Agriculture Reference
In-Depth Information
2005). Rotting is mostly inhibited at freezing temperatures
(0-1°C) (Plaza
et al
. 2004a).
Chemical control
Post-harvest reduction of green mould inoculum is of
primary importance due to an increase in pathogen resist-
ance to chemicals. Post-harvest application of selected
fungicides can aid by delaying green mould development,
especially in combination with immediate cooling of fruit
after packing (Olsen
et al
. 2000). Therefore, different com-
binations of applications and chemicals are continuously
evaluated for effective control, for example the increased
effectiveness of Imazalil (Smilanick
et al
. 1997) when fruit
was treated with heated aqueous solutions of the fungicide
and the control of a Thiabendazole (TBZ)-resistant isolate
of green mould with TBZ at a reduced rate but at higher
temperatures of 50°C (Schirra
et al
. 2008). New chemicals
are regularly evaluated and three new fungicides (azox-
ystrobin, fludioxonil and pyrimethanil) are being registered
for post-harvest use against
Penicillium
decays of citrus fruit
in the United States (Kanetis
et al
. 2008a). The chemicals
belong to different chemical classes, and it was shown that
even
P
.
digitatum
isolates resistant to Imazalil or TBZ were
sensitive to the new compounds.
Common food preservatives (i.e. potassium sorbate)
were also evaluated alone or combined with fungicides and
were found more effective when applied at high tempera-
tures. Control was also achieved with potassium sorbate in
combination with Imazalil and TBZ respectively even
when tested against a resistant isolate of
P. digitatum
(Montesinos Herrero
et al
. 2009). The advantage of testing
edible films and coatings are that most of them are classi-
fied as food additives or generally regarded as safe (GRAS)
compounds by EU or US regulations. El-Mougy
et al
.
(2008) also found that potassium sorbate and sodium ben-
zoate have potential as nontoxic post-harvest fungicide
applications. Sanitizing agents also have an antimicrobial
effect; sodium hypochlorite and hydrogen peroxide in
combination showed a significant delay in fungal infection
by
P. digitatum
. The post-harvest treatment of citrus with
essential oil amended coatings were active against
P. digi-
tatum
(Du Plooy
et al
. 2009), as also were cyprodinil treat-
ments at 150 mg/L (Schirra
et al
. 2009).
Transmission
The fungus survives in the field on soil debris in the form
of conidia (FFTC 2003). Cooler autumn and winter
temperatures favour fungal development, during which large
numbers of spores are produced and carried by wind currents
towards fruit surfaces in the tree canopies (Brown 2003a). In
addition, airborne spores will contaminate the pack house
and its equipment, storage rooms, transit containers, and
even the retail marketplace. Spores will also accumulate in
water drenchers and soak tanks (Brown 2003a).
Symptomology
Initial symptoms of green mould are similar to those of
sour rot and blue mould. The small decayed area appears
as a soft watery spot that first grows on the peel and later
turns green due to the large number of olive green spores
produced (Timmer
et al
. 2000). White mycelia surround
the sporulating area whilst the outer region of the lesion is
composed of softened rind. White mycelia appear on the
surface of the rind and after it reaches approximately
2.5 cm in diameter, olive green spores are produced
(Syngenta 2007). The entire fruit is soon encompassed by
a mass of olive green spores, which are easily dispersed by
any physical motion or air currents (Brown 2003a). If the
relative humidity is low, decayed fruit become soft and
shrink. If the relative humidity is high, fruit become soft
and decompose due to opportunistic moulds and bacteria
(Timmer
et al
. 2000; FFTC 2003). More mature and over-
ripe fruit also result in more decay, especially if left in the
sun after harvest (Howard 1936).
Control
Preventative measures
Control measures of green mould are to keep conidial
inoculum levels low and careful handling of fruit during har-
vest and post-harvest (FFTC 2003). Stringent sanitary prac-
tices (i.e. removal and repacking of diseases fruit in a remote
area that has been cleaned) must be enforced in order to
limit airborne spore populations (Rada 2009). The pallets,
pack house, packing line and washer brushes should be san-
itized daily to eradicate inoculum accumulation. Aqueous
solutions in drenchers and soak tanks should be treated con-
tinuously with a sanitizer (i.e. chlorine) to prevent the accu-
mulation of green mould inoculum (Brown 2003a). Exhaust
fans must be used to remove mould spores from pack houses
and dumping areas must be where the air flow will not carry
the mould spores back to the packing area (Rada 2009).
Biological control
Two species of
Candida
assessed for control against
P. digitatum
showed protective rates of up to 80% due to
the fact that these yeasts actively increase and persist in
wounds and on the surfaces of citrus fruit (Taqarort
et al
.
2008). Isolates of
Pseudomonas
spp., cultured from the
fruit and leaf surfaces of citrus, in combination with a hot
sodium bicarbonate treatment could provide a practical
alternative for green mould control (Zamani
et al
. 2008).