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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
192 Postharvest Diseases of Fruits and Vegetables
conidia (Barkai-Golan, 1973) (Fig. 32). Similarly, the LD50 temperature
for sporangiospores of Rhizopus stolonifer exposed to hot water for 4 min,
was 49°C, whereas that for germinating spores was only 39°C (Eckert
and Sommer, 1967).
The moisture content of spores before or during exposure to heat can
considerably influence heat transfer and spore survival (Edney and
Burchill, 1967). Comparisons between the heat sensitivities of moist and
dehydrated conidia of Penicillium digitatum revealed that exposure to
70°C for 30 min killed 90% of the moist spore population but only 10% of
the dry spore population. Dry spores that survived this treatment were
capable of infecting citrus fruit, although the onset of symptoms was
delayed 24 hours. The resistance of the dry spores to high temperatures
may explain their ability to survive from season to season without losing
their pathogenicity to citrus fruit (Barkai-Golan, 1972).
Possible mechanisms of pathogen control by heating include: pectic
enzyme inactivation or denaturation of other proteins; lipid liberation;
destruction of hormones; depletion of food reserves; or metabolic injury, with
or without accumulation of toxic intermediates. More than one of these
mechanisms may act simultaneously (Barkai-Golan and Philhps, 1991).
2 4 6
Hours at 25°C
Fig. 32. Sensitivity to heat (2 min at 42°C or 46T) of freshly harvested and
germinating spores of Alternaria alternata. (Reproduced from Barkai-Golan,
1973 with permission of the Mediterranean Phytopathological Union).
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