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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
10 Postharvest Diseases of Fruits and Vegetables
that stimulation increased during the last month of fruit ripening. This
period corresponds with the large increase in sugar content of the
exudates, along with the increased susceptibility of grapes to infection
(Kosuge and Hewitt, 1964; Padgett and Morrison, 1990).
Various fungi, which penetrate through wounds, encounter the
moisture required for germination of their spores in fresh wounds in the
cuticle and epidermis of the host. The fractured cells in the injured area
also supply the nutrients required for the germination and infection
phases. Pathogens which penetrate through the host lenticels can feed on
the nutrients secreted from the cells adjoining the lenticels, especially
after injury following tissue senescence (Eckert, 1978).
Atmospheric gases. Additional environmental factors, such as
oxygen (O2) and carbon dioxide (CO2) levels in the atmosphere in which
the fruits or vegetables are stored, can also affect germination. Reducing
the O2 level in the air to below 21% or increasing the CO2 level to more
than 0.03% can inhibit fungal spore germination, although various
species react differently to the depressing levels. Generally, an
atmosphere of 15-20% CO2 directly inhibits spore germination of
Rhizopus stolonifer, Botrytis cinerea, and Cladosporium herbarum, with
inhibition rising as the gas level increases. However, such an atmosphere
does not affect spores of Alternaria alternata; inhibition of these spores
requires that the gas level be increased to more than 32%. On the other
hand, spore germination of Fusarium roseum accelerated under the effect
of high concentrations of CO2, but when the concentration was over 32%
the germination of these spores, too, was inhibited (Wells and Uota,
1970). (See Fig. 21a).
Exposure of spores to low oxygen levels, too, might damage their
germination ability, but a significant delay normally occurs only with the
reduction of oxygen level to 1% and below (See Fig. 21b). The capacity of
high CO2 levels and low O2 levels, or a combination of the two, to inhibit
spore germination is used for creating a modified or controlled
atmosphere suitable for the prolongation of the postharvest life of
harvested fruits and vegetables (See the chapter on Means for
Maintaining Host Resistance - Modified and Controlled Atmospheres).
Spores encountering conditions suitable for germinating, including
available water or moisture combined with the required nutrients,
adequate temperature and other environmental conditions, will swell,
develop germ-tubes and be ready for the next stage: penetrating into the
host.
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