Citrus - Crop Post-Harvest: Science and Technology - page 63

conidia are unicellular, hyaline and biguttualte whilst the
beta conidia or stylospores are filiform and hooked and do
not germinate (predominate in older pycnidia). On twigs
that have almost stopped producing pycnidia, perithecia
(125-160 m m) develop that are spherical and have tapered
beaks (200-800 m m long). The ascospores are hyaline; two
celled and contains two guttulae (Timmer et al . 2000).

(Sommer et al . 2002). Later season fruit are more prone to
stem-end rot than early season fruit.
Warm temperatures associated with ethylene de-
greening after harvest is more favourable for development
of Diplodia , than for growth of Phomopsis . Phomopsis
becomes more prevalent than Diplodia later in the season
when naturally coloured fruit do not require de-greening
(Brown 2003e). Also, the incidence of Phomopsis stem-
end rot will increase in fruit grown on older trees where
more inoculum is likely to be present.

Disease cycle and epidemiology
The fungus produces ascospores (sexually produced) and
pycnidiospores (asexually produced). Ascospores are
formed on decaying wood on the soil or on dead branches
are produced in relatively small numbers. Pycnidiospores
are produced abundantly on dead branches, within a
pycnidium (Kucharek et al . 2000). These spores are
transported by rainfall to the immature fruit during late
spring and summer months (Brown 2003e). On early sea-
son immature fruit, infections of the fruit surface cause
small pustules to be produced (melanose). The fungus also
colonizes dead tissue on the surface of the button. This
infection remains quiescent until after harvest after which
the fungus will invade the stem end of the fruit when the
button senesces and the fungus can enter by natural open-
ings. Symptoms usually begin to appear after 10 days.
Sporulation rarely occurs on infected fruit; therefore,
contamination of handling and packing equipment by the
fungus is not a major problem in comparison to green and
blue mould. Also, Phomopsis stem-end rot does not spread
from infected to healthy fruit in packed containers
(Brown 2003e).

Control
Stem-end rot is reduced by utilizing numerous control
measures in the field during production and during harvest
and post-harvest handling periods. As the calyx of imma-
ture fruit on the tree are also infected during periods of
melanose infection, fungicide sprays and removal of dead
wood for melanose control will contribute to Phomopsis
stem-end rot control. In the pack house the use of fungi-
cides in a post-harvest application are necessary regardless
of prior control measures (Brown 2003e). In addition, a
'reduced risk' pesticide with fludioxonil as the active
ingredient can also be applied as a dip, drench, flood or
post-harvest spray application (Syngenta 2007).

Alternaria rot
Alternaria rot usually occurs at the stem-end rot of fruit
stored for long periods of time but can also be found on the
stylar end of fruit in the orchard (imperfections at navel
infection) where it can cause premature fruit drop. Only a
small amount of rot causes a bitter taste and the black frag-
ments spoil the appearance of fruit (Timmer et al . 2000).
Black rot and brown spot are morphologically indistin-
guishable pathogens of citrus: one causes rot by macerat-
ing tissues and the other causes necrotic spots by producing
a host-specific toxin (Atsunorl et al . 2001).

Transmission
Ascospores are windborne and can easily spread over long
distances. However, the pycnidiospores are spread over
short distances within a tree or to an adjacent tree by water
(Brown 2003e).

Alternaria Black rot
Alternaria citri Ellis & N. Pierce in N. Pierce causes
Alternaria black rot. There are many strains of the patho-
gen Alternaria citri, but the strain that causes black rot is a
non-toxin-producing strain and the strains on mandarin
causing brown spot have been referred to as A. alternata
pv. citri (Adaskaveg et al . 2008).

Symptomology
In initial stages of infection, stem-end rot caused by
Phomopsis is indistinguishable from that caused by Diplodia
without isolation and culture of the causal organism (Brown
2003e). The optimal temperature for Phomopsis stem-end
rot is 23-24°C, with a minimum temperature of 10°C
(Sommer et al . 2002). Infections are softening of the rind,
with a tan or brown discolouration. The internal core of the
fruit becomes infected and turns dark. Phomopsis seldom
grows so rapidly through the core of the fruit that decay is
exhibited at both ends, as with Diplodia . Also, the advanc-
ing margin of Phomopsis lesions are even, compared to
fingerlike lobes developed at the margins by Diplodia

Morphology
On potato dextrose agar, the mycelia produced appear yel-
lowish or olivaceous hyaline. The conidia are short-clavate,
oblong and dark olive brown (15-22 × 25-40 m m) and have
four to six septa (Timmer et al . 2000).