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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
210 Postharvest Diseases of Fruits and Vegetables
growth rate of the surviving fungal population was markedly reduced.
Such a delay in the initiation of Botrytis infection may also indirectly
reduce the levels of contact infection and "nesting^' during storage.
A report from South Africa (Du Venage, 1985) indicates that
irradiation of strawberries under commercial conditions prolonged the
summer shelf life of the fruit from 3-12 days at ambient temperatures up
to 50 days at 2'^C. However, for most commodities, even doses which are
sublethal for the pathogen but are capable of retarding or temporarily
halting its growth, may inflict radiation damage on the host.
Radiation doses required for direct suppression of postharvest
pathogens are generally above the tolerance level of the fruit and result
in radiation damage (Barkai-Golan, 1992). Decay control by sublethal
irradiation doses, which are beneath the damage threshold of the
commodity, may be possible in two instances: a) when decay suppression
is caused indirectly by delaying the ripening and senescence processes of
the commodity; and b) when irradiation is combined with other physical
or chemical treatments.
Decay Suppression via Delay of Ripening and Senescence
For several fruits a reduction in the incidence of fungal diseases has
been recorded after exposure to relatively low radiation doses, that are
incapable of directly suppressing, or even temporarily retarding,
pathogen growth but are sufficient to delay the ripening and senescence
of the commodity. Since the susceptibility of fruits and vegetables to
postharvest infection increases as the ripening and senescence processes
progress (see the chapter on Factors Affecting Disease Development -
The Fruit Ripening Stage), one should not be surprised that radiation
doses capable of retarding these processes may contribute towards
disease suppression by maintaining the natural resistance to infection
characteristic of younger tissues. It was thus found that doses of 50-850
Gy are capable of inhibiting the ripening of mango, papaya, banana and
other tropical and subtropical fruits (Thomas, 1985; Barkai-Golan, 1992).
The success of the treatment in each of these fruits depends upon the
balance between the dose needed to retard ripening and the dose
tolerance of the fruit. Alabastro et al. (1978) found that irradiation of
mature green Carabao mangoes with doses of up to 220 Gy delayed the
appearance of anthracnose and stem-end rots by 3 to 6 days without any
adverse effect on fruit appearance. The suppression of decay by such low
doses suggests that no direct fungicidal effect on the pathogens is
involved. Similarly, doses of 50-370 Gy on Cavendish bananas were
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