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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Physical Means 207
more resistant to gamma radiation than the unicellular spores of other
fungal species (Sommer et al., 1964b). The multicellular or bicellular
spore presumably exhibits resistance when one of the cells escapes injury
and remains capable of forming a colony.
For a given fungal species, the inhibiting effect of radiation increases
with the radiation dose; however, for a certain dose, the rate of
irradiation application may affect both spore survival and mycelial
growth. It has generally been found that higher rates of irradiation
increased the efficacy of the radiation, enabling the dose levels needed for
pathogen inactivation to be reduced (Beraha, 1964). The greater
radiobiological effect of a rapidly applied dose has been attributed to the
lack of, or fewer opportunities for repair.
The number of fungal spores or mycelial cells in the inoculum exposed to
radiation may affect the radiation dose required for their inactivation.
Increased spore density in the inoculum has generally necessitated an
elevated radiation dose. In oranges inoculated with Penicillium digitatum
and P. italicum, the reduction in spore concentration resulted in the
prolongation of the incubation period of the disease (Barkai-Golan, 1992).
The presence of oxygen in the atmosphere at the radiation site is an
important factor in enhancing the effectiveness of a given dose. It was
thus found that the dose required to reduce survival of Rhizopus
stolonifer spores to 1% was much less in the presence of oxygen than that
under anoxia (Sommer et al., 1964a). The increased antifungal effect was
attributed, among other factors, to the formation of peroxides, which
caused cell injury.
Another factor affecting the radiation sensitivity of microorganisms is
the water content of their cells. This may be the reason why vegetative
cells are more sensitive to radiation than spores (Barkai-Golan, 1992).
This is true for both fungi and spore-bearing bacteria. The high water
content of bacterial vegetative cells may favor the production, within the
cytoplasm, of a variety of harmful radicals that enhance the effects of
radiation injury (Grecz et al., 1983).
The timing of irradiation may also affect pathogen sensitivity: extending
the time lag between inoculation and irradiation may increase dose
requirements for pathogen suppression (Zegota, 1987). Since many of the
postharvest pathogens enter the host via wounds incurred at harvest, the
time lag between harvest and irradiation would parallel the time between
inoculation and irradiation. During this period, the infection may be
initiated and the size of population exposed to irradiation may increase.
For quiescent fungi, an extension of the time interval between harvest and
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