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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Means for Maintaining Host Resistance 145
with Colletotrichum spp., fruits treated with calcium chloride and
calcium propionate exhibited delayed formation of acervuli (aggregated
conidial structure of Colletotrichum) by the fungi, and markedly
reduced lesion development. Reduced rates of infection were
demonstrated in field trials, when fruits treated with weakly diluted
applications of calcium salts were inoculated with conidia of either
C. gloeosporioides or C. acutatum.
Biggs et al. (1997) demonstrated the direct effect of 18 calcium salts
(at 600 mg calcium per liter) on the colony growth of Monilinia fructicola,
the cause of brown rot, mainly on stone fruits. All the salts, except
calcium formate, calcium pantothenate and dibasic calcium phosphate,
reduced fungal growth on amended potato-dextrose agar after 7 days at
20°C. Calcium propionate was the most inhibitory compound, reducing
growth by 90%. Calcium hydroxide, calcium oxide, calcium silicate and
calcium pyrophosphate reduced growth by approximately 65% compared
with the control, and were not significantly different from each other. In
general, salts that were inhibitory on amended agar medium were also
inhibitory in a liquid culture (potato-dextrose broth). However, the effect
of calcium salts on fungal growth in the liquid culture, but not on the
agar medium, was correlated with the incidence and severity of disease
in detached non-wounded fruit inoculated by spraying with Monilinia
spore suspension. It was suggested that calcium salts are more likely to
remain in continuous contact with fungal mycelium in the liquid culture
system and that future tests should utilize growth in a liquid medium for
preliminary determination of toxicity levels (Biggs et al., 1997). All the
salts examined, except dibasic calcium phosphate and calcium tartrate,
inhibited polygalacturonase (PG) activity of M fructicola. The greatest
inhibition of PG was caused by calcium propionate (85%), followed by
calcium sulfate, tribaric calcium phosphate, calcium gluconate and
calcium succinate. These four salts reduced PG activity by approximately
75%. Regarding these results, it is interesting to note that disease
incidence and severity following calcium treatment were correlated with
PG activity. The ability of calcium salts to reduce fungal growth and PG
activity directly suggested the possibility that the effects of calcium on
the incidence of brown rot and lesion diameter may result partly from
suppressed pathogen activity.
Calcium propionate, which proved to be the most active salt inhibitor
of fungal PG, has been extensively used as a food additive, and is known
as an inhibitor of molds and bacteria in stored grain (Raeker et al., 1992).
Furthermore, a substance such as calcium propionate, that has no
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