Nematodes

Plant-parasitic nematodes are microscopic, unsegmented roundworms that feed on plant roots by puncturing cell walls and withdrawing cell contents by means of a protrusible hypodermic structure called a stylet. They live within root tissues and in the water films that surround soil particles and roots.

The types of plant-parasitic nematodes that become established in a vineyard are determined by the nematodes present in the soil at planting, the nematodes in irrigation water, sanitation and cleanliness of nursery stock, susceptibility of the selected rootstock, the nematode host status of cover-crops and native vegetation, and the movement of nematodes with soil by vehicles. Of the many genera of plant parasitic nematodes detected in soils from California vineyards, dagger, ring, and root lesion nematodes are the most prevalent in north and central coast vineyards, and in the San Joaquin Valley.

As the name implies ring nematodes have a deeply striated cuticle, which gives the appearance of rings. Ring nematode occurs in sandy or fine clay soils, especially waterlogged soils. Root knot, dagger and citrus nematodes occur most commonly in the San Joaquin Valley and southern California. Populations of root-knot nematodes are best adapted to loamy sand and sandy loam soils; citrus nematodes favors sandy and clay loam soils. The needle nematode is mainly found in southern California.

Other nematodes associated with grape in California include stubby root nematode, Paratrichodorus minor; spiral nematode, Helicotylencus pseudorobustus; sheath nematode, Hemicycliophora spp. and pin nematode, Paratylenchus hamatus.

Plant-parasitic nematodes feed on roots, reducing vigor and yield of the vine usually in irregular patterns across the vineyard. Damage patterns are frequently associated with soil textural differences. The plant-parasitic nematodes that afflict grapevines have a wide diversity of feeding habits and invoke a variety of host responses at the cellular, tissue, root and whole plant levels. The damage caused by nematodes may be exacerbated by climatic conditions, geographic region, moisture and nutrient stress, and horticultural practices.

Root knot nematodes are sedentary endoparasites penetrating into roots and inducing giant cell formation, usually resulting in root galls. Giant cells and galls disrupt uptake of nutrients and water, and interfere with plant growth.

Both dagger and needle nematodes cause slow, gradual decline. Xiphinema americanum, the most common species of dagger nematode found in California, weakens vines by feeding near the root tip and is a vector of tomato ringspot virus (causal agent of grapevine yellow vein disease ). Xiphinema index can cause yield reduction in some varieties but is more important for its transmission of grapevine fanleaf virus (causal agent of grapevine fanleaf degeneration disease). Both ring and dagger nematodes feed from outside the roots (ectoparasites), but can reach the vascular tissues with their long stylet. Ring nematodes cause general aboveground lack of vigor and reduced vine growth and yields; in severe cases, more than half the fruit and leaf buds along a cordon or spur may be absent.

Root lesion nematodes are migratory endoparasites that restrict the growth of roots as they feed and migrate in and out of roots.

Young female and juvenile stages of citrus nematodes feed initially from the outside of roots before penetrating deeper into root tissues. They establish feeding sites with their heads embedded in cortical tissue and their posterior ends outside the roots. Their feeding disrupts the uptake of nutrients and water, and interferes with plant growth.

Aboveground symptoms of nematode damage to roots are generally non-specific and may be manifested as unthrifty vines, poor growth, reduced yields, nutrient deficiency, and greater sensitivity to stress. These effects of damage to the root system are often misdiagnosed as water stress or nutrient deficiency. The symptoms described below are indicative of a nematode problem, but most are not diagnostic as they could result from other causes as well. Generally, nematode infestations are seen in areas of the vineyard with vines that lack vigor and have restricted growth and reduced yields.

Root knot nematodes produce small swellings or galls on young feeder roots (about 0.125 inch in diameter) or secondary rootlets. Larger galls may result from multiple infections. The galling response varies across cultivars and rootstocks; it also differs with nematode species (see Grape Pest Management Manual, 3^rd edition, UC ANR Publication #3343). When galls are broken apart, tiny, glisterning white bodies of mature females can be detected with the aid of a hand lens. Gelatinous egg masses are often attached to the female bodies. Root-knot nematodes can be found wherever roots occur; they are most prevalent within a soil depth of 6 to 35 inches.

