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

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Postharvest Disease Summary 297
on the tree. However, the fungus can penetrate via the lenticels and
remain quiescent until after harvest. The quiescent stage has been
related to the synthesis of the phytoalexin, benzoic acid in the tissues;
after harvest, with the loss of the phytotoxic activity of the phytoalexin,
the pathogen resumes active growth and decay is developed
(Swinburne, 1983). The resistance of apple tissues to decay may,
however, change because of the development of N, galligena mutants
resistant to benzoic acid (Seng et al., 1985).
F. Colletotrichum gloeosporioides (Penz.) Sacc. [perfect state:
Glomerella cingulata (Stonem.) Splaud. & v. Schrenk] and
Colletotrichum acutatum J.H. Simmonds
These two species are involved in bitter rot of apples, which may cause
heavy losses in warm, wet growing areas (Sutton, 1990).
Some strains of C. gloeosporioides are capable of exhibiing both the
sexual state (perithecia with ascospores) and the asexual state (acervuli
with conidia), while others produce only asexual spores. Both types of
spores can take part in disease initiation (Sutton and Shane, 1983).
Infection takes place in the orchard during the growing season, in warm,
wet weather, the optimal temperature for growth being about 26°C.
Spores are washed down onto the fruit by rain, and infection can take
place by direct penetration of germinating spores into the intact fruit
(Brook, 1977). At a later stage, completely decayed fruits turn into
'mummies', a source of immense quantities of conidia which serve as
inoculum for the following year (Brook, 1977). Bitter rot is also found in
storage following fungal penetration through wounds sustained during
harvesting, handling and marketing.
Bitter rot symptoms appear as circular lesions with the fungal
reproductive structures (acervuli, perithecia or both) often in concentric
rings on the fruit lesion (Shi et al., 1996). Isolates of the two
Colletotrichum species exhibit a considerable variation in colony color,
but they generally produce acervuli with typical orange to salmon-pink
conidial masses (Jones et al., 1996). Apple isolates of C. gloeosporioides
differ from apple isolates of C. acutatum in the morphology of their
conidia and by a faster rate of colony growth (Shi et al., 1996). The
frequency of occurrence of the various pathogens on apples varies among
orchards and may be influenced by many variables including
environmental factors, sampling date, host cultivar and management
practices.
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