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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Factors Affecting disease Development 45
investigators believe that weed pollens with high sugar contents are
likely to lead to further stimulation of the pathogen on fruit in the
orchard (Fourie and Holz, 1998).
3. THE FRUIT RIPENING STAGE
The susceptibility of harvested fruits and vegetables to decay agents
depends mainly on their ripening stage at the time of picking; it
increases as ripening progresses. Many types of fruits are more
susceptible to injuries as they ripen and, therefore, become more
susceptible to pathogen attack (Eckert, 1975). However, in general, the
susceptibility of the fruit to pathogen invasion increases regardless of its
susceptibility to injury. Various tissue characteristics, such as the acidity
level, the turgor state of the tissues, or nutrient availability, change
throughout the senescencing and ripening stages and might, separately
or in combination, enhance the susceptibility to disease. Other factors
affecting the impact of the ripening stage on disease susceptibility
involve the enhanced virulence of the pathogen, on the one hand, and
weakened host resistance and protection, on the other hand. These
factors include:
a. increased availability of compounds that induce formation of
pectolytic enzymes by the pathogen;
b. enhanced susceptibility of the host cell walls to the pectolytic
activity of the pathogen;
c. reduced concentrations of compounds toxic to the pathogen or of
those that inhibit its enzymatic activity within the host tissues;
d. changes in nutrient availability during ripening.
One of the primary factors enhancing the susceptibility of the fruit to
infection is the enhanced susceptibility of the plant cell walls to the
activity of pectolytic enzymes produced and secreted by the pathogen.
The pectic substances that comprise the cell wall of the young fruit are
present in the form of insoluble protopectin. The insolubility of
protopectin stems from both its high molecular weight and its close links
with the cell wall cellulose, which comprise bridges of neutral
polysaccharides and perhaps proteins as well (Eckert, 1978). At this
stage, the cell walls withstand the pectolytic activity, which constitutes a
most significant attack mechanism for many pathogens. As the fruit
ripens, the links connecting the pectic substances to the cell wall break,
so that the increased solubility of the pectic substances increases, and
the tissue starts to soften (Eckert, 1978; PauU et al., 1999). At this stage,
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