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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
144 Postharvest Diseases of Fruits and Vegetables
Applying CaCl2 solution to developing table grapes cv. Italia during
the season increased calcium content in the whole berries and in the peel,
resulting in reduced decay incidence during cold storage and the
subsequent shelf life (Miceli et al., 1999). In this case, calcium-treated
berries infected by B, cinerea showed an increase in the cellulose content
of cell walls, but no changes in the water-soluble pectin or hemicellulose
contents. However, a higher protopectin content was recorded in the cell
walls of Ca-treated berries after infection, indicating the involvement of
Ca ions in the stabilization of the cell-wall structure that led to increased
resistance to the pathogen (Miceli et al., 1999).
Although prolongation of storage life as a result of calcium application
is thought to be due mainly to the role of calcium in ameliorating
physiological disorders and thus indirectly reducing pathogen activity,
direct effects of calcium on the pathogen have also been recognized.
Calcium interfered with spore germination and germ tube elongation of
P. expansum and B. cinerea (Conway et al., 1994a). At low concentrations,
calcium may also directly inhibit polygalacturonase activity. Conway et
al. (1994a), who studied the commercial potential of Ca for maintaining
the quality of apples in storage, found that the Ca content of fruits from
trees that had been sprayed with CaCb solutions throughout the growing
season was significantly increased but not enough to be expected to
retard pathogens infecting wounded fruits. Pressure-infiltrating fruits
after harvest with high CaCl2 concentrations (4-8%) resulted in a tissue
Ca level that would potentially maintain fruit firmness and retard decay,
but it was suggested that proper sanitation, to reduce fungal infection
that could arise from the pressure-infiltration procedure, should also be
implemented before this technique could be seriously considered for
commercial use. Furthermore, such concentrations stimulate pectate lyase
activity; in order to inhibit this activity, higher calcium concentrations
would be required (Conway et al., 1994a).
Biggs (1999), who studied the potential role of preharvest calcium
supplementation of apples in reducing pathological diseases, found that
calcium salts directly suppress bitter rot caused by Colletotrichum
gloeosporioides and C. acutatum. Calcium chloride and calcium
propionate at 1000|ag of calcium per milliliter had no effect on conidial
germination, but markedly inhibited germ-tube growth of these
pathogens. These two salts, as well as calcium silicate, also inhibited
mycelial growth: fungal dry weight in liquid culture media was reduced.
When calcium salts were applied to wounded apples prior to inoculation
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