Agriculture Reference
In-Depth Information
main branches, as well as impregnation of the wood
beneath with gum. The lesion starts as a small blister-like
area from which the bark lifts in scales. The lesion enlarges
by extension of scaling around the margins, an apparently
seasonal phenomenon. Decline of affected branches and
loss of fruit production can be associated with the blockage
of the xylem vessels with gum. Psorosis B is characterised by
rampant and rapidly expanding bark lesions, and sloughing
of large portions of bark. Fruit from trees infected by
psorosis B sometimes bear chlorotic ringspots on the rind.
stunted and the trunk below the union is reduced in
size. The disease affects
Poncirus trifoliata
, Rangpur lime
and occasionally citranges. It is carried symptomlessly
in other varieties of citrus and its effects are manifested
when budwood of infected scions is propagated on these
sensitive rootstocks. Symptoms generally do not appear
until trees are three to four years old. Rough lemon,
sweet orange and mandarin rootstocks are tolerant and
do not show symptoms when grafted with infected
budwood.
Symptoms do not appear until trees are at least eight years
old. Symptoms seen in immature leaves are transient, and
consist of faint f flecks between and parallel to lateral veins
or pale wavy lines forming an oak-leaf pattern. These
symptoms are only seen for a few days in spring or
occasionally in the autumn f flush. Foliage of trees with
psorosis B can also show persistent patterns and ringspots
in mature foliage. Ringspots can also occur on fruit.
Symptoms of psorosis B are most commonly seen within
the shaded canopy of the tree.
Accurate diagnosis of psorosis cannot be based on
symptoms alone, and requires testing with indicator plants
and/or serological or molecular laboratory methods.
Source of infection and spread
The viroid is graft transmissible, including naturally
occurring root grafting.
Citrus exocortis viroid
is also
transmitted mechanically as a contaminant on budding
knives, secateurs and hedging and pruning tools.
Importance
Scaly butt is rarely seen, because viroid-free propagating
material has been available from the Auscitrus budwood
scheme for decades.
Management
•
Source of infection and spread
Psorosis is graft transmitted, but seed transmission has
also been demonstrated. An insect vector is not known;
however, in some countries, natural spread by unknown
means has occurred.
Plant trees from nurseries that use viroid-free planting
material obtained from Auscitrus.
Treat cutting tools with 10% commercial bleach
(i.e. 1% available chlorine) to prevent mechanical
transmission (note: bleach is corrosive so treat tools
accordingly after use).
•
Importance
Psorosis is rarely seen, because pathogen-tested
propagating material has been available from the Auscitrus
budwood scheme for decades.
TRISTEZA STEM PITTING
■
Cause
Citrus tristeza virus
(Closteroviridae).
Management
•
Plant trees from nurseries that use Auscitrus budwood
and seed.
Symptoms
On grapefruit, symptoms range from deep, ropy
furrowing to fine pitting of the wood, seen by peeling
back the bark. Fine pits have a more adverse effect on tree
health. Severely affected trees can have small, lopsided
fruit, are stunted and bushy with twig dieback and yellow
mottling or nutrient deficiency-like symptoms on leaves
during growth f lushes. Symptoms often start in one
branch and spread throughout the tree. Occasionally,
trees decline rapidly.
Remove infected trees from the orchard.
•
SCALY BUTT (EXOCORTIS)
■
Cause
Citrus exocortis viroid
(Pospivoroidae).
Symptoms
The typical symptoms of scaly butt are cracking and
scaling of the bark below the bud union, with the bark
often peeling off in vertical strips. Affected trees are
The orange stem-pitting strains stunt infected trees and
reduce fruit size significantly. Leaves are distorted with the
leaf margins curled upwards and crinkled. Branches
