Date: 25 March 2007
From: ProMED-mail <promed@promedmail.org>
From: Richard Hamilton <rihamilto@shaw.ca>
Source: USDA Agricultural Research Service Press release [edited] <http://www.ars.usda.gov/is/pr/2007/070212.htm>

Superior lettuces fend off 2 destructive viruses

Iceberg lettuce ranks as one of America's top-5 favorite veggies.

However, this delicately flavored, slightly sweet crisphead and its relatives -- the romaine of Caesar salads, the softer textured leaf lettuces, or the creamy butterheads like Boston and bibb -- are vulnerable to attack by an impressive array of stealthy viruses and other natural enemies.

But 5 kinds of superior iceberg lettuces, developed several years ago by Edward J. Ryder of the Agricultural Research Service (ARS), are today still holding their own against attack by 2 of these daunting

villains: big vein virus and lettuce mosaic virus. A world-renowned lettuce breeder, Ryder, now retired, did the work while based at the ARS Crop Improvement and Protection Research Unit in Salinas, California.

In 2004, the plants became the 1st publicly available iceberg lettuces to boast resistance to both diseases. That's why lettuce breeders and seed companies in California and elsewhere were quick to request samples of the tiny black seeds.

Dual-resistance enhances survival because a lettuce field can easily be besieged by both viruses at once. That's according to research horticulturist James D. McCreight, who is in charge of ARS research at Salinas.

Equipping lettuce plants with genes that enable strong, natural resistance is still the most economical, eco-friendly way to defend vulnerable plants from the viral diseases.

Lettuce big vein gets its name from the unhealthy, enlarged appearance of veins in infected lettuce leaves. These lettuces may be bushy-looking or undersized.

The likely culprit is Mirafiori lettuce big vein virus, which makes its way to lettuce roots via a soil-dwelling, fungus-like microbe.

Lettuce mosaic, caused by a virus of the same name, results in stunting and unattractive mottling. Green peach aphids can spread the virus as they move about a lettuce field, sipping plant juices.

Read more about the research in the February 2007 issue of Agricultural Research magazine <http://www.ars.usda.gov/is/AR/archive/feb07/lettuce0207.htm>.

ARS is the U.S. Department of Agriculture's chief scientific research agency.

[Byline: Marcia Wood]

--
ProMED-mail
<promed@promedmail.org>

Most isolates of Lettuce mosaic virus (LMV) can infect and are seed-borne in cultivars containing the mo1 gene. A reverse transcription and polymerase chain reaction (RT-PCR)-based test was developed for the specific detection of LMV-Most isolates. Based on the complete genome sequences of 3 LMV isolates belonging respectively to the Most type, the Common type and neither of these 2 types, 3 different assays were compared: (i) presence of a diagnostic restriction site in the region of the genome encoding the variable N-terminus of the capsid protein, in the 3' end of the genome, (ii) RT-PCR using primers designed to amplify a cDNA corresponding to a portion of the P1 coding region, in the 5' end of the genome and (iii) RT-PCR using primers designed to amplify a central region of the genome. The assays were performed against a collection of 21 isolates from different geographical origins and representing the molecular variability of LMV. RT-PCR of the central region of the genome was preferred because its results are expected to be less affected by natural recombination between LMV isolates, and it allows sensitive detection of LMV-Most in situations of single as well as mixed contamination

LMV-Common and LMV-Most are 2 seed-borne types of Lettuce mosaic virus (LMV), genus Potyvirus. LMV-Most, but not LMV-Common, overcomes the resistance afforded to lettuce by 2 recessive genes, mo1 1 and mo1 2. An RT-PCR-based assay thought to be specific for LMV-Most also amplified LMV-Tn2, previously typified as LMV-Common. The sequence of selected regions along the genome indicated that LMV-Tn2 is a natural recombinant between LMV-Most and LMV-Common isolates, with a putative recombination site located within the P3 coding region. This is the 1st evidence of a naturally occurring LMV recombinant isolate.

SYMPTOMS

Symptoms of lettuce mosaic vary greatly. Leaves of plants that are infected at a young stage are stunted, deformed, and (in some varieties) show a mosaic or mottling pattern. Such plants rarely grow to full size; head lettuce varieties infected early fail to form heads. Plants that are infected later in the growth cycle will show a different set of symptoms. These plants may reach full size, but the older outer leaves will be yellow, twisted, and otherwise deformed.

On head lettuce, the wrapper leaves often will curve back away from the head. Developing heads may be deformed. In some cases brown, necrotic flecks occur on the wrapper leaves.

COMMENTS ON THE DISEASE

There are several sources of the lettuce mosaic virus. Since the virus is seedborne in lettuce, infected seed is a primary way of introducing lettuce mosaic to fields. The virus can infect numerous crops and weeds, thereby creating reservoirs of the virus. Lettuce mosaic virus can also be vectored by aphids, which spread the virus within a lettuce field and introduce it into lettuce fields from infected weeds and crops outside the field.

