Agriculture Reference
In-Depth Information
Another biological control agent,
Bacillus subtilis
also
showed the ability to suppress growth of
P. digitatum
in the
post-harvest protection of citrus (Leelasuphakul
et al
. 2008).
Canamas
et al
. (2008) indicated that it is also possible
to control
P. digitatum
, using bacteria such as
Pantoea
agglomerans
as a pre-harvest treatment.
Different treatments (i.e. sanitary practices, 'reduced
risk' fungicides (GRAS) and biological control agents)
have shown to decrease the incidence of
Penicillium
decay.
These treatments do not necessarily show the same effec-
tiveness as fungicides applied in pack houses and must
therefore be used in integrated approaches, such as the use
of an in-line recirculating drench application with a fungi-
cide-sanitizer (sodium bicarbonate) to increase fungicide
efficacy and to minimize the selection for resistant isolates
of the pathogen (Kanetis
et al
. 2008b).
Transmission
Conidia of
P. italicum
are airborne and large numbers of
these blue spores are produced by the fungus that can
cover the entire surface of the infected fruit (Timmer
et al
.
2000; Sommer
et al
. 2002), and are easily dispersed by
physical movement or by air currents. These conidia con-
taminate the pack house, equipment, water used in drench-
ers and soak tanks, storage rooms, transit containers and
even the retail market area (Brown 2003b). The fungus
survives in the field on soil debris and produces spores
that infect split and injured fruit in the tree and on the
ground. During favourable conditions (cooler autumn and
winter temperatures), large numbers of spores are pro-
duced and transported by air currents to surfaces of fruit in
tree canopies (Brown 2003b). In contrast to
P. digitatum
,
P. italicum
conidia are able to spread from fruit to fruit in
packed containers, through uninjured skin, and create
nests of decayed fruit (Eckert & Eaks 1989). Spoilage
occurs when masses of conidia produced on infected fruit
contaminate surfaces of healthy fruit in the carton, there-
fore soiled fruit must first be cleaned before retail sale
(Brown 2003b).
Blue mould
Blue mould is caused by the fungus
Penicillium italicum
Wehmer (1894), and causes much less decay than green
mould (Brown 2003b).
Morphology
Penicillium italicum
produces a pale grey-green colony on
artificial media (pale brown to yellowish or orange brown
on the reverse side) with a characteristic sweet odour
(Onions & Brady 1987; Pitt 1991). Some isolates of this
species produce a clear exudate on the colony and a
brown soluble pigment in the artificial media (Pitt 1991).
Conidiophores are smooth and produce conidia in long
disordered chains that are typically cylindrical at first,
becoming elliptical or ovate at maturity (2-3 × 3-5
m
m)
(Carlos 1982; Pitt 1991; Timmer
et al
. 2000).
Symptomology
Symptoms of early blue mould infections are identical
to green mould and sour rot with the exception of a blue
conidial mass that forms on decaying fruit. Diseased
tissue becomes soft, watery and slightly discoloured.
A white powdery growth of mycelium develops on
the surface of the lesion and eventually blue spore
mass forms (Timmer
et al
. 2000, Syngenta 2007). The
area of sporulation is surrounded by a narrow band of
white mycelium, which is encompassed by a defi-
nite band of water-soaked rind (Timmer
et al
. 2000;
Brown 2003b).
Disease cycle and epidemiology
Disease cycle and epidemiology is similar to that of green
mould. However, when Valencia oranges or Clementine
mandarins are stored for long periods at temperatures
below 5°C, blue mould becomes more important since it
grows faster than green mould at temperatures below
10°C (Whiteside
et al
. 1988; Timmer
et al
. 2000). Blue
mould is therefore most prevalent in cold stored fruit
where it is able to develop slowly at cold temperatures
(Sommer
et al
. 2002). The infection and sporulation cycle
can also be repeated several times in a pack house and
during extended storage. This prolific conidia production
ability of
P. italicum
enables it to eventually develop
resistant strains against chemical fungicide treatments
(Brown 2003b).
Control
Preventative measures
Preventative measures to control blue mould are the same
as for green mould, e.g. keeping conidial inoculum levels
low and careful handling of fruit during harvest and post-
harvest (FFTC 2003). Similar stringent sanitary practices
used for green mould control are also applicable to blue
mould.
Chemical control
Blue mould can be controlled with the same chemical
treatments as used for green mould. Blue mould can
also predominate in fruit treated with benzimidazole
fungicides, since resistance to these materials occur more
