Dagger Nematodes [Xiphinema spp] in the Pacific Northwest (PNW)
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This article with Educational Purposes is provided by AGNEMA, Nematode Assay Laboratory, as a brief introduction to one of the major vectors of plant viruses, Dagger Nematodes [Xiphinema], in the Northwest of the United States.
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Dagger nematodes (their long stylet looks like a dagger, Images 1 and 2) or Xiphinema comprises of 234 species. The first species, X. americanum was described by Cobb in 1913.
The role of Xiphinema as a direct pest of crops is more uncertain, but some evidence suggests that it can cause considerable damage. Dagger nematodes as virus vectors are far more important threat, and several of them are common species in our region. Direct feeding by these nematodes can cause substantial growth and yield reductions. The tips of roots are often attacked sometimes resulting in terminal swellings (Image-3) and a proliferation of roots behind these root caps. These nematodes more often feed in the root hair zone where nematodes may aggregate. Attacked root tips may become hook-shaped or swell to form terminal galls. Root tissues darken with cortical hyperplasia and lateral root proliferation; secondary and feeder roots are often lost. On strawberry, X. americanum can cause strawberry decline marked by extensive necrosis of feeder roots, and X. bakeri can destroy strawberry root systems and cause 40-50% reduction in root and top growth of raspberry plants.
Besides of direct damages that these nematodes inflict on their hosts, they are also capable of transmitting certain viruses (nepoviruses) to their host plants. These may include peach rosette mosaic virus, arabis mosaic virus, tobacco and tomato ringspot viruses, cherry raspleaf virus, and grape fanleaf virus. The most obvious symptoms of the infestation of a plant are therefore referable to the symptoms of the virus infection rather than nematode itself. One example is the presence of a brown line at the graft union on apple and plum infected with tomato ringspot virus transmitted by X. americanum. Virus symptoms commonly include: chlorotic mottling and defoliation; bright yellow discoloration of foliage; chrome yellow flecks along main veins; spots, blotches and crinkling of leaves; and stunting.
Fortunately, out of 234 species of dagger nematodes, there are only four species of these nematodes that have been reported from the Northwestern states namely:
- americanum, a common species in the region, and vectors Tomato ring spot (Torsv) to various plants (images 4-10) and Tobacco ring spot virus (TRSV) to gape, (Image11)). It also transmits Peach Rosette Mosaic Virus to peach tree (Image-11). This last virus [has not been reported in NW. X. americanum by itself is an important pathogen on grape, but the viruses it transmits to grape are far more important.
- rivesi causes root damages to cherry an d apple, and transmits Cherry Raspleaf Virus (CRLV) to these trees (Image-12 and 13).
- bakeri is root damaging nematode of red cranberry and transmits Arabis Mosaic Virus (though has not been reported in NW)
- pachtaicum is a very common dagger nematode in the State’s vineyards. Although it causes root damages but fortunately has not been associated with viruses.
Luckily X. index (vector of grape fanleaf virus) has not been reported from the northwestern region. Though a grape leaf showing fanleaf symptoms was depicted in Good Fruit Grower in 2011 (Image 14) is probably through plant material. Severe symptoms on grape that are depicted in images 15 and 16 have not been reported from the NW region. However, the grape growers should remain alert and report any such symptoms to WSU virologist to prevent the spread of disease.
Management of dagger nematodes:
Management of dagger nematodes in established orchards and vineyards is rather difficult if it is not impossible. Planting rapeseed and canola and incorporating them into orchard soil may reduce dagger nematode number, but at the end will not make the orchard soil cured. It seems that prior to establish a new orchard or a vineyard the site soil must be thoroughly surveyed and sampled and if necessary properly fumigated and resampled to ascertain that transmitting vector nematodes are controlled.
Obviously, precautions should be taken to prevent reintroduction of the vector to treated farms.
For more information about Sampling click here.
Table-1: Plant viruses vectored by dagger nematodes in the Northwest USA.
Note: Xiphinema index is not reported in Pacific Northwest.
AGNEMA, plant-parasitic nematode assay and research laboratories, provided this information for Educational and Non-profit purposes for interested people in Gardening, Farming and AG industry to increase their awareness about either presence of pest problems or potential problem that might threaten plant community in the Northwest of the United States.
We appreciate feedback of all kinds. Get in touch with us to give us your comments and suggestions.
call: +1 (509) 255-3744