Agriculture Reference
In-Depth Information
The bacteria enter through the open flowers during cool
weather. They are carried to the flowers by nectar feeding
insects (Pegg
et al
. 1995). Nectar is probably an energy
source for the bacteria. Once inside the flower, they remain
latent in the nectary gland or stylar canal and locule until
the fruit matures, sugar concentrations increase and
translucence occurs (Rohrbach and Schmitt 1994). Drought
before flowering, followed by rainfall during flowering,
increases disease incidence.
The bacteria are thought to be part of a group of organ-
isms that habitually live on the surface of the pineapple
plant, and which are carried by insects and mites to the open
flowers from infected rotting fruit. High temperatures kill
the bacteria (Pegg
et al
. 1995). The disease appears to be
limited to pineapple production areas where fruit develops
under cooler conditions since the disease rarely occurs in the
lowland tropics. Pink disease bacteria cannot survive fruit
temperatures greater than 38°C. Thus pink disease only
occurs when flowering occurs during cool weather or a rainy
season (around 18°C) and fruit mature during periods when
air temperatures do not exceed 29°C (Rohrbach 1989).
Infection usually occurs through the open flower, but
may also occur through fruit surface growth cracks during
the latter stages of fruit development. Bacteria may be
vectored to the flowers by insects. Application of surfactants
prior to and during flowering significantly increases disease
in Hawaii, indicating that the bacteria are ubiquitous on the
plant. The bacteria remain quiescent in the flower and
developing fruit until approximately one month before fruit
maturity. Low fruit acid and sugars are associated with high
levels of the disease (Rohrbach & Schmitt 1994).
The most common symptom is a yellowish to reddish
brown to very dark, dull brown discolouration of fruit tissue
internally (Plate 7.3). Infected tissues generally become
hardened, granular, brittle and speckled with colour
variations with a woody consistency. External symptoms
are not seen in the affected fruit, however, severely infected
fruit may be identified by a 'woody' sound when tapped
(Rohrbach & Schmitt 1994). The disease may affect
multiple fruitlets or the entire fruit, but occasionally only
single fruitlets are involved. Frequently, the speckled
appearance will occur in vascular tissue to the core of the
fruit. Symptoms develop during the last month of fruit
maturation (Rohrbach and Schmitt 1994).
Marbling disease is similar to pink disease but is charac-
terized by a brown granular appearance and consistency of
infected fruit tissue without sterilization. In contrast to pink
disease, marbling occurs when fruit are initiated, flower, and
mature under warm conditions (> 21-27°C). While moisture
does not appear to be critical for infection, disease is
enhanced with rainfall during flowering and when fruit
matures under dry hot conditions followed by rainfall during
the last 6-8 weeks of development (Rohrbach 1989).
Control
Controlling the vectors with insecticides is the primary
means of managing pink disease. Applications, starting at
the red-bud stage and followed by three additional
applications at 5-day intervals (throughout flowering),
have resulted in the highest level of control. The disease
has been controlled in the Philippines by applying insecti-
cides during flowering (Kontaxis 1978). Susceptibility to
the disease can be reduced by the use of potash fertilizer.
Cultivars and hybrids vary from highly resistant to very
susceptible. Growing of relatively resistant groups such as
Smooth Cayenne, provides control of the disease
(Rohrbach & Schmitt 1994). Control is not warranted in
Queensland, Australia (Pegg
et al
. 1995).
Control
No practical control measures are known. Affected fruit
having clearly visible symptoms at harvest are readily
eliminated during processing (Pegg
et al
. 1995). Processing
costs can be reduced by excluding diseased fruit from the
cannery operation. Infected fruit can he detected by
examining the external appearance and by testing fruit
firmness. Differences in cultivar susceptibility have also
been noted; Smooth Cayenne is moderately resistant
(Rohrbach & Schmitt 1994).
Marbling
Strains of the bacteria,
Acetobacter peroxydans
,
Acetobacter
sp. and
Erwinia herbicola
var. ananas are the
causative organisms of this disease (Pegg
et al
. 1995).
Though the disease occurs in all pineapple growing
countries, it is serious only in countries where pineap-
ples are produced under lowland tropical conditions,
where temperatures remain above 21°C (Rohrbach &
Schmitt 1994). In Thailand, 5-20% of the slices in
canneries are marbled, and high incidences in October
and November can even stop canning operations. In
Hawaii, the highest levels of marbling occur in April and
May (Rohrbach & Schmitt 1994).
Yeasty rot
Several fungi in the genus
Saccharomyces
cause yeasty rot.
The disease is widespread and associated with ripe fruits,
and observed mainly during spring in overripe or damaged
fruit (Pegg
et al
. 1995). In spring, rapid changes in fruit
growth resulting from the shift from cold, dry to warm, wet
