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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Physical Means 219
illumination was applied before inoculation with P. digitatum, without
direct contact of the pathogen with the UV light. Moreover, illumination
of previously inoculated fruit failed to prevent decay development (see
Fig. 14).
The experiments with kumquats clarified that the fruit/pathogen
interaction depends on the rate of fungal growth against resistance
development. Inoculation of the fruits before UV treatment gives an
advantage to the pathogen, whereas illumination applied 2 days before
inoculation was sufficient to improve fruit resistance. Host resistance is
thus dependent on the existence of a lag period between illumination and
inoculation, during which phytoalexin can be accumulated and can
suppress pathogen development (Rodov et al., 1992). Induced resistance
in UV-treated grapefruits was found to reach its maximum level 24-28 h
after the exposure to UV light, after which the level decreased (Droby et
al., 1993a). Resistance was affected by the temperature at which the fruit
was stored 24 h after UV treatment and before P. digitatum infection.
Fungal development in the UV-treated fruit was characterized by
sporadic mycelium and a marked inhibition of sporulation. All these
findings in citrus fruits emphasize the fact that decay suppression
following UV treatment is not connected with its germicidal properties
alone, but also with its ability to induce biochemical and physiological
changes in the tissues. This is especially marked when low doses of UV
are involved.
Further studies with UV-illuminated grapefruits (Porat et al., 1999)
indicated that UV-treatment induced the accumulation of chitinase
protein, while the combination of UV and wounding induced both
chitinase and P-l,3-endoglucanase. These two enzymes, which are
included among the 'pathogenesis-related' (PR) proteins, are believed to
be involved in plant defense mechanisms against fungal infection (El
Ghaouth, 1994).
UV illumination of carrots was found to induce the accumulation of the
antifungal phytoalexin, 6-methoxymellein, and to enhance root
resistance to infection by Botrytis cinerea and Sclerotinia sclerotiorum
(Mercier et al., 1993). The direct relationship between the accumulation
of methoxymellein and the development of resistance to infection by B,
cinerea, has previously been exhibited in freshly cut slices of carrot roots
treated with heat-killed conidia of 5. cinerea (Harding and Heale, 1980).
A recent study on table grapes (Nigro et al., 1998) found a significantly
lower incidence of infected berries and a reduced diameter of S. cinerea
lesions among artificially inoculated berries treated with UV-C doses of
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