November 5, 2004
From: ProMED-mail<promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes [edited] <http://www.apsnet.org/>

First report of lettuce big-vein virus and Mirafiori lettuce virus in Chile
I. M. Rosales, P. Sepulveda, and A. Bruna, Instituto de Investigaciones Agropecuarias (INIA), Centro Regional de Investigacion La Platina, Casilla 439/3 Santiago, Chile. Plant Dis. 88:1286, 2004; published on-line as D-2004-0914-01N, 2004. Accepted for publication 11 Aug 2004.

Lettuce big-vein disease (BVD), a species in the genus _Varicosavirus_, is a serious virus disease of lettuce. Recent evidence has brought into question the role of Lettuce big-vein virus (LBVV) in the etiology of BVD, and suggested that Mirafiori lettuce virus (MiLV), a species in the genus _Ophiovirus_, and not LBVV is the causal agent of BVD (1,2). Lettuce plants (_Lactuca sativa_ L.) with symptoms similar to those of BVD were observed during the winter of 2003 in open-field and hydroponic-grown lettuce plants located in the Chacabuco Province of central Chile.

Symptomatic plants exhibited leaves with chlorotic vein banding that became ruffled and distorted. Symptoms were usually accompanied by reduced plant size and absence of head formation. Roots from symptomatic plants were analyzed by light microscopy-acid fuchsin staining. Zoosporangia and resting spores of _Olpidium brassicae_ were identified on the basis of their morphology and structure.

Additionally, soil transmission experiments were performed with 50 healthy lettuce seedlings replanted into contaminated soil collected from lettuce fields having symptomatic crops. After 3 weeks, one-half of the seedlings
showed differing degrees of big-vein symptoms, and the presence of spores of _O. brassicae_ was confirmed in the roots by light microscopy. Seedlings raised in sterilized soil showed no symptoms after the same period of time.

On the basis of nucleotide sequences of LBVV and MiLV from the GenBank database, primers specific to the coat protein genes of each virus were designed as follows:
MiLVV-CP1: 5(prime)-CAAATCTGTCCACAATTCC-3(prime);
MiLVV-CP2: 5(prime)-TCTCACTTGAAAACCTTCC-3(prime);
MiLVV-CP3: 5(prime)-TTGCAACGTGATGAAACC-3(prime);
MiLVV-CP4: 5(prime)-AAAGAAGAGAAGCCTGTTCC-3(prime);
LBVV-CP1: 5(prime)-AAGCTTTCCGTACTGTCC-3(prime);
LBVV-CP2: 5(prime)-CCTTGATACAGTTTTTGACC-3(prime);
LBVV-CP3: 5(prime)-GTATGCTGATTTCTGTAGACC-3(prime);
LBVV-CP4: 5(prime)-TAGATGTTCTTCGCCACC-3(prime).

The primers were used in reverse transcription-polymerase chain reaction (RT-PCR) assays with dsRNA as a template. Amplicons of the expected size were obtained in each of the 5 symptomatic plants analyzed using each of the designated primer sets: MiLVV-CP1/MiLVV-CP2: 562 bp; MiLVV-CP3/MiLVV-CP4: 743 bp; LBVV-CP1/LBVV-CP2: 485 bp; and LBVV-CP3/ LBVV-CP4: 570 bp. No amplicons were obtained from healthy lettuce plants.

The identity of both viruses was verified by cloning and sequencing of the amplicons. Nucleotide sequences were compared with those in the GenBank database. Sequences derived from the Chilean isolates resulted in identities of 87 to 97 percent for MiLV and 97 to 99 percent for LBVV. All samples analyzed were from the Chacabuco Province, where 43 percent of the lettuce crops in Chile are grown. Thus, the impact that BVD may have on lettuce availability for local consumption may be significant.

To our knowledge, this is the 1st report of Lettuce big-vein disease and Mirafiori lettuce virus infecting lettuce, and the 1st report of BVD, in Chile.

References:
(1) H. Lot et al. Phytopathology 92:288, 2002.
(2) P. Roggero et al. Eur. J. Plant Pathol. 109:261, 2003.

[The gist of this piece is that MiLV, not LBVV, causes big-vein symptoms in lettuce. Moreover, LBVV apparently plays no etiological role in disease expression and does not affect the outcome of MiLV infection. LBVV and MiLV
are vectored by the soil-inhabiting fungus _Olpidium brassicae_. LBVV virions are fragile, somewhat rigid rods (320-360 nm long). Those of MiLV are highly kinked filaments, about 3 nm in diameter, that form masses of 2
classes of particles differing in length that appear to be closed circles, suggestive of tenuiviruses.

A recently described virus, Ranunculus white mottle virus, is similar to known tenuiviruses such as citrus psorosis-ringspot virus (CPsV) and tulip mild mottle mosaic, members of the genus _Ophiovirus_, family _Bunyaviridae_. Comparison of the core RNA-dependent RNA-polymerase (RdRp) motifs of negative-stranded RNA viruses supports grouping CPsV, Ranunculus white mottle virus (RWMV), and MiLV in the _Ophiovirus_, creating a
monophyletic group separated from all other negative-stranded RNA viruses.

Disease management basically requires strict measures to kill the vector in the plastic growing trays. Other strategies include developing resistant cultivars using a wild lettuce (_Lactuca virosa_) in crosses with cultivated lettuce and a range of biological control agents.

Additional links: <http://www.hri.ac.uk/site2/research/path/virus/virus2.htm>
<http://www.apsnet.org/phyto/pdfs/2002/0118-01R.pdf>
- Mod.DH]