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 Introduction
 Host range and symptoms
 Diagnostic tests
 Figures






Group Title: Plant pathology fact sheets
Title: Tristeza. Plant pathology fact sheet SP-155.
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Permanent Link: http://ufdc.ufl.edu/UF00066806/00001
 Material Information
Title: Tristeza. Plant pathology fact sheet SP-155.
Series Title: Plant pathology fact sheets
Physical Description: Book
Creator: Roberts, P. D.
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 2001
 Notes
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00066806
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Table of Contents
    Introduction
        Page 1
    Host range and symptoms
        Page 2
    Diagnostic tests
        Page 3
    Figures
        Page 4
        Page 5
Full Text



Plant Pathology Fact Sheet


Tristeza

P.D. Roberts, R.J. McGovern, R.F. Lee, and C.L. Niblett, Respectively, Assistant
Professor, Southwest Florida Research and Education Center, Immokalee, Fl
34120; Associate Professor, Gulf Coast Research and Education Center,
Bradenton, Fl 34203; Professor, Citrus REC- Lake Alfred, Fl 33850 and Professor
Plant Pathology Department, University of Florida, Gainesville, Fl 32611. Re-
vised April 2001
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Causal Agent and Epidemiology


Tristeza, caused by the citrus tristeza vi-
rus (CTV), is one of the most important factors
limiting citrus production worldwide. The vi-
rus originated in Asia and has been spread by
man to most citrus producing areas. Devastat-
ing epidemics have occurred in the western
hemisphere beginning with Brazil and Argen-
tina in the 1930s, when both severe CTV strains
and the efficient aphid vector, Toxoptera citricida,
were introduced by importation of living cit-
rus material. Tristeza was first reported in
Florida in the early 1950s, but serious losses
were not experienced until the 1980s after CTV
strains causing decline (CTV-D) on sour orange
rootstock had become widespread. The epi-
demic of CTV-D on sour orange rootstock in
Florida increased with the arrival of T. citricida
in 1995 and has effectively eliminated sour or-
ange as a rootstock for Florida citrus.

It is essential that steps be taken to pre-
vent the introduction and spread of more dam-
aging CTV strains such as those that cause stem-
pitting of grapefruit, lime, and sweet orange sci-
ons regardless of the rootstock. The presence
of such strains could severely limit production
of grapefruit, lime and vigorous sweet orange
varieties such as Hamlin and pineapple.


Citrus tristeza virus, a member of the
closterovirus group, is the largest known plant
virus, having the shape of a long flexuous rod
about 11 X 2,000 nm in size (Figure 1). The ge-
netic material of the virus consists of a single
strand of ribonucleic acid (RNA) which is en-
closed by a protein coat. Virus spread occurs
through use of CTV-infected budwood and by
aphids. CTV is not transmitted in seed, and it
is unlikely that it moves in the field on contami-
nated tools and equipment.

CTV is transmitted by several aphid spe-
cies in a semi-persistent manner. Aphids may
acquire and transmit the virus during short
feeding probes, but longer feeding times (up
to 24 hours) increase the efficiency of virus
transmission. Viruliferous aphids are capable
of transmitting CTV for 24 hours after its acqui-
sition. Four aphid vectors of CTV are present
in Florida: 1) the melon aphid, Aphis gossypii,
which is yellowish-green to blackish-green; 2)
the spirea aphid, A. spiraecola, which is light
green; 3) the black citrus aphid, Toxoptera
aurantii, which is shiny black; and 4) Toxoptera
citricida, commonly called the brown citrus
aphid, which is also shiny black. T. citricida can
an be differentiated from T. aurantii by anten-
nae color and wing veination using a 20x hand
lens (Figure 2). T. citricida was first found in
Florida in 1995 and has spread throughout the
state. T. citricida is the most efficient aphid vec-


SP-155







tor of CTV, about 20 times more effective in vi-
rus transmission than A. gossypii, the next most
efficient vector. Even more alarming is the dem-
onstrated ability of T. citricida to transmit se-
vere stem pitting CTV strains that are not
readily transmissible by the other three aphid
vectors. In addition, T. citricida reproduces very
rapidly and reaches much higher populations
than other vectors of CTV.

The introduction of stem-pitting CTV
strains now, in the presence of T. citricida, could
have the same disastrous impact on Florida cit-
rus production that occurred in several South
American countries.


