Agriculture Reference
In-Depth Information
and navel oranges and in grapefruit. Black spot causes
substantial economic losses in all major citrus producing
countries subject to summer rainfall. Countries in which
CBS occurs include Argentina, Brazil, Peru, Venezuela,
Uruguay, summer-rainfall regions of South Africa, Nigeria,
Uganda, Kenya, Zimbabwe, Swaziland, Mozambique,
India, China, Hong Kong, Taiwan, Japan, Philippines and
coastal regions of Australia (Meyer
et al
. 2006). The dis-
ease is absent in Mediterranean regions subject to winter
rainfall (i.e. Europe) and the state of California, United
States (Kotze 1996; FFTC 2003).
mainly present on dead leaves but may also be observed on
fruit. The fungus has been intercepted in the United States
on fruit from several countries, however, only the pycnidi-
ospores form on the fruit and these are not airborne.
Therefore, the risk of CBS spread on fruit is relatively low
(Whiteside
et al
. 1988).
Symptomology
Citrus black spot produces a spectrum of symptoms both
on leaves and fruit and is primarily a pre-harvest fruit
disease (Timmer
et al
. 2000). However the disease can
lead to significant post-harvest losses. Fruit sometimes
show no visible symptoms when picked but later develop
the characteristic red spot lesions of the disease. This stage
of symptom development starts during storage or in retail
outlets, depending on temperature and humidity condi-
tions. Therefore, citrus fruit from areas affected by this
latent infection represent a risk for introduction of
G. citri-
carpa
into disease-free areas (Agostini
et al
. 2006). This
resulted in the European Community and the United States
regulating importation of fresh citrus fruit from countries
where the disease is prevalent (Baayen
et al
. 2002). These
regulations may restrict potential market access for mostly
southern hemisphere countries and reduce the availability
of the fruit for northern hemisphere consumers in the
off-season.
Morphology
Ascocarps are aggregated and globose (100-175 μm). Asci
are clavate-cylindrical, bitunicate and eight-spored.
Ascospores are hyaline, aseptate, multiguttulate, cylindri-
cal and swollen in the middle (4.5-6.5 × 12.5-16 μm).
Conidia are obobate to elliptical, hyaline, aseptate and
multiguttulate with a colourless subulate appendix (5.5-7 ×
8-10.5
m
m) (Timmer
et al
. 2000).
Disease cycle and epidemiology
The citrus black spot fungus has two sexual stages.
Guignardia citricarpa
is the teleomorph (sexual) stage
during which ascospores are produced and
Phyllosticta
citricarpa
(asexual) stage during which conidia or pycnidi-
ospores are produced (Kotze 1962). The disease life cycle
starts with infected fallen leaves on the orchard floor
(Magarey & Borchert 2003). Ascospores are the major
source of inoculum, whilst conidia are a minor source
(McOnie 1964, 1967). Infection by ascospores occurs in the
presence of moisture when spores germinate and produce
appressoria (Obagwu 2003). A thin infection peg penetrates
the cuticle and expands, forming a small mass of mycelia
between the cuticle and epidermal wall (McOnie 1967).
This stage represents a latent infection. In some countries
the citrus trees have evergreen leaves and in others, season-
ally, which can have an effect on the availability of inocu-
lum. The most critical period for infection occurs at fruit set
and can persist for the first few months after petal fall. Early
symptoms of black spot are not evident for more than six
months after fruit set (Kotze 1981). Symptom expression
can also appear post-harvestly as typical red spot.
Control
Preventative measures
In areas where citrus is produced mainly for processing,
disease control is aimed at reducing the inoculum source,
thereby eliminating the possibility of premature fruit drop.
Since the disease is mainly cosmetic and does not affect the
internal characteristics of the fruit or the quality of the
juice (Ayres 2001) symptomatic fruit can be used for pro-
cessing. Once the pathogen is introduced into the orchard,
eradication is almost impossible. Therefore a preventive
disease control approach is important to ensure that the dis-
ease does not spread to CBS free production areas.
Preventive measures include:
• Nursery inspections and certification schemes.
• Access control for people, vehicles, machines and imple-
ments to nurseries and orchards.
• Washing and disinfection of vehicles, machines, equip-
ment and material used for harvesting.
• Maintaining plant health and ensuring good nutritional
and health conditions.
• Inspecting groves frequently.
• Elimination of plants in an advanced state of decline.
Transmission
Guignardia citricarpa
conidia are dispersed naturally over
short distances. Both spore types are dispersed with the aid
of moisture and air currents (Kotze 1962). Whilst the
conidia may cause infection, the primary source of infec-
tion are the ascospores. Dark brown-black pycnidia are
