatures (6 to 7 weeks at
about 6°C), a form of external CI is expressed which is of
a very similar appearance to that observed at low tempera-
ture, which can sometimes be seen in fruit that are quite
soft (nearly ripe) at the point of removal from storage.
'Pulp spot', a low-temperature disorder, is commonly
observed in 'Fuerte' fruit as small dark spots in the flesh,
and blackening of a region surrounding cut vascular
bundles. Swarts (1984) reported the incidence to be higher
early in the season. Mesocarp discolouration results in
an overall grey to brown flesh discoloration, usually most
intense in the distal half of the fruit, and the symptom is
more predominant towards the end of the season. Both
disorders involve browning reactions implicating
particularly the enzyme polyphenol oxidase (PPO) (Kahn
1975) and phenolics (Kahn 1977). Application of
exogenous ethylene, irrespective of the method of
application, caused intensification of mesocarp discolour-
ation in the fruit of several avocado cultivars during cold
storage (Pesis
et al
. 2002). 'Ettinger' fruit treated with
Ethrel (2-chloroethyl phosphonic acid) (a chemical that
PHYSIOLOGICAL DISORDERS AND
THEIR CONTROL
A range of physiological disorders affect avocado fruit and
most of these occur following long storage periods (2-4
weeks) (Zentmyer 1984). The key disorders are flesh
greying, vascular browning and pulp spot, which are all
symptoms of internal CI (Eaks 1976). When fruit are
stored for excessively long periods the flesh may fail to
ripen evenly, and become increasingly susceptible to path-
ogens. The timing of expression of internal CI and its
severity depends on temperature management, initial ripe-
ness, cultivar, production area and fruit maturity. External
CI may occur if fruit are stored at low temperatures (0°C to
3°C) or for long periods (>6 weeks) at standard storage
temperatures. Skin pitting, scalding and blackening are the
main external CI symptoms on mature-green avocado kept
at 0-2°C for more than 7 days before transfer to ripening
temperatures. Avocados exposed to 3-5°C for more than
two weeks may exhibit internal flesh browning (grey pulp,
pulp spot and vascular browning), failure to ripen and
increased susceptibility to pathogen attack. The timing of
CI development and its severity depend on cultivar,
production area, and maturity-ripeness stage. Hatton
et al
.
(1965) observed CI characterized by a grey-brown
discolouration of the vascular system in addition to uneven
ripening and development of off-flavours in avocado fruits.
Florissen
et al
. (1996) reported symptoms as mesocarp
discolouration, hardening of vascular strands and 'off
flavours'. Chaplin
et al
. (1982) and Couey (1982) reported
symptoms as grey or dark discolouration of the mesocarp.
Engelbrech and Koster (1986) found that storage for longer
than 5 days at 5.5°C caused abnormalities in subsequent
respiratory patterns during ripening at 18°C. Kosiyachinda
and Young (1976) found that lower temperatures were
