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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Host Protection and Defense Mechanisms 81
B, cinerea, Sclerotinia fructigena, and Aspergillus niger, which do not
produce protease in the infected tissue and do not induce the accumulation
of benzoic acid, can rot immature fruit (Swinburne, 1975).
Inoculating lemon fruits with Penicillium digitatum, the pathogen
specific to citrus fruits, results in the accumulation of the phytoalexin
scoparone (6,7-dimethyloxycoumarin). The induced compound has a
greater toxic effect than that of the preformed antifungal compounds
naturally found in the fruit tissue, such as citral and limetin, as indicated
by the inhibition of P. digitatum spore germination (Ben-Yehoshua et al.,
1992). A considerable increase in the scoparone level was found in lemons
which had been preheated at 36°C for 3 days, and then inoculated with P.
digitatum spores. The increased concentration of scoparone in the fruit
was in good correlation with the enhanced antifungal activity of the fruit
extract. This finding led to the supposition that scoparone plays an
important role in the increased infection resistance of heated fruit. It is
worth mentioning that various citrus fruits (lemon, orange, grapefruit,
etc.) differ from one another in their ability to produce scoparone in
response to fungal infection (Ben-Yehoshua et al., 1992).
Scoparone production can also be induced in the peel of various citrus
fruits by ultraviolet (UV) illumination (Rodov et al., 1992). In
UV-irradiated (254 nm) kumquat fruits the accumulation of scoparone
reached its peak (530 |Lig gi dry weight of the flavedo tissue) 11 days after
illumination. However, its level then declined rapidly, returning to the
minimal trace levels characteristic of the untreated fruit, 1 month after
treatment (Fig. 13). Increasing the radiation dose and raising the storage
temperature (from 2 to 17°C) enhanced scoparone production.
A correlation has been drawn between the level of phytoalexin
accumulated in the flavedo of irradiated fruits and the increased
antifungal activity of the flavedo. It has also been found that irradiating
kumquat fruits prior to their inoculation with P. digitatum resulted in
decreased incidence of green mold decay, whereas irradiating fruits
which had previously been inoculated with P. digitatum failed to prevent
decay development (Fig. 14). The finding that decay reduction was
achieved when irradiation was applied to the fruit prior to its
inoculation, and therefore without any direct exposure of the pathogen to
the radiation, led to the suggestion that disease inhibition stems from
increased resistance of the fruit to infection and not from the direct
fungicidal effect of UV on the pathogen (Rodov et al., 1992).
http://arab2000.forumpro.fr