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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Attack Mechanisms of the Pathogen 57
The virulence of a pathogen has been related to the total PG activity or
to specific PG isozymes. The production of PG isozymes may be
influenced by various factors, such as growth conditions of the pathogen,
nutrients and fungal strains (Cleveland and Cotty, 1991; Tobias et al.,
1993). Recently an increase in total PG activity and in that of PG
isozymes has also been correlated with reactivation of latent infections
and the beginning of tissue maceration by the reactivated pathogen
(Zhang et al., 1997).
Endo-pectate lyase, which attacks the pectic acid at random, is
produced by various pectolytic bacteria and is probably the major cause
of cell wall decomposition of potatoes infected by these bacteria (Hall and
Wood, 1973), but enzymes of the endo-PG group may also be involved in
tuber maceration (Beraha et al., 1974). The involvement of pectate lyase
(PL) in fungal decay has been described for anthracnose (Colletotrichum
gloeosporioides) in stored avocados (Wattad et al., 1994); PL was found to
cause softening in unripe avocado fruits, whereas the addition of
antibodies to this enzyme inhibited tissue maceration.
Saprophytic fungi and bacteria, which are not included in the
postharvest pathogens' category, may also produce pectolytic enzymes on
various substrates; some of these are capable of macerating discs of
potato tuber or of other plant organs (Ishii, 1977). Therefore, the fact
that a given microorganism can produce pectolytic enzymes in vitro
cannot serve, by itself, as evidence of pathogenic capability. To determine
the capability of a pathogen to decompose the pectic compounds of plant
cell walls, its pectolytic activity should be examined in the host tissues
themselves.
Simmonds (1963) suggested that the failure of the pathogen to produce
adequate levels of pectolytic enzymes in the host may be related to the
establishment of infections that remain quiescent until the ripening
process leads to suitable changes in the cell wall structure. An
inadequate level of pectolytic activity in the plant tissue may be caused
by several factors (Swinburne, 1983): (1) Pectolytic enzymes are not all
constitutive and they may need inducers for their production (Bateman
and Basham, 1976); the inducing substrates could be supplied to the
pathogen only as the fruit ripens. (2) Even if such enzymes were
produced, access to the labile sites within the wall could be blocked by
cation cross-linking. (3) Enzymes may be inactivated by inhibitors
present at higher levels in the immature fruit than in the mature one
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