Agriculture Reference
In-Depth Information
Symptomology
Alternaria alternata
attacks young fruit, leaves and twigs,
producing brown-to-black lesions. Severe fruit infections,
especially shortly after petal fall, result in the drop of
young fruitlets. The remaining fruit usually have lesions
that vary in size and form (dots to large, corky pock marks
on the peel). On occasion,
A. alternata
is able to penetrate
the citrus rind and cause localized necrosis, but this is rela-
tively rare (Dewdney & Timmer 2009a).
Ascospores are hyaline, slightly curved and nonseptate
(3.5-5 × 12-22 μm) (Timmer
et al
. 2000).
Disease cycle and epidemiology
Conidia of
C. gloeosporioides
are disseminated to the fruit
surfaces during the warm, rainy summer months. Here the
conidia will germinate on the fruit surface to produce
appressoria which form germ tubes that will penetrate the
healthy rind within 12 hours (Eckert & Eaks 1989). The
appressoria are highly resistant to some commonly used
fungicides and can remain dormant for many months until
the fruit is susceptible for infection (Taverner 2003). Early
season cultivars can develop significant high levels of
anthracnose symptoms following ethylene de-greening
which stimulates the remaining appressoria to germinate.
However the decay will not spread from infected to healthy
fruit in packed containers (Ritenour
et al
. 2003).
Control
Preventative measures
Planting with disease-free nursery trees helps prevent
brown spot. Even though conidia are air-borne, they tend
not to travel long distances, therefore plantings of healthy
trees can remain healthy for long periods. Highly suscepti-
ble cultivars should be planted at a wider spacing to pro-
mote rapid drying of the canopy and make the disease more
manageable. It is also important not to promote excessive
vegetative growth by over watering or excessive nitrogen
fertilization (Dewdney & Timmer 2009b).
Transmission
Colletotrichum gloeosporioides
is a saprophyte that grows
on dead plant parts in the citrus grove. During rainfall,
spores are produced and dispersed by air currents and rain
to developing fruit (Eckert & Eaks 1989).
Chemical control
Fungicides are the primary means of controlling brown
spot. The preferred method to time fungicide sprays is with
the use of a weather-based model such as the one used in
Florida (the ALTER-RATER). The model is based on the
fact that brown spot is most severe when rainfall is greater
than 2.7 mm, daily leaf wetness duration exceeds 10 h and
average daily temperature is between 20°C and 28°C
(Dewdney & Timmer 2009b).
Symptomology
The fungus is usually present, but some seasonal factors,
i.e. increased fruit sensitivity and high levels of ethylene
during de-greening, stimulate symptom expression
(Taverner 2003). Disease symptoms on uninjured de-
greened fruit start as a silvery grey and leathery decay on
the stem-end or on the side of the fruit (Eckert & Eaks
1989). The same degree of firmness and elevation is
retained as the adjacent healthy rind. The bruised or injured
rind becomes brown to reddish brown or black spots
(1.5 cm or more in diameter) appear. The decay may appear
firm and dry or, if sufficiently deep-seated, may cause the
fruit to become soft. Under high humidity, masses of pink
or salmon-coloured conidia develop on the lesion surface
(Taverner 2003). Under drier conditions, these conidia
appear brown or black. 'Anthracnose tear staining' mainly
occurs in grapefruit when cold water (from heavy dews)
runs down the surface of fruit, injuring the rind (Barkley
2003).
Anthracnose
Anthracnose is caused by the fungus
Colletotrichum gloe-
osporioides
(Penz.) Penz. & Sacc. in Penz. The expression of
this disorder is commonly associated with the use of ethylene
during the de-greening of citrus (Taverner 2003). Anthracnose
can be an important pathogen on Robinson tangerine and
certain other tangerine hybrids such as Fallglo and Sunburst
that are harvested early in the season and that usually require
long periods of ethylene de-greening to give the fruit an
attractive appearance (Eckert & Brown 1986). The disease is
most severe on the more poorly coloured fruit following de-
greening. Anthracnose is, however, a minor problem on
oranges, grapefruit and lemons (Eckert & Eaks 1989).
Control
Preventative measures
Good cultural practices that produce healthy trees with
minimal amounts of deadwood, or removal of deadwood
by pruning can aid in the control of anthracnose. Regular
Morphology
The fungal colony varies from white to grey or black.
Acervuli are erumpent and superficial and 90-270 μm in
diameter. The conidia are oval or oblong, (10-16 × 5-7 μm).
