Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
46 Postharvest Diseases of Fruits and Vegetables
the fruit can provide pectic compounds that induce the formation of
pectolytic enzymes and their activity, and the plant tissue becomes more
susceptible to the pectolytic enzymes secreted by the pathogen. Recent
studies with papapya cell walls suggested that, in addition to pectin
hydroljdsis, the modification of another cell-wall compound - the
hemicellulose - is also involved in fruit softening (PauU et al., 1999).
Another factor that changes during the ripening process is the ability
of the tissues to produce toxic compounds, which inhibit the pathogen
growth; this diminishes as the tissues ripen. These compounds include:
tomatine, which can be found in high concentrations in the green tomato
skin (Verhoeff and Liem, 1975); tannins in young bananas (Green and
Morales, 1967; van Buren, 1970); monoene and diene compounds in
young avocado skin; resorcinols in the skin of mangoes in their early
developmental stages (Prusky and Keen, 1993); and various toxic
compounds found in the oil glands of young citrus fruit rind (Rodov et al.,
1995b). Polyphenols and tannins, highly concentrated in the younger
fruits, have been described both as germination and growth inhibitors of
microorganisms, and as suppressors of enzymatic activity.
Regarding the nutrient availability changes during the advanced
development or ripening of the fruit, a direct relationship was found
between the sugar contents in nectarines and plums and the late-season
susceptibility of these fruits to B. cinerea infection (Fourie and Holz,
1998). During their early developmental stages, the young unwounded
fruits are resistant to the pathogen. It is not until the early phase of
rapid cell enlargement of the nectarine and the last phase of rapid cell
enlargement of the plum that the fruit becomes susceptible to infection,
and disease development occurs (Fourie and Holz, 1995).
Studying the sugar content in exudates of immature plum and
nectarine fruits, Fourie and Holz (1998) found that fructose, glucose and
sorbitol were the only sugars present, whereas sucrose was first detected
during fruit maturation, which occurs approximately two weeks before
harvest. The sugars were exuded at low concentrations by immature
fruits but their concentrations increased as the fruit ripened. In
nectarines there was a pronounced increase in the sugar concentrations,
especially that of sucrose, near the picking-ripe stage. Fungal growth was
found to increase when the reducing sugars or sucrose were added to
culture media in excess of 0.27 or 0.14 mM and when sorbitol was
supplied in excess of 0.66 mM. With the progress in fruit maturity during
the last two weeks before harvest, total sugars in plum exudates
approached these values, whereas in nectarine exudates they far
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