Download Lettuce chlorosis virus

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Lettuce chlorosis virus
IDENTITY
Name: Lettuce chlorosis virus
Taxonomic position: Viruses: Closteroviridae: ?Crinivirus
Common names: LCV (acronym)
Notes on taxonomy and nomenclature: Duffus et al. (1996) have partially characterized Lettuce chlorosis virus (LCV)
as a new distinct member of the Closteroviridae family. In particular, it differs from Lettuce infectious yellows crinivirus
(LIYV; EPPO/CABI, 1996) in its host range, insect transmission and serology. LCV is transmitted by whiteflies but it
apparently has a monopartite genome. Within the family Closteroviridae, the genus Closterovirus was proposed to
include monopartite members which are aphid-transmitted and the genus Crinivirus was proposed to include bipartite
and whitefly-transmitted viruses (Wisler et al., 1998). Whitefly-transmitted but monopartite, LCV has not yet been
assigned to a particular genus.
EPPO computer code: LCV000
EPPO A1 list: No. XXX
EU Annex designation: not specifically included, but could be covered by the broad category of viruses transmitted by
non-European populations of Bemisia tabaci.
HOSTS
The disease caused by LCV has been observed on lettuce (Lactuca sativa) and sugarbeet (Beta vulgaris). Several other
crops (e.g. Daucus carota, Spinacia oleracea, Nicotiana tabacum) and weeds (e.g. Conyza canadenis, Helianthus
nutalli, Lactuca serriola, Malva parviflora, Physalis wrightii, Rumex crispus, Solanum elaeagnifolium) are mentioned
as host plants of the virus. In field trials, all tested lettuce cultivars (cvs Acacia, Empire, Gilaben, Fallgreen, New
Dominion, Winterhaven) were found susceptible. It must be stressed that Cucurbitaceae are not host plants, in contrast
to LIYV (Duffus et al., 1996 ; McLain et al., 1998).
GEOGRAPHICAL DISTRIBUTION
LCV occurs in USA, in the southern deserts of California (Imperial Valley).
EPPO region: Absent.
North America: USA (California).
EU: Absent.
Distribution map: none.
BIOLOGY
Since 1981, in the desert regions of southwestern USA, lettuce infectious yellows crinivirus (LIYV) has been a limiting
factor of several vegetable crops (lettuce, sugar beet and cucurbits). Bemisia tabaci biotype B was then introduced and
these new populations were much poorer vectors of LIYV. In addition, they were so destructive to melons planted in
autumn that virtually no melons have been planted since 1992. This has resulted in a very low incidence of LIYV in the
period 1991-1994. Since 1991, yellowing symptoms on weeds, lettuce and sugar beet crops revealed a number of virus
isolates that did not react to antiserum used against LIYV. It was found that these virus isolates belonged to a new and
distinct member of the Closteroviridae which was partially characterized and called lettuce chlorosis virus (Duffus et
al., 1996). LCV is transmitted by B. tabaci biotypes A and B.
DETECTION AND IDENTIFICATION
Symptoms
Symptoms of LCV are virtually identical to those caused by LIYV. On lettuce and sugarbeet, symptoms are
characterized by interveinal chlorosis, stunting, rolling and brittleness of affected leaves. Affected lettuce plants show
stunted heads.
Morphology
The virus has been purified and virions are flexuous filamentous particles of variable length. Duffus et al. (1996)
reported an average length of 750-800 nm with a diameter of approximately 12 nm and McLain et al. (1998) reported a
similar range of 750-950 nm. It is estimated that the virus has a capsid protein of 32kD (Wisler et al., 1997). A single
high molecular weight dsRNA has been found. LCV probably has a monopartite genome (McLain et al., 1998)
Detection and inspection methods
Serological tests using polyclonal antisera can be used to identify LCV (ELISA and Western blot). This virus can be
inoculated with B. tabaci to indicator plants such as Chenopodium capitatum, C. murale, Nicotiana benthamiana
(Wisler et al., 1997, McLain et al., 1998).