Host Range and Symptoms

CTV infects almost all citrus species,
hybrids, and relatives. Many strains of the vi-
rus exist and contribute to the diversity of
symptoms associated with CTV infection. Mild
strains of CTV produce essentially no notice-
able symptoms in their hosts. Severe strains
produce a range of symptoms including one or
more of the following: seedling yellows; decline
on sour orange rootstock; stem-pitting of grape-
fruit; and stem-pitting of sweet orange. The
CTV-D strains have eliminated new plantings
using sour orange rootstock and is rapidly
eliminating groves planted with sour orange
rootstock in Florida. Trees on sour orange ex-
hibiting decline are routinely "pushed" or re-
moved from production, causing major eco-
nomic losses. Rootstock reactions to CTV-D
strains presently found in Florida vary from
highly susceptible, resulting in tree decline and
death (sour orange and Citrus macrophylla), to
tolerant, wherein no noticeable effect on tree
growth is expressed (Carrizo citrange,
Cleopatra mandarin, Swingle citrumelo, etc.),
or resistant, wherein the virus cannot multiple
in the rootstock (Trifoliate orange). The CTV-
D strains cause death of the phloem at the bud
union. Phloem death produces a girdling ef-
fect which may result in overgrowth of the scion


at the bud union, paucity of feeder roots, stunt-
ing, yellowing of leaves, reduced fruit size,
poor growth, dieback, wilting, and death. De-
clining trees on sour orange rootstock may also
exhibit pin holing or honeycombing on the in-
ner face of the bark, or brown discoloration at
the bud union.

However, the most virulent and damag-
ing CTV strains are those causing stem-pitting
(deep pits in the wood under depressed areas
of bark) in the trunk, branches and twigs of sci-
ons regardless of rootstock, resulting in trees
with low vigor. Twigs on infected trees are
brittle and break easily when intentionally bent
or blown by the wind under fruit load. Stem-
pitting may not be apparent until the bark is
peeled from twigs (Figure 3). Rope-like exter-
nal symptoms can also be cause by stem-pit-
ting strain in trunks of grapefruit and/or sweet
orange scions (Figures 4, 5). Stem-pitting of
scions due to CTV results in decline but does
not usually cause tree death. The economic
impact of these strains results from reductions
in fruit set, size, and quality (Figure 6). Stem-
pitting strains are transmissible by T. citricida
but not usually byA. gossypii, A. spiraecola, or T.
aurantii. These CTV strains commonly occur in
Asia, Australia, Southern Africa, Brazil, Colum-
bia, Venezuela and other areas.

Stem-pitting strains have not yet been
identified in Florida, however a severe strain
of CTV was detected in dooryard plantings and
commercial plantings of Meyer lemon. The
strain from Meyer lemon caused moderate to
severe stem-pitting on grapefruit in biological
assays. Imported citrus material may harbor
CTV strains capable of causing stem-pitting, but
their presence may not be obvious if the intro-
duced material is tolerant to the virus, such as
lemon or one of the mandarins. There is a pos-
sibility that stem-pitting strains of CTV may
have been unknowingly imported into Florida
before the Florida Budwood Registration Pro-
gram was established in 1952 by the Division
of Plant Industry (DPI) of the Florida Depart-







ment of Agriculture and Consumer Services, or
through illegal importation thereafter. Al-
though such strains have not yet been detected,
the movement of such "sleeping" severe strains
would be greatly accelerated now that T.
citricida is present.


Diagnostic Tests

Mexican lime has been the standard in-
dicator bioassayy plant) used to confirm the
presence of CTV. However, it is not useful for
differentiation of seedling yellows, decline on
sour orange, or stem-pitting. The enzyme-
linked immunosorbent assay (ELISA) is a rapid
laboratory test that detects CTV in plant tissue
by means of antibodies reactive with the virus.
The use of polyclonal antibodies enables de-
tection of all CTV strains. A more selective
monoclonal antibody has recently been devel-
oped which detects some severe CTV strains
such as those causing decline on sour orange
rootstock or stem-pitting.

However, there is currently no rapid
method to identify and differentiate the strains
that cause stem-pitting on grapefruit and sweet
orange. Differentiation of CTV strains relies on
graft transmission to a battery of bioassay plants
including Mexican lime (reactive to most
strains), a sweet orange scion grafted onto sour
orange (for seedling yellows), Madam Vinous
(for stem-pitting on sweet orange), and Duncan
grapefruit (for stem-pitting on grapefruit) seed-
lings. The biological assays for stem-pitting
require 10 to 12 months under controlled green-
house conditions. Development of additional
diagnostic tests to differentiate CTV strains on
a more timely basis using strain-specific anti-
bodies, nucleic acid probes, and polymerase
chain reaction protocols is under way.


