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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Factors Affecting disease Development 47
exceeded them. Fourie and Holz (1998), therefore, concluded that the
stimulatory effect of fruit exudates on B. cinerea growth coincided with
the period of rapid sugar release from the fruit and the exhibition of fruit
susceptibility to decay.
4. EFFECTS OF ETHYLENE
A strong link between the ripening stage of a fruit and its sensitivity
to decay may explain why conditions or chemical substances that
stimulate ripening, generally also enhance decay. A classic example of
conditions that stimulate ripening is the exposure of various citrus fruit
cultivars to low concentrations (50 ppm) of ethylene for degreening.
Ethylene treatments are applied commercially at the beginning of the
citrus fruit picking season, to degreen the fruits that have reached
maturity but have not yet developed the desired color. However, this
economically important procedure, which enhances chlorophyll
decomposition and exposes the yellow, orange or red color in the fruit
peel, is accompanied by enhanced sensitivity of the fruit to decay (Brown,
G.E. and Lee, 1993). It was found that along with the enhancement of
ripening, ethylene also stimulates senescence and disruption of the
stem-end ^button', thus activating the quiescent infection of Diplodia
natalensis at this location (Brown, G.E. and Wilson, 1968; Brown, G.E.
and Burns, 1998) and leading to increased incidence of stem-end rot
(McCornack, 1972).
An increase in rot incidence generally follows the use of ethylene at
concentrations above those needed for adequate degreening. In vitro
studies showed that the pathogen growth was stimulated by exposure to
high ethylene concentrations (Brown, G.E. and Lee, 1993). However, the
increased growth rate of the pathogen in response to high ethylene is not
the only factor responsible for enhanced susceptibility of the fruit to
disease. Following the finding of the correlation between ethylene
treatment and the activities of the enzymes, polygalacturonase (PG) and
cellulase (Cx) during citrus abscission (Riov, 1974; Ratner et al., 1969), it
was recently found that the increase in disease incidence caused by the
use of high concentrations of ethylene may be related to the activity of
the abscission enzymes (Brown, G.E. and Burns, 1998). Enzymes formed
during abscission degrade pectic substances in the middle lamella and
lead to cell separation in the abscission zone. The activity of these
enzymes in oranges was found to increase rapidly on exposure to high
ethylene concentrations (0.055 ml l-i) and a larger number of cells were
degraded within the abscission layer by these enzymes than by those
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