MEANS OF MOVEMENT AND DISPERSAL
LCV is efficiently transmitted by both A and B biotypes of B. tabaci (whereas LIYV is very inefficiently transmitted by
the B biotype). Virus-vector relationships were studied (Duffus et al., 1996). LCV is transmitted in a semipersistent
manner. A minimum acquisition period of 1 h on an infected plant has been observed, but the virus is transmitted with
more efficiency after a longer feeding period (e.g. 84 %, 96% efficiency after 24h, 48 h acquisition period, respectively).
The persistence of LCV in Bemisia was determined. The insects retained the virus for 4 days and they remained rather
efficient vectors during 3 days. LCV could not be transmitted mechanically.
PEST SIGNIFICANCE
Economic impact
Natural infections of sugarbeet (Beta vulgaris) were reported in California (US) in 1997 (Wisler et al., 1997).
Apparently, the disease occurred in a limited area. The report concerned two sugarbeet fields in Imperial county which
had shown yellowing symptoms during winter 1995-1996. However, there is no data on damage or losses. Data is also
lacking on the impact of LCV on lettuce crops. Field trials were conducted on various lettuce cultivars (McLain et al.,
1998). Lettuce plants showing pronounced LCV symptoms had a significantly reduced mean head weight (486 g for
diseased lettuces compared to 697 g for healthy ones). However, asymptomatic virus-infected plants show little yield
losses. In this trial, it was difficult to conclude whether yield losses were due to the presence of the virus, as infected
plants were also infested by B. tabaci.
In California, it appears that LCV has gradually replaced LIYV. Wisler et al. (1998) noted that LIYV is still present
in California but at a very low incidence since 1992 (less than 0.1 %). It seems that LCV also occurs at a rather low
incidence and that it is not considered as a significant problem. This is probably because weeds are largely eliminated
and insecticides (e.g. imidacloprid) are widely used to control whiteflies. However, it is stressed that any changes in
cropping patterns (e.g. re-introduction of late summer cucurbit crops for LIYV), whitefly vector populations and
pesticide use could result in a serious future outbreak of either virus at any time (Wisler et al., 1998).
Control
The control of the disease is essentially based on the reduction of whitefly populations and the removal of weeds which
act as major reservoirs for LCV (Duffus et al., 1996).
Phytosanitary risk
Lettuce is an important crop in Europe both outdoor and under protected conditions, and sugar beet is also an important
field crop. B. tabaci, the vector of the disease is present in many countries of the EPPO region. LCV could present a
threat particularly to field crops in southern Europe and protected crops in northern Europe. However, data on the extent
and severity of the disease is lacking and under the current crop and population vector situation prevailing in California,
it appears that the disease is not currently considered as a major problem.
PHYTOSANITARY MEASURES
The main pathway of entry of LCV is probably through viruliferous B. tabaci. Therefore host plants of B. tabaci from
areas where LCV occurs should come from a place of production free from this virus and B. tabaci (or treated against B.
tabaci) during the last growing season.
BIBLIOGRAPHY
Duffus JE, Liu HY, Wisler GC, Li R (1996) Lettuce chlorosis virus - A new whitefly-transmitted closterovirus. European Journal of
Plant Pathology, 102(6), 591-596.
EPPO/CABI (1996) Lettuce infectious yellows 'closterovirus'. In: Quarantine pest for Europe. 2nd edition (ed. Smith IM, McNamara
DG, Scott PR, Holderness M). CABI International, Wallingford, UK.
McLain J, Castle S, Holmes G, Creamer R (1998) Physicochemical characterization and field assessment of lettuce chlorosis virus.
Plant Disease, 82(11), 1248-1252.
Wisler GC, Duffus JE, Gerik JS (1997) First report of lettuce chlorosis virus naturally infecting sugar beets in California. Plant
Disease, 81(5), p 550.
Wisler GC, Duffus JE, Liu H-Y, Li RH (1998) Ecology and epidemiology of whitefly-transmitted closteroviruses. Plant Disease,
82(3), 270-279.