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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Means for Maintaining Host Resistance 125
as 32% had no effect on A, alternata spore germination. Similarly, growth
of A, alternata, B, cinerea and C. herbarum were inhibited by 50% in an
atmosphere of 20% CO2, whereas growth of F, roseum was inhibited by
50% only when the CO2 level was elevated to 45%. Growth of colonies of
the two bacteria, E, carotovora and E. atroseptica was also inhibited by
high CO2 concentrations. On the other hand, in the absence of CO2 no
growth of these fungi occurred at any O2 concentration (Wells, 1974).
Combining low O2 with high CO2 concentrations does not necessarily
result in additive effects. For certain fungi, such as Sclerotinia
sclerotiorum, growth retardation induced by low O2 concentrations is not
enhanced by elevating the CO2 content of the atmosphere. On the other
hand, certain fungi, such as A. alternata and Penicillium expansum have
been found to grow more slowly in CAs composed of low O2 (2.3%) and high
CO2 (5%) than in air, although neither low O2 nor high CO2 alone caused
any significant growth retardation (El-Goorani and Sommer, 1981).
In contrast to the fact that CA usually retards the growth of the
pathogen, it has been found that atmospheres low in O2 stimulated the
fungus, Geotrichum candidum. Furthermore, a combination of low O2
concentrations with 3% CO2 increased the stimulatory effect of the low
O2 alone (Wells and Spalding, 1975). CO2, at concentrations higher than
0.03%, has been found to stimulate the growth of several pathogenic
fungi even when the O2 level was sufficiently low to limit fungal
development, and the CO2 concentrations were not in themselves
inhibitory (Wells and Uota, 1970). The growth stimulation by CO2 under
these conditions has been attributed to fixation of CO2 into acids of the
Krebs cycle, by fungal cells, and utilization of the derived energy for
fungal growth.
Several investigations have addressed the effects of CA on the
enzymatic activity of the pathogen. Holding R, stolonifer cultures in a
I0W-O2 atmosphere retarded their growth and also repressed fungal
enzymatic activities. Pectin methyl-esterase (PME), polygalacturonase
(PG) and cellulase (Cx) activities were directly related to fungal growth
on potato-pectin culture filtrates, with the highest activities found in the
normally aerated cultures (Wells, 1968). Similarly, a controlled
atmosphere of 5% CO2 and 3% O2, which may be suitable for storing
various apple cultivars, markedly reduced mycelial growth as well as the
enzymatic activities of PG and polymethylgalacturonase (PMG) produced
by Gloeosporium album and G. perennans (Edney, 1964).
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