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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Means for Maintaining Host Resistance 127
The ability of modified or controlled atmospheres to extend the
physiological life of fruits have been closely related to their action on the
host cell walls. As fruits ripen the pectic substances of the middle
lamella, which are initially laid down by the cell in a relatively insoluble
form, protopectin, become more soluble, and the tissue begins to soften.
The increased solubility of the pectic substances and the softening render
the tissue more vulnerable to maceration by pectolytic enzymes released
by the pathogen. Storing the fruit in CA may retard changes in the pectic
constituents, with longer retention of a firm texture, which is less
vulnerable to pathogen attack (Smock, 1979).
Under CA storage, when ripening and senescence are retarded, the
fruit may retain its ability to produce antifungal compounds, normally
characteristic only of young tissues. Such effects lead to disease
inhibition and delay the transformation from quiescent to active
infections, by prolonging the pathogen latency (Barkai-Golan, 1990). An
exception to the association between ripeness and disease development
is the bacterial soft rot of tomatoes caused by Erwinia carotovora,
which develops even more rapidly in mature-green fruits than in
mature ones (Parsons and Spalding, 1972). This phenomenon may
explain why CA conditions, which delay ripening and reduce decay by
Rhizopus and Alternaria in mature-green tomatoes do not result in
bacterial soft rot control.
In general, atmospheres containing about 2.5% O2, a level which is
commonly maintained in CA storage, are most likely to suppress decay
indirectly, by acting upon host resistance, rather than by acting directly
on the pathogen, since most pathogenic fungi grow under these
conditions. In order to suppress the main pathogens directly, O2 should
be lowered to 1% or less. However, excessively low O2 concentrations
result in anaerobic respiration and the accumulation of alcohols and
aldehydes in the tissues, which leads to the development of off-flavors
and irreparable damage to the tissues (Sommer, 1982). Similarly,
elevating the CO2 level of the atmosphere above 5% suppresses fruit
respiration, but has almost no direct effect on fungal growth. In order to
inhibit fungal growth significantly, the level of CO2 must be increased to
about 10% or more (Wells and Uota, 1970) (see Fig. 23B). However, only
a few fruits can tolerate very high CO2 levels for extended storage or
transit periods. Most fruits are injured under these conditions and
develop off-flavors.
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