Citrus - Crop Post-Harvest: Science and Technology - page 64

Disease cycle and epidemiology
Alternaria citri is a saprophyte that grows on dead citrus
tissue and produces airborne conidia. The pathogen estab-
lishes a quiescent infection in the button or stylar end of the
fruit. Infection occurs mainly through growth cracks from
the stylar end. The pathogen will not colonise the fruit
from the button until senescence sets in and the fruit has
been weakened by adverse conditions in the field, during
storage or when the fruit has becomes over mature (Olsen
et al . 2000). Large numbers of conidia are produced in
infected fruit and this then becomes the survival mecha-
nism of the pathogen.

damaged fruits, especially oranges with split navels.
Preventing stress can reduce the incidence of splitting and
the occurrence of black rot. It is also important to remove
infected fruit from sites and to properly dispose of them
(Cals 2007). Delaying harvest until infected fruit have
fallen has been used as a strategy to prevent inadvertent
inclusion of infected fruit in harvested crops. However,
unaffected fruit should be harvested at optimum maturity.

Chemical control
There are no pre-harvest chemicals that are presently rec-
ommended for control and fungicide treatments are usually
ineffective. Post-harvest treatments with imazalil, 2,4-D,
or both have provided some control. The growth regulator
2,4-D delays senescence, thereby restricting colonization
of the host (Adaskaveg et al . 2008). Further studies on the
effect of different chemical treatments to control black rot
illustrated that the combined use of sodium carbonate, thia-
bendazole, 2,4-D and storage at 6°C decreased the pres-
ence of black rot (Reqaeian & Aboutalebi 2007).

Transmission
Alternaria citri produces airborne conidia which can attach
and grow on the blossom end of the fruit (Cals 2005).

Symptomology
Premature colouring and fruit drop are the best signs of
infection (Cals 2005). The rot is not always evident on the
outside of the fruit. Eventually (frequently not until after
harvest) a dark slightly sunken spot appears on the blossom
end (as opposed to the stem end) of the fruit (Olsen et al,
2000). Diseased fruit have a brown to blackish discoloura-
tion at the blossom end and the discolouration and decay
may be restricted to the blossom end or it may extend deep
into the central cavity (Olsen et al . 2000). When fruit is cut,
the infected tissue becomes soft and darkens, and the
surfaces reveal a soft, thin coating of mycelium
(Phytopathology 2009). Downy mycelium development
begins already within a few hours after having the fruit cut.
First it is white in colour, than darkens quickly as the
conidia and conidiophores are formed. This rotten spot
may eventually cover as much as one-quarter of the fruit
and the juice of the entire fruit has an unpleasant taste (Cals
2007). It is therefore an important disease for the process-
ing industries because of the juice being contaminated by
masses of black fungal mycelium found in the interior of
the infected fruit. In lemons, the disease is most common
during storage. Certain environmental factors that cause
splitting often predispose navel oranges to infection (Olsen
et al . 2000). The incidence of splitting is higher in sun-
burned fruit and in trees stressed by drought and frost
injury.

Alternaria brown spot
Brown spot is caused by Alternaria alternata Fr. (Keissler)
pv. citri and severely affects the tangerine cultivars
(Minneola, Orlando, Nova and Lee, Murcotts and Sunburst
tangerines). Minneolas are the most susceptible cultivar of
the tangerine hybrids and control is the most difficult. This
disease does not affect oranges, but may cause some spot-
ting on grapefruit if they are adjacent to heavily infested
tangerines or tangelos (Dewdney & Timmer 2009a).

Morphology
Characteristic of the genus is dark-coloured multi-celled
conidia that are produced in chains. The conidiophores are
septate and brown (Timmer et al . 2000).

Disease cycle and epidemiology
Conidia are produced on leaves 10 days after symptoms
appear, primarily on old lesions of mature leaves. Conidia
production continues for up to 50 days after infection. In
addition, conidia are produced in lower numbers on fruit
and twigs remaining on the tree. For an infection to occur,
temperatures must be favourable (20-29°C) and the length
of the wetting period about 8-10 h (Timmer et al . 2000).

Control
Preventative measures
Proper fertilization and irrigation will significantly reduce
the incidence of this disease (Cals 2005). Healthy, good-
quality fruit are more resistant to black rot than stressed or

Transmission
The release of conidia into the air is triggered by rainfall or
by a sharp change in relative humidity. Once the spores are
released, they are distributed by air currents to susceptible
tissue (Dewdney & Timmer 2009a).