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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Physical Means 193
Ultrastructural changes in heat-treated, non-germinated spores of
Monilinia fructicola were seen as progressive destruction of the
mitochondria, disruption of vacuolar membranes and formation of gaps
in the conidial cytoplasm (Margosan and Phillips, 1990). The site most
sensitive to heat in dormant conidia of M fructicola may be the
mitochondria, probably in the inner membrane. In germinating conidia,
exposure to heat results in changes in the nuclei, in the cell wall, or both
(Baker and Smith, 1970).
As can be seen from the in vitro studies (see Fig. 28), Botrytis cinerea
is very sensitive to high temperatures, and heat treatments have found
to control it on various fruits and vegetables, including apples (Fallik et
al., 1995; Klein et al., 1997), sweet peppers (Fallik et al., 1996), tomatoes
(Fallik et al., 1993) and strawberries (Garcia et al., 1996).
Short-term, hot water dips (50°C for 3 min) completely inhibited, or
significantly reduced, decay development in artificially inoculated or
naturally infected sweet red peppers (Fallik et al., 1996). Higher
temperatures or longer exposures to 50°C resulted in heat damage. The
mode of action of hot water dips on decay development appears to be
interaction with fungal pathogens, as was exhibited by the inhibition of
spore germination and germ-tube elongation of B. cinerea and A. alternata,
the two main fungi responsible for postharvest decay of peppers.
Long-term, hot water immersion (38°C for 3 days) of Botrytisinoculated
tomatoes totally prevented gray mold decay under shelf-life
conditions, with no damage to the fruit (Fallik et al., 1993). It was shown
that heat treatments at 38*^C directly suppressed spore germination of B.
cinerea within 24 h following their exposure to heat stress. Longer heat
treatments inhibited hyphal growth and prevented expansion of the
colony. The direct effect of long-term heat treatment has been exhibited
also for the more heat-resistant fungus, Penicillium expansum (Fallik et
al., 1995). Heating to 38, 42 or 46°C directly arrested spore germination
and mycelial growth in culture, the necessary duration of exposure being
in inverse proportion to the temperature. However, the direct effect is not
the only way in which the long-term heating (38°C, 96 h) inhibits decay
development in P. expansum-inoculated apples: a similar inhibition of
spore germination also occurs when the spores are incubated in peel
extracts derived from fruits heated under the same conditions without
any direct exposure of the spores to the heat. In this case, decay
inhibition is the result of the indirect effect of heat on the pathogen, via
the heat-treated host.
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