Agriculture Reference
In-Depth Information
citrus trees can carry these grapefruit stem pitting strains
but only grapefruit show obvious symptoms.
The impact of grapefruit stem pitting in Australia has been
reduced by mild-strain cross protection. This involves
inoculating grapefruit trees with a mild isolate of
Citrus
tristeza virus
to protect against severe stem pitting strains.
Grapefruit budwood supplied by Auscitrus has been
pre-immunised with a mild protective isolate of
Citrus
tristeza virus
.
Orange stem pitting is only known to occur in Queensland.
There is legislation preventing the movement of citrus trees
and budwood to other states from Queensland. A cross-
protecting strain is not currently available to protect
against orange stem pitting viral strains.
Fig 6.49 Symptoms of Australian citrus dieback in leaves. Note the
yellowing in veins and leaves is similar to the symptoms of nutrient
deficiency.
Management
•
Use pre-immunised grapefruit budwood from the
Auscitrus budwood scheme.
abnormally high rainfall in spring and autumn. Disease
incidence suggests an insect vector that only occasionally
visits citrus from nearby native vegetation.
Do not take citrus budwood or trees from Queensland
into other states.
•
Importance
Grapefruit and Smooth Flat Seville orange are more readily
affected than sweet orange. Symptoms have not been reported
on rough lemon, Rangpur lime, or Eureka and Lisbon lemons.
UNDETERMINED CAUSE
AUSTRALIAN CITRUS DIEBACK
(A PHYTOPLASMA-ASSOCIATED DISEASE)
Management
•
Plant trees from nurseries that use pathogen-tested
planting material obtained from Auscitrus.
Symptoms
Season, cultivar, age and nutritional status of the tree all
affect the range and severity of symptoms of Australian
citrus dieback. Symptoms are at first localised in one
branch, with chlorotic, nutrient deficiency-like symptoms
developing in the young growth. Leaves present irregular
blotching or small round spots of green surrounded by
yellow tissue, yellowing of veins and/or yellowing of the
entire surface. Defoliation is severe on badly affected trees.
Trees decline chronically, but do not die. Development of
these symptoms is particularly severe in autumn. Fruit are
smaller on infected trees, but unlike huanglongbing, are
not distorted in shape or bitter, and unlike tristeza stem
pitting, are not lopsided. Symptoms of Australian citrus
dieback can resemble huanglongbing, root rot, tristeza
stem pitting, nutrient deficiency and nematode infestation.
Remove affected trees from the orchard.
•
SHELL BARK OF LEMONS
■
Symptoms
Trees younger than 10 years are rarely affected. Cracking
and peeling of the bark on the trunk and main limbs
occur with a variable amount of gumming. The outer bark
dies and peels off in strips but the inner bark stays alive
and replaces the outer layer. After healing, the condition
may not recur for some years. Growth may be checked and
considerable damage results when subsequent fungal
infection occurs. Symptoms of shell bark can resemble
those of collar rot (see Phytophthora in this chapter).
Source of infection and spread
The cause of this disease is unknown; however, infected
trees are associated with a phytoplasma. Graft transmission
is poor, while the spread of the disease coincides with
Source of infection and spread
The cause of this disease is unknown; however, affected
trees often carry a number of viroids that may include
Citrus exocortis viroid
.
