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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
164 Postharvest Diseases of Fruits and Vegetables
The relative fitness of imazalil-resistant and imazalil-sensitive wild
biotypes of P. digitatum was evaluated by Holmes and Eckert (1995) in
Californian citrus groves and packinghouses. Both resistant and
sensitive biotypes of the fungus were found to be stable with respect to
their responses to imazalil over several disease cycles in non-treated
lemon fruit and over several generations on culture medium. According
to Holmes and Eckert (1995), the fact that imazalil-resistant biotypes
have reached high frequency in packinghouses with a history of heavy
imazalil application, could have indicated that the resistant biotypes
have an advantage in relative fitness over the sensitive wild biotype in
imazalil-treated fruit. On the other hand, the absence of resistant
biotypes from groves where imazalil has never been applied, despite the
regular introduction of resistant spores on picking boxes, suggested that
resistant biotypes may be less fit than sensitive biotypes of P. digitatum
in non-treated fruit. Upon studying the competition between sensitive
and resistant biotypes of P. digitatum in resistant/sensitive mixtures
(1:1), it was found that both on non-treated fruit and in culture medium
without imazalil, resistant biotypes were generally less competitive than
sensitive biotypes. Holmes and Eckert (1995) considered that such
results should encourage an evaluation of resistance-management
programs involving the rotation of imazalil with a non-selective
fungicide. However, the continued intensive use of imazalil could
adversely affect this strategy by the development of increased fitness
among fungicide-resistant fungi (Holmes and Eckert, 1995).
In most packinghouses in California, citrus fruits are routinely treated
with sodium o-phenylphenate, imazalil and TBZ, to control Penicillium
decay. A recent study on the sensitivity o{ Penicillium spp. to postharvest
citrus fungicides indicated that the intensive use of the three chemically
unrelated fungicides has resulted in the proliferation of triple-resistant
biotypes of P. digitatum (Holmes and Eckert, 1999). This did not lead,
however, to an increase in the level of resistance to any of the fungicides
in the various isolates of Penicillium collected during several years. On
the other hand, the proportion of isolates that were resistant to all three
fungicides increased from 43% in 1948 to 74% in 1994. It was also found
that while imazalil-resistant biotypes of P. digitatum were frequently
isolated in packinghouses, resistant P. italicum was rare. It was
suggested that a fundamental difference in the biology of P. italicum and
P. digitatum might be involved. The difference could result from smaller
populations of P. italicum available for selection in the packinghouse,
greater imazalil sensitivity of resistant mutants, and the lower parasitic
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