Nematodes are translucent, microscopic roundworms, typically measuring 0.25-1 mm in length and only about 0.1-0.2 mm in diameter. Their abundance ranges from about 1,000 per litre of degraded or infertile soil, up to 50,000 per liter of highly fertile soil.
Most nematodes in soil are beneficial components of the soil ecosystem where they feed on bacteria, fungi and other microinvertebrates, stimulating and regulating the turnover of nutrients.
Some nematode species, however, parasitize plant roots and are economically important plant pathogens. All plant-parasitic nematodes use hollow spearlike mouthparts called "stylets” to feed on plant roots. Vines severely damaged by plant-parasitic nematodes are stunted and have reduced yields.
The main nematode species of significance to winegrape production in B.C. are ring nematodes (Mesocriconema xenoplax), dagger nematodes (Xiphinema species), root-knot nematodes (Meloidogyne hapla) and root-lesion nematodes (Pratylenchus penetrans). Vineyards in B.C. often have more than one species.
A few nematode species do not cause much direct damage on their own but can transmit viruses while they are feeding (dagger nematodes). Most species cause direct damage, including gall-like malformations that impair root function (root-knot nematodes) or dieback of root tips and formation of lesions on the roots (ring nematodes and root-lesion nematodes).
When nematode population densities are large, root systems will be sparse, there will be few fine roots, and the damaged roots will appear reddish, brown or black rather than a healthy white colour. If root-knot nematodes are present, it may be possible to see characteristic swellings or galls on feeder roots.
At the field level, nematode damage is often overlooked because the above-ground symptoms are non-specific and difficult to distinguish from effects of other factors. Nematode populations usually exist in patches of high population densities — rarely will entire fields be uniformly affected. Thus, typical symptoms include poor vine vigour in patches of one to dozens of vines. This patchy, uneven growth caused by plant-parasitic nematodes may occur within a vineyard with uniform soil conditions.
Areas with high nematode population densities may also be correlated with other soil factors that can affect vine vigour. For example, many species of plant-parasitic nematodes reach higher populations and cause more damage in sandy soils than in finer-textured soils. As a result, poor vine growth in sandy soils can be the result of low water or nutrient availability as well as the interaction of these additional stresses with nematode damage.
These nematodes are ectoparasites that use long stylets to extract the contents of root cells, especially at root tips. They cause necrosis or blackening of roots like root-lesion nematodes but their preferential feeding at root tips also causes stunting and sometimes formation of swollen or malformed root tips without obvious necrosis.
Preferred hosts for ring nematodes include cherry, peach, apricot and plum, in addition to grape. These nematodes are known to cause significant damage to grape in California and Oregon. Ring nematodes have been found to be relatively widespread in the Okanagan Valley, and preliminary research at the Summerland Research and Development Centre indicates that they can cause significant growth reduction of young grapevines, particularly self-rooted vines, under Okanagan growing conditions.
These nematodes are endoparasites — infective juveniles invade root tips and establish permanent feeding sites. As the nematodes feed and mature in the root, galls or knots of root tissue form around them. These galls, which typically range in size from 3-12 mm in diameter, interfere with water and nutrient uptake.
Several species of root-knot nematodes are recognized to be significant pests of grape in most grape-growing regions. To date, only one species has been found in B.C. vineyards, Meloidogyne hapla. The damage caused by M. hapla has not been studied as extensively as other species, and its impact on grapevine health under Okanagan growing conditions is not clear.
Xiphinema americanum and several closely related species of Xiphinema are commonly found in B.C. vineyards. These nematodes are ectoparasites. They do not burrow into roots but insert their long stylets deep into root tips, where they feed on root tip cells. Dagger nematode feeding causes limited necrosis and stunting and swelling of root tips, but they seldom cause significant damage on their own.
Dagger nematodes are mostly important as vectors of viruses: X. americanum and related species transmit Tomato ringspot virus, Tobacco ringspot virus, Peach rosette mosaic virus and several other minor viruses.
