Agriculture Reference
In-Depth Information
for human use. Biological control systems include various
modes of action such as: production of antibiotics (Fravel
1988), competition for nutrients and space (Janisiewicz
et al
. 2000), induction of host resistance (Droby
et al
.
2002; Poppe
et al
. 2003), synthesis of phytoalexins and/or
the accumulation of an extra-cellular matrix (Janisiewicz
1988; Lima
et al
. 1998; Chan & Tian 2005), siderophore
production and direct interaction with the pathogen
(Neilands 1981; Schwyn & Neilands 1987; Buyer
et al
.
1989) and/or volatile production (Fravel 1988). During the
last decade research on citrus, biocontrol focused on
microorganisms colonising the wound site and competing
with pathogens for nutrients.
In citrus, several bacterial antagonists such as
Bacillus
spp
. have been reported to reduce post-harvest decay
(Huang
et al
. 1992; Obagwu & Korsten 2003).
Bacillus
subtilis
tested by Yang and Ye (2006) showed that the
higher concentration inhibited the incidence of rot on
navel orange fruit better than the lower concentrations.
A formulated product of
Pantoea agglomerans
in combi-
nation with heated sodium bicarbonate solutions also
showed excellent decay control in mandarins and oranges
inoculated with both
P. digitatum
and
P. italicum
. This
treatment showed no rind injuries or residues on treated
fruits (Torres
et al
. 2007). Two biological control strains
of
Serratia plymuthica
suppressed
P. digitatum
and
P. italicum
when used in combination. These two strains
showed different modes of action against the pathogens
(Meziane
et al
. 2006).
Commercial testing of yeasts to control post-harvest dis-
eases on citrus fruit, have also been evaluated in combina-
tion with conventional fungicide treatments at lower rates.
The yeast
Kloeckera apiculata
could effectively control the
decay of several citrus cultivars and did not alter the quality
parameters of fruit (Long
et al
. 2007).
Candida oleophila
strain O effectively controlled
P. digitatum
in a post-harvest
application (Lahlali
et al
. 2005). Biofumigation with the
volatile-producing fungus
Muscodor albus
also showed
promising results for use in storage rooms or shipping
packages (Mercier & Smilanick 2005).
Commercial biopesticide products available for use on
citrus are
Pseudomonas syringae
(BioSave 10 LP and 11
LP) which are applied in pack houses to prevent post-har-
vest fungal diseases during storage of citrus (Stockwell &
Stack 2007), and
Candida oleophila
(Aspire) to control
Penicillium
on citrus and pome fruit. BioSave has been reg-
istered with the US Environmental Protection Agency for
plant disease suppression (Stockwell & Stack 2007) and
Aspire by Ecogen Inc. in the United States (Shachnal
et al
.
1996).
Bacillus amyloliquefaciens
(PPCB004) has been
shown to be effective for the control of post-harvest citrus
diseases (Arrebola
et al
. 2009) and a dossier has been pre-
pared for product registration in South Africa. A possible
mode of action has been reported as iturin A, which resulted
in abnormal conidial germination and germ tube develop-
ment (Arrebola
et al
. 2009).
Natural products
Plant extracts from
Withania somniferan
and
Acacia seyal
,
were evaluated as potential natural biopesticides on citrus.
Inoculated fruit did not show decay symptoms after 21
days of storage when tested against
P. digitatum
(Mekbib
et al
. 2007). Chitosan, extracted from the exoskeleton of
crustaceans, can be used as a fruit coating (Harrison 2009).
Chitosan has the mode of action to boost the ability of
plants for defence against fungal infections. This resulted
in the decrease of
P. digitatum
,
P. italicum
and
A. citri
inci-
dence on Valencia fruit and it has been suggested that it can
be commercially used for the control of post-harvest dis-
eases (Abd-El-Aziz & Mansour 2006). Other natural prod-
ucts include essential oils extracted from the epicarp of
C. sinensis
which show fungitoxicity against ten post-
harvest pathogens (Sharma & Tripathi 2006) and essential
oils of oregano, fennel, aremisia, laurel and lavender which
also showed different degrees of control against
P. digitatum
(Soylu
et al
. 2005).
Cultural and physical requirements
Cultural and physical activities represent nonchemical dis-
ease control strategies that require manipulation of the
environment in order to decrease disease pressure. In field
management systems, soil drainage improvement, use of
ridges (to allow air movement and draining during the ini-
tial phase of crop growth), use of block raising techniques
for better spacing and removal of the inoculum sources are
amongst the most prominent practices used in citrus pro-
duction (Dixon 1984).
At fruit harvesting, maximum care is required to prevent
punctures, bruises, and abrasions on fruit rind. Harvesting
by clipping reduces the possibility of inflicting wounds as
compared to pulling (Claypool 1983). Citrus fruit sub-
jected to dehydration at low relative humidity after harvest
is prone to stem-end rind breakdown, a physiological
injury which can predispose fruit to decay (Wardowski &
Brown 2001). Therefore, temperature and humidity man-
agement in the post-harvest arena is crucial to avoid dete-
rioration of produce and initiation of infection. Hong
et al
.
(2007) showed that different treatments above 50°C
effectively decrease the manifestation of stem-end rots,
mould decay and black rots on satsuma mandarins.
