December 9, 2005
From: ProMED-mail<promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes, December 2005 [edited]
<http://www.apsnet.org/pd/searchnotes/2005/PD-89-1361B.asp>

First report of the stubby-root nematode Paratrichodorus teres from potato in the Columbia Basin of Washington State
E. Riga, Washington State University, IAREC, Prosser, WA 99350; R. Neilson, Scottish Crop Research Institute, Dundee, DD2 5DA, Scotland, UK. Plant Dis. 89:1361, 2005; published on-line as DOI: 10.1094/PD-89-1361B. Accepted for publication 2 Sep 2005.

Stubby-root nematodes of the genus _Paratrichodorus_ are migratory ectoparasites that cause stunted and deformed roots. They also transmit Tobacco rattle virus, the causal agent of corky ringspot disease in potato.
Thus, they are important to the potato industry in the Pacific Northwest, including the Columbia Basin of Washington state, which is one of the largest potato-producing regions in the United States.

Stubby-root nematode species are difficult to identify because the morphological differences between the species are very small (4). _Paratrichodorus teres_ is a morphologically similar species and has been reported mainly from temperate regions within Europe. _P. teres_ has also been reported in potato fields in eastern Oregon (3). The morphological similarity between _P. allius_ and _P. teres_ creates problems for routine identification because trichodorid species have a conservative morphology and are separated by the following primary characteristics: females, type of genital system and vulva position; males, number of ventromedian precloacal supplements and cervical papillae and habitus (2).

As with most soil nematodes, females are more prevalent than the males. Unfortunately, in the generic key (2), female _P. allius_ and _P. teres_ belong to subgroup 1-12 and are separated by a secondary characteristic,
vaginal shape that could be affected by fixation or handling. Male _P. allius_ and _P. teres_ are separated by the former species having one fewer ventromedian precloacal supplement. Thus, the morphological similarity between _P. allius_ and _P. teres_ creates problems for routine identification. This might be one of the reasons that _P. teres_ has not been diagnosed in Washington until now.

Using nematode morphological diagnostic keys (1,2) and comparing 18S rDNA sequence data obtained from specimens with those listed on public sequence databases, e.g., NCBI GenBank, stubby-root nematodes from 2 potato fields close to Moses Lake, Grant County, WA, were diagnosed as _P. teres_. The 2 adjacent potato fields are irrigated and are situated approximately 100 miles from the eastern Oregon site where _P. teres_ was first described (3). Soil samples were collected from several Columbia Basin potato fields that had corky ringspot disease and stubby-root nematodes.

Corky ringspot disease is an emerging disease in Washington, and the presence of a new viruliferous nematode species might be of concern to the potato industry.

References:
(1) W. Decraemer. Page 615 in: Manual of Agricultural Nematology. W. R. Nickle, ed. Marcel Dekker, Inc., New York, 1991.
(2) W. Decraemer and P. Baujard. Fundam. Appl. Nematol. 21:37, 1998.
(3) H. J. Jensen et al. Plant Dis. Rep. 58:269, 1974.
(4) S. MacFarlane et al. Adv. Bot. Res. 36:169, 2002

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[Stubby root nematodes are becoming of increasing importance to the potato industry in Washington State, because they are vectors of Tobacco rattle virus (TRV), the causal agent of Corky Ringspot disease. In addition to the presence of _Paratrichodorus allius_ (Pa) in Washington State, a 2nd morphologically similar stubby root nematode species (_Paratrichodorus teres_, Pt) has been identified. The current method for determining if the
nematodes are viruliferous for TRV takes several weeks, because it requires a glasshouse bioassay. Therefore, a rapid molecular assay has been developed using RT-PCR to identify viruliferous nematodes. The technique can detect TRV in as few as 5 nematodes.

Corky ringspot disease (CRS) in potatoes is caused by tobacco rattle virus (TRV), which is vectored by stubby root nematode species (_Paratrichondorus_, _Tricondorus_ spp.). Both virus and vector have wide host ranges, leading to persistence of the disease in spite of the use of expensive chemical control agents.

Use of resistant varieties can be a key factor for managing this disease. However, it is difficult to confirm resistance of parents and selected progeny genotypes due to erratic incidence and symptom expression in field
screening plots. Screening nurseries require many replications and large plots, but escapes are still common.

Recent work at Scottish Crop Research Institute (SCRI) (_Xenophontos_ et al., (1998) Potato Research, 41, 255-265; Dale et al., (2000) European Journal of Plant Pathology, 106, 275-282) has shown that some potato cultivars, (Wilja and King Edward), can become persistently systemically infected with TRV without developing spraing symptoms (reddish brown spots, rings, or arcs in tuber tissue), and that such plants can serve as sources
for virus acquisition by vector nematodes. Stocks of these cultivars can become chronically infected, and show effects on yield and quality, with the production of smaller tubers with extensive secondary growth and growth
cracking. Preliminary experiments suggest that more cultivars than the 2 mentioned above are affected in this way.

Links:
<http://image.fs.uidaho.edu/vide/descr808.htm>
<http://www.dias.kvl.dk/Plantvirology/TRVforweb.htm>
<http://plpnemweb.ucdavis.edu/nemaplex/Plntpara/pltvirus.htm>
<http://www.scri.sari.ac.uk/SCRI/Web/Site/home/ResearchAreas/MGOE/HPC/Tobacco_Rattle_Virus.asp>
<http://www.findarticles.com/p/articles/mi_qa4069/is_200311/ai_n9319072>
- Mod.DH]