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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Host Protection and Defense Mechanisms 71
However, because of the marked antifungal activity of tomatine and
other saponins, these preformed compounds are behoved to be involved
in host resistance towards saponin-sensitive fungi. Tomatine presumably
owes its toxic properties to its ability to bind to 3p-hydroxy sterols in
fungal membranes (Steel and Drysdale, 1988). Most tomato pathogens,
on the other hand, can specifically degrade tomatine and detoxify its
effect through the activity of tomatinase (Osbourn et al., 1994).
In a recent study, Sandrock and VanEtten (1998) examined 23 strains
of fungi for their sensitivity to a-tomatine and to its two breakdown
products, P2-tomatine and tomatidine. The two saprophytes and the five
non-pathogens of tomato included in this study were found to be
sensitive to a-tomatine, while all the other fungi tested, except for two
tomato pathogens, were determined to be tolerant to a-tomatine. All the
tomato pathogens tested except Phytophthora infestans and Pythium
aphanidermatum were able to degrade a-tomatine. These included
Alternaria alternata, the common postharvest pathogen of tomato fruit.
A strong correlation was found between tolerance to a-tomatine, ability
to degrade this compound, and pathogenicity to tomato. However,
P2-tomatine and tomatidine, which were less toxic to most pathogens
because of their inability to complex with membrane-bound SP-hydroxy
sterols (Steel and Drysdale, 1988), were inhibitory to some of the
non-pathogens of tomato, suggesting that tomato pathogens may have
multiple tolerance mechanisms to a-tomatine (Sandrock and VanEtten,
1998).
Tannins in young banana fruits (Green and Morales, 1967) and
benzylisothiocyanate in unripe papaya fruits (Patil et al., 1973) are
additional examples of in-fruit toxic compounds. Since the concentration
of these compounds decreases with fruit ripening, it has been considered
that they have a role in resistance to decay in young fruits.
Antifungal substances isolated from unripe avocado fruit peel include
monoene and diene compounds, of which diene is the more important
(Prusky and Keen, 1993). Resistance of unripe avocado fruits to
postharvest pathogens has been suggested to be closely related to the
presence in the peel of the antifungal diene compound (l-acetoxy-2
hydroxy-4-oxo-heneicosa 12,15) (Prusky and Keen, 1993). This compound
inhibits spore germination and mycelial growth of Colletotrichum
gloeosporioideSy at concentrations lower than those present in the peel
(see Fig. 3). The concentration of diene diminishes considerably as the
fruit ripens and, in parallel, the dormant fungus in the peel starts its
development (Prusky et al., 1982) (see Fig. 4). The decrease in the
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