Citrus - Crop Post-Harvest: Science and Technology - page 60

frequently in isolates of P. italicum than in P. digitatum
(Gutter 1975). Resistance problems can be minimized with
the use of effective sanitation procedures. Resistance can
also be minimized with the use of two or more chemically
unrelated fungicides. Sanitizing agents (i.e. hydrogen
peroxide, calcium chloride) and chitosan produced com-
mercially from chitin, were tested against P. italicum and
showed a reduction in linear growth and spore germination
(El-Mougy et al . 2008). Similar results were obtained for
P. digitatum . Other alternatives to fungicides, such as
essential oils of clove and thyme, reduced the decay per-
centage of P. italicum, to the same level as was obtained
with Imazalil (Karimi & Rahemi 2008). Azizi et al . (2008)
obtained similar results when green mould, blue mould and
brown spot were exposed to different concentrations of
essential oils of medicinal plants. In addition, a nonprotein
amino acid, ß-aminobutyric acid (BABA), significantly
reduced disease incidence and the lesion diameter of
P. italicum (Tavallali et al . 2008). This can be due to
induced resistance against the pathogen infection in orange
fruit by BABA treatment and shows promise for alternative
disease control.

Disease cycle and epidemiology
The pathogen is commonly present in soils and spreads to
fruit surfaces. As the fruit mature, it becomes more suscep-
tible to sour rot infection (Baudoin & Eckert 1982, Timmer
et al . 2000). High levels of rind moisture (such as with
fruit harvested early in the morning following irrigation or
rainfall) play an important role in enhancing conidia ger-
mination and growth. The fungus penetrates the fruit only
through injuries, particularly deep injuries that can be
caused by insects or mechanical means, such as thorn or
stem punctures or by plugging at harvest. Disease develop-
ment depends on high humidity and temperatures above
10°C, with the optimum range between 25°C and 30°C
(Timmer et al . 2000; Brown 2003c). Upon infection, the
sour odour associated with the advanced stages of sour rot
attracts flies ( Drosophila spp.), which can disseminate the
fungus and cause other injured fruit to become infected.

Transmission
Geotrichum candidum is present in soil from where it is
dispersed to fruit surfaces by air currents or water (Syngenta
2007). Higher populations of the fungus are recovered
from fruit located in the lower part of the tree, and from
fruit surfaces where soil is entrapped (i.e. scarred surfaces
or areas under the button). Fruit dropped to the ground also
contain higher fungal populations due to adhering soil
(Brown 2003c). Conidia are also removed from the fruit as
it comes in contact with the packing line, as well as any
area where water is recirculating. If high conidia levels in
the water are not controlled, all fruit is exposed and small
wounds are likely to become infected. Contaminated water
can therefore spread conidia to dip tanks, drenchers,
washer brushes, belts and to other fruit on the packing
line. This fungus can also spread by contact after pack-
ing to create a nest of infected fruit in boxes (Taverner
et al . 2009).

Biological control
Prado et al . (2008) illustrated the potential use of
Saccharomycopsis schoenii to control P. expansum , P. digi-
tatum and P. italicum . Similarly, Obagwu and Korsten
(2003) showed the effectiveness of Bacillus spp in control-
ling Penicillium decay in post-harvest treatments. Droby
et al . (2007) showed that the performance of biocontrol
agents could be increased by combination with disinfect-
ants and additives. Other physical treatments that can
contribute towards more effective control of P. italicum
include storage temperatures of 18°C or less. Blue mould
can also be inhibited with biocontrol treatments and curing
of fruit at 40°C for 18h, storing for five days at 5°C
and storing for seven days at 20°C (Brown 2003b; Nunes
et al . 2007).

Symptomology
Sour rot can cause significant losses in high rainfall years
(Mercier & Smilanick 2005). The sour rot infection has the
most unpleasant smell of all decays known and the initial
symptoms are similar to those of green and blue moulds.
The cuticle is more susceptible in comparison to the lesions
formed by Penicillium -induced moulds (Sommer &
Edwards 1992). Extracellular enzymes produced by the
fungus degrade the rind, segment walls and juice vesicles,
causing the fruit to disintegrate into a slimy, watery mass.
The lesions first appear water soaked, light to dark yellow
and slightly raised (Brown 2003c; Syngenta 2007). At high
relative humidity, the lesions may be covered with a yeasty,

Sour rot
Sour rot is caused by Geotrichum candidum Link ex Pers.
(anamorph) which is a common inhabitant of citrus soils
(Brown 2003c). Sour rot develops on fruit that is wounded
during harvesting and handling (Eckert & Eaks 1989).

Morphology
Geotrichum candidum grows rapidly on potato dextrose
agar, producing a dull grey-white colony with chains of
arthrospores (Butler & Eckert 1962). Conidia vary between
2-8 × 3-50 m m and 3-6 × 6-12 m m (Timmer et al . 2000).