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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Means for Maintaining Host Resistance 109
We have seen that the minimum temperature for growth of some
fungal species is around 0°C, while other species are capable of growth,
although at a very slow rate, even at temperatures as low as -2°C or less
(see Table 2). For these pathogens, storing the commodity at 0°C would
not arrest their growth nor prevent disease development, but would only
delay the appearance of the disease or, in other words, would prolong its
incubation period. We have also seen that the closer the storage
temperature approaches to the minimum for growth of the pathogen, the
longer is the incubation period of the disease and the slower the progress
of decay (see Fig. 8). It should be emphasized, however, that even if the
pathogen is not eradicated at 0°C or at temperatures close to it, it is very
significant that its rate of growth under these conditions is very slow;
this enables us to understand our general desire to lower the
temperature of the storage room atmosphere as much as possible.
However, the possibility of lowering the storage temperature is limited
by the sensitivity of the fruit or vegetable to chilling injury. This
sensitivity varies among fruit and vegetable species, and even among
cultivars of the same species, or depends on the state of maturity of a
given cultivar.
Storing cold-sensitive fruits and vegetables at temperatures below
their resistance threshold results in the development of chilling injuries
- physiological injuries caused by low, but above-freezing, temperatures.
The severity of the damage depends on the cultivar sensitivity, on the
temperature and on the duration of exposure to it (Ryall and Lipton,
1979). Chilling injuries are generally associated with the destruction of
groups of cells, which leads to the formation of sunken areas and to
external or internal browning. The development of chilling injuries in
cold-sensitive crops has been hypothesized to derive from changes in
lipids, which leading, in turn, to changes in the permeability of cell
membranes and the accumulation of toxic intermediate compounds,
which may cause cell destruction or death (Lyons, 1973).
Following chilling injury, the sensitivity of the fruit or vegetable to
decay increases considerably, and decay incidence might reach higher
levels than those in fruits held at higher temperatures even though the
latter are unable to reduce decay. The increased decay in the chill-injured
host may result from the easy penetration of the pathogen through
damaged tissues (see the chapter on Postharvest Disease Initiation -
Pathogen Penetration into the Host). Such increased decay in
cold-sensitive fruits was exhibited in Navel oranges which had to undergo
cold-sterilization against insect infestation, under Japanese quarantine
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