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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
122 Postharvest Diseases of Fruits and Vegetables
accumulate in the storage atmosphere. Of these compounds, ethylene is
apparently the most important. Since the accumulation of ethylene
above certain levels may hasten the ripening and enhance senescence of
many fruits and vegetables, its removal from the atmosphere may help
to suppress the physiological processes related to ripening and
senescence.
However, for many fruits and vegetables, the factor limiting the
extension of their useful life is the development of postharvest diseases.
Many studies have indicated that modifications of the storage
atmosphere, apart from their effects on the physiological processes of the
host, can also retard postharvest disease development during storage (El
Goorani and Sommer, 1981; Barkai-Golan, 1990). The effect of low O2
levels or high CO2 levels on postharvest disease development can be
direct - by suppressing various stages of the pathogen growth, and its
enzymatic activity - or indirect - by maintaining the resistance of the host
to infection by keeping it in a superior physiological condition.
1. CONTROLLED ATMOSPHERE
Direct Effects on the Pathogen
Oxygen is required for normal respiration and growth of the pathogen.
Suppression of growth by low O2 is most likely due to effects of electron
transport on the cytochrome system, although other oxidative enzyme
systems present in cells may also be suppressed by low oxygen (Sommer,
1985). In most fungi, no growth occurs in the absence of molecular O2,
but the reduction in the level of O2 required to inhibit the various stages
of fungal growth varies considerably among species. In general, however,
lowering the level of O2 from 21 to 5% has little or no effect on fungal
growth. In order to obtain appreciable reduction of spore germination,
mycelial growth and sporulation in many fungal species, O2
concentrations of less than 1% are required (Wells and Uota, 1970),
although spore sensitivity to low O2 does not necessarily match hyphal
sensitivity. Wells and Uota (1970) showed that spore germination of
Rhizopus stolonifer and Cladosporium herbarum in 1% O2 was about
50% of that in air, and they found that the rate decreased gradually as
the O2 concentration decreased from 1 to 0.25%. Spore germination of
Alternaria alternata, Botrytis cinerea and Fusarium roseum, on the other
hand, decreased significantly only when the O2 concentration was 0.25%
or less (Fig. 23A). All cultures resumed normal growth when returned to
air. Mycelial growth (as indicated by mycelial dry weight) of these fungi
was inhibited by more than 50% when the O2 concentration was 4%.
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