MANAGEMENT

Cultural Control

A lettuce mosaic control program is a good example of an integrated way of controlling a plant disease. Plant lettuce seed that has been tested for the virus and that contains no infected seed/30 000 seed tested (Imperial and Monterey counties mandate via ordinance that only tested, clean seed can be planted in the county). Remove potential virus reservoirs (see lists below) by practicing good weed control within and outside lettuce fields, and by plowing down harvested lettuce fields in a timely manner. A lettuce-free period during winter months helps break the virus cycle (again, some counties mandate such periods via county ordinances). Good aphid management should be practiced. Some resistant varieties are available.

Lettuce big-vein is a soilborne viral disease characterized by vein clearing and leaf shrinkage, resulting in plants of poor quality and reduced marketable value. The disease commonly occurs at temperatures below 20 C, typical of temperate climates. Recently, Mirafiori lettuce virus (MiLV), a possible ophiovirus, has been found to be the causal agent of the disease, whereas the historically associated Lettuce big-vein virus (LBVV, genus Varicosavirus) causes only a latent infection (Roggero et al., 2003). No synergism has been observed between these 2 serologically unrelated viruses, although both are transmitted by the chytrid, _Olpidium brassicae_. MiLV has been detected in samples from several European countries, USA and Japan. Mixed infection by both MiLV and LBVV are common in samples from lettuce (_Lactuca sativa_) crops in northern Italy and France (Roggero et al., 2003). Molecular sequences for both viruses are now available (Sasaya et al., 2002; Van Der Wilk et al., 2002).

During recent years, typical big-vein symptoms have been observed in Sao Paulo State, Brazil on various lettuce cultivars grown both directly in soil or hydroponically. Symptoms were observed mainly during the cooler season, with day temperatures ranging from 18 to 22 C and night temperatures from 10 to 16 C. Samples collected in different areas of Sao Paulo State (Biritiba-Mirim, Embu-Guacu, Guarulhos, Itapecerica da Serra, Mogi das Cruzes, Monte Alegre do Sul, Vargem Grande Paulista) were tested by DAS-ELISA with antisera specific for MiLV and LBVV (Roggero et al., 2003) and examined by electron microscopy (EM) using negative staining for the presence of viruses. Most samples with big-vein symptoms tested positive by ELISA for both viruses and a few only for one of the 2. Using EM, only LBVV virus particles were observed; virions were observed in only a few samples, confirming the difficulty in EM visualization of both LBVV and MiLV particles. These surveys in this subtropical area of Brazil had similar findings to those in the temperate climate of Europe, where both viruses are commonly found. The presence of _O. brassicae_ was already reported in Brazil (Lin, 1979). This is the 1st report of the occurrence of lettuce big-vein disease and the associated viruses MiLV and LBVV in a subtropical area.

Most batches of lettuce seedlings taken over an 18-month period from a vegetable nursery were infested with lettuce big-vein disease (LBVD) with an up to 31 percent incidence. Using lettuce seedlings in bait tests, contamination was detected at the nursery in potting mix composted for different periods and in dirt from under the benches, and at the bark supplier's site in this ingredient of the potting mix and waste "bark" from the ground. In a field experiment in which lettuce seedlings from the infested nursery were inoculated with infested roots or left uninoculated before transplanting into subplots on land with no history of lettuce planting, disease progress followed a sigmoid curve with the former but an almost straight line with the latter. However, significant clustering of symptomatic plants was found only in the subplot with the uninoculated plants. Leaf symptoms of LBVD were more severe in lettuces infested later, whereas symptoms in those infested earlier were obvious initially but then became milder. The disease impaired formation of hearts: the proportion of symptomatic plants that lacked hearts was 24-36 percent when leaf symptoms 1st appeared 5-7 weeks after transplanting but 14-16 percent after 8-9 weeks. When leaf symptoms 1st appeared at 5-6 weeks, there was a fresh weight loss of 14-15 percent for heads (all plants) and 39 percent for hearts (excluding plants without hearts). When leaf symptoms 1st appeared 7 weeks after transplanting, there was no significant yield loss for heads and only a 14 percent loss for hearts. At 8-9 weeks, there were no significant yield losses for heads or hearts.

Links:
<http://www.bspp.org.uk/ndr/july2003/2003-19.asp>
<http://www.springerlink.com/content/gf41k9228dffc7ee/>
<http://ipm.ucdavis.edu/PMG/r441101011.html>
<http://image.fs.uidaho.edu/vide/descr440.htm>
<http://www.dcn.davis.ca.us/vme/agri-analysis/lmv-lettuce.html>
<http://publish.csiro.au/paper/AR03146.htm>.

- Former Mod.DH]

[see also in the archive:

2005
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Lettuce mottle virus - Chile: 1st report 20051013.2990

2004
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Mirafiori lettuce virus - Chile (Chacabuco Province) 20041105.3002