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

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124 Postharvest Diseases of Fruits and Vegetables
For R, Stolonifer, however, a 50% inhibition was caused only at 2%
O2 and some growth was still recorded at 0% (FoUstad, 1966, Wells and
Uota, 1970). Differences between the sensitivity of spores and sporangia
to low O2 were recorded for R, stolonifer: whereas fungal sporangia
remained viable after 72 h of anoxia, only a few of the sensitive
sporangiospores survived under these conditions (Bussel et al., 1969).
The response of fungal sporulation to low O2 may also differ with the
species: B. cinerea produces an abundance of aerial mycelium when
grown in 1% O2, but no spores develop in this level of O2; sporulation of
A. alternata and C. herbarum, on the other hand, was found even in
0.25% O2, although mycelial growth was markedly reduced (FoUstad,
1966). Lowering of O2 concentrations also suppressed sclerotium
formation in species that naturally produce them during their life cycle.
It was thus found that sclerotium production by Sclerotinia minor was
more sensitive to low O2 than was radial growth and at 1% O2 no
sclerotia were produced, although some growth was recorded (Imolehin
and Grogan, 1980).
Low oxygen tension of 1-3%, a range tolerated by many agricultural
commodities in storage, also reduces growth of various decay-causing
bacteria, such as Erwinia carotovora, E, atroseptica and Pseudomonas
fluorescens, although some growth was recorded even at 0% O2 (Wells,
1974).
Carbon dioxide is essential for the growth of many aerobic
microorganisms, since it is fixed in lactic, fumaric, citric and other acids
of the Krebs cycle. However, although these microorganisms can fix CO2
for their use, they cannot use it as a sole source of carbon for
metabolism. High concentrations of CO2 may directly suppress fungal
growth by retarding various metabolic functions, so causing lowered
respiration (Sommer, 1985). The early studies of Brown, W. (1922b) and
Brooks et al. (1932) had already demonstrated the inhibitory effect of
high-C02 atmospheres on mycelial growth and spore germination of B,
cinerea, R. stolonifer, Mucor spp. and other fungi. The retarding effect of
CO2 on fungal growth is greater in the early phases of growth and at low
storage temperatures (Brown, W. 1922b).
Similarly to O2 concentration, that of CO2 required to inhibit spore
germination and mycelial growth varies with the species (Wells and
Uota, 1970) (Fig. 23B): spore germination of R. stolonifer, C. herbarum,
and B. cinerea was inhibited by over 90% at 16% CO2, but levels as high
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