Management

Management strategies for CTV are
based on the presence and incidence of the vi-
rus and its vectors. Quarantine against impor-
tation of CTV conducted by DPI is important
in preventing the introduction of new severe
strains, such as those which cause stem-pitting.

The Florida Budwood Registration Pro-
gram maintains and makes available to nurs-
erymen tested virus-free budwood for use in
citrus propagation. Only budwood from trees
registered as virus-free should be used for
propagation. Unfortunately, budwood trees
grown in outdoor increase blocks are largely
infected already with CTV and some of the reg-
istered sites can no longer provide budwood.
Trees grown in screenhouses or with other
physical barriers that exclude aphids can en-
sure virus-free budwood. Currently, five com-
mercial nurseries and two agencies (DPI and
the Florida Citrus Foundation) produce virus-
free budwood in greenhouses in Florida. Their
capacity at present enables only limited quan-
tities of budwood.

Removal of declining trees on sour or-
ange and replacement with trees budded on a
tolerant rootstock is recommended when a tree
is no longer economically viable. Replacement
trees should be virus-free. Whether to remove
and replace the declining trees yearly or to re-
place an entire block at once is dependent upon
each individual situation. The decision must
be consistent with the production goals, eco-
nomic considerations, and management prac-
tices of each unique citrus producer.

Suppression and elimination of severe,
stem-pitting CTV strains is extremely impor-
tant to prevent and delay their distribution.
Production managers and grove workers
should constantly monitor trees for infection by








stem-pitting strains. Trees exhibiting symp-
toms of stem-pitting should be reported to DPI
in Gainesville for confirmation and appropri-
ate action.

Mild strain cross protection may be
implemented immediately to help prolong the
economic life of groves which are exposed to
severe strains of CTV. Mild strains of CTV have
been selected which reduce the damage by se-
vere CTV strains on sour orange rootstocks.
Joint research with international scientists in-
dicated that some of these same mild CTV
strains may provide cross protection against
stem-pitting strains of the virus. Intentional
introduction of these known and well-charac-
terized mild CTV strains into propagative ma-
terial would, over a period of several years,
result in their widespread prevalence. Prolif-
eration of these mild strains would thus lessen
the probability that severe CTV strains could
be acquired and transmitted by aphid vectors.


Progress is also being made on the in-
troduction of virus resistance into desirable cit-
rus varieties. Immunity to CTV present in
Poncirus trifoliata has been transferred into sweet
orange type plants suitable for use as breeding
parents by conventional plant breeding tech-
niques. In addition, cells of commercial citrus
varieties have been fused with CTV-resistant
but non-sexually compatible relatives by so-
matic hybridization. This process provides an
approach to utilize resistance genes not previ-
ously available in citrus. It also is now pos-
sible to genetically change (transform) citrus
through the introduction of foreign genes. Such
genes, often from the virus itself, have been
shown to induce resistance to the virus in the
transgenic plants. While rapid progress is be-
ing made in these areas, promising plants must
be evaluated for virus resistance and horticul-
tural performance over a number of years be-
fore being used on a commercial scale. These
approaches will all aid in the future manage-
ment of CTV.

A. ANVI-.NN.1L

SV t Toxptera citricidus
SF(h.MFNTS I, II, & III

Toxoptera aurantii

B. WINGS
LIGHT PTEROSTIGMA SEGMENT
-M I
M 2
M3+4


Toxoptera citricidus


UARK PFEROSTIGMA SEGMENT


-M 3 + 2


Figure 1. Citrus tristeza virus as seen with a
transmission electron microscope after positive
staining. The bar indicates 500nm.


-A~


r T7xoptera aurantii
Figure 2. Diagnostic characteristics of Toxoptera
citricida and T. aurantii which permit field
identification using a 20x hand lens.






























Figure 3. Stem-pitting on small branches of a
Pera sweet orange tree in Brazil.


Figure 5. Stem-pitting in a grapefruit tree in
South Africa not obvious until a patch of bark
was removed.


Figure 4. Rope-like external symptoms caused
by a stem-pitting CTV strain in the trunk of a
grapefruit tree in Australia.


Figure 6. Small fruit from a grapefruit tree
infected with a stem-pitting strain of CTV
in Columbia.




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