Xiphinema index transmits the devastating Grapevine fanleaf virus. This nematode species can also cause significant direct damage in the absence of the virus. Plant protection measures that require treatment of imported planting stock have prevented Xiphinema index from being introduced into B.C.
These nematodes are migratory endoparasites, burrowing into roots to feed and migrating freely between root tissue and soil. Root-lesion nematodes cause symptoms that range from many small lesions on young feeder roots to abnormal darkening and necrosis or death of the roots. Severely damaged root systems will have few newly formed feeder roots and sparse tufts of fine roots that will be blackened and crumbly.
Most research documenting impacts of root-lesion nematodes has been conducted on Pratylenchus vulnus, a species that does not occur in B.C. The species most commonly found in the Okanagan Valley is P. penetrans. While this nematode clearly damages roots, the relationship between P. penetrans population densities and overall vine vigour is not well understood.
Nematodes are best managed before planting. Prevention includes testing for the presence of pathogenic nematodes followed by pre-plant soil fumigation to reduce nematode populations.
There are no chemical treatments for nematodes that can be used on established vines in Canada.
Consider testing your soil for plant-parasitic nematodes before planting new vineyard blocks. If damaging populations are present, consider soil fumigation before planting.
Sampling strategies and interpretation of the results depend on the nematode species suspected and environmental conditions and should be developed in consultation with a diagnostic laboratory. Populations of all nematode species vary seasonally.
Since some nematode species migrate between roots and soil, it is usually necessary to collect both soil and root samples for testing. The laboratory analysis includes extraction of nematodes from soil and plant tissue, identification of the species present and enumeration. The absence of recognized nematode pathogens in diagnostic samples can be used to rule out nematodes as the cause of poor crop growth.
Soil analysis for the presence of nematodes is provided by the following laboratories:
Soil fumigation is an option for reducing the population of pathogenic nematodes in soil prior to planting. They will also help reduce the levels of soil-borne plant pathogens and viable weed seeds.
However, soil fumigation is expensive. Before considering, have samples of your soil analyzed for nematodes. (See the testing section above.)
Soil must be well prepared before fumigation and should be moist. As many old grape roots as possible should be removed.
These products release toxic gases on contact with moist soil. Following application, the soil surface should be sealed by watering and rolling or with polyethylene sheets. Refer to the product labels for detailed information on application and safety precautions.
Soil solarization is a non-chemical technique that will help to reduce the population of nematodes and other soil-borne pests. Solarization involves capturing the heat of the sun by covering the soil with transparent polyethylene plastic sheets during warm sunny months. The soil temperatures under the plastic increase to levels lethal to many soil-borne plant pathogens, weed seeds, seedlings and nematodes.
It is usually necessary to take the land to be solarized out of production for a year. Soil should be tilled before solarization and should also have a good soil moisture level. The area to be treated should be free of weeds, plant debris and large clods that might raise the plastic off the ground.
Cover the area with a double layer of clear polyethylene sheet, seal the edges with soil and leave it in place for 4-6 weeks during the heat of the summer (mid-June through mid-August). If possible, leave the poly in place until replanting to prevent re-contamination. Black plastic is less effective than clear plastic.
Rootstocks that are tolerant or resistant to plant-parasitic nematodes are available. These rootstocks have been evaluated against nematode species found in California and Mediterranean regions of Europe. Consequently, it is not clear if they would express the same tolerance or resistance to nematode species found in B.C.
Rootstocks reported to be tolerant or resistant include: Dogridge, Harmony, Freedom, Ramsey, Teleki 5C, K51-32, and 1613C. Two other rootstocks, 420A Mgt and 101-14 Mgt, are reported to be resistant to ring nematodes, which are probably the most important nematodes in B.C. vineyards overall.
Other cultural management techniques include green manuring and/or rotating crops to reduce the numbers of pathogenic nematodes. These tactics do not guarantee a reduction in nematodes as populations may be sustained on other species of plants including weeds.