The dagger nematode, X. index, feeds on root tips causing swelling and bending in a manner similar to the nodosities caused by phylloxera as well as many dead feeder roots if population levels are high; multiple prolonged attacks can result in darkened, necrotic spots that spread through the root tip. Virus transmission by dagger nematode produces symptoms on leaves such as yellowing of veins, mosaic, and malformation with symptom expression less apparent among white varieties and in warmer regions.

Infestation by root lesion nematode restricts top growth of young vines. If young vines are planted in soil infested with root lesion nematode, root systems may be severely restricted, including an absence of major roots, many dead feader roots and on rare occacion brown lesions at feeding sites.

The citrus nematode causes death of feeder roots. The bodies of adult females protrude from the roots; females deposit eggs in a gelatinous matrix to which soil adheres giving citrus nematode infested rootlets a dirty appearance.

The ring nematode can cause a reduction of small feeder roots and there may be abnormal tufted growth of small roots; it does not enter the root but feeds deep into root tissues using a long stout stylet. Population densities of ring nematode are highest in areas where grape roots are most abundant, within the top 18 inches of soil.

To make management decisions, it is important to know the nematode species present and to estimate the size of the population. If a previous orchard or vineyard had problems caused by nematodes that are also listed as pests of grape, population levels may be high enough to cause damage to the young vines.

If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification. The best time to sample differs according to region, type of nematode, and cultivar of grapes. Bloom and harvest times, which influence nematode populations, also differ according to region. Research has shown that populations of citrus nematode are highest in the Coachella Valley from February through March and again in October. In the San Joaquin Valley, X. index populations are most likely to be detected in November through February. Root knot nematodes are found at any time of the year.

Sample when the soil is moist, (irrigate three days before sampling or wait for three consecutive days of rain). Select areas that are different due to cropping history, soil texture, or crop injury. Place samples in plastic bags, label with your name, address, location, the previous crop/cultivar, and the current cultivar grown, keep sample cool (do not freeze), and send to a diagnostic lab as soon as possible.

Preplant situation: Take 10 soil samples down to 3 feet and include roots if previously planted to another crop.

Established vineyards: Include soil and roots sampled 12 to 18 inches from around the vine trunk to a depth of 30 inches, discarding the top 3 inches. Select five vines, each from a different area, mix thoroughly the soil sampled from each vine and make a composite sample of 1 quart (1 liter) for each block. Observe root irregularities. Consult UC ANR Publication 3343, Grape Pest Management, 3rd edition for sampling procedures and how to interpret the results.

Look for nematode symptoms in the vineyard late in the growing season to prepare for future management.

Effective nematode management programs rely upon a suite of practices. Every precaution should be taken to exclude plant-parasitic nematodes if the area is free of them; once established, nematode infestations are permanent. Nematode management requires consideration of the host ranges, life cycles, survival strategies and longevity of nematdoe species present. Knowledge of the host-status of rootstocks and cover crops to the resident nematode species, and an appreciation for the importance and stewardship of natural regulators of populations of pest species is also required.

Only certified nematode-free stock should be planted. There is a high risk of introducing nematodes in rooted cuttings that have been propagated in soil that is not known to be free of plant-parasitic nematodes. Nematodes are easily moved and introduced on soil adhering to vehicles, implements and animals; ensure that such potential sources are eliminated by appropriate sanitation practices. Nematodes are also moved and introduced by water, either through flooding or drainage, through irrigation with water drained from a neighboring field or drawn from rivers that have passed through infested areas. Tactics for reducing infestation levels of irrigation water, include redesign of the supply system, settling ponds, use of water from deep wells, and ozone treatment.

Vineyard Preparation

Deep tillage may be necessary to disrupt restrictive layers that may be naturally occurring or that have resulted from previous cultural practices. Some nematodes are susceptible to soil disturbance. For example, population levels of large-bodied dagger nematodes, Xiphinema spp., can be reduced by repeated tillage of fallow soil. However, this practice can be detrimental to soil structure and does not affect nematodes below the tilled profile. Old vines must be removed with equipment that removes the greatest amount of root mass from the soil profile. There may be advantages in applying herbicide to vine stumps some time before root removal so that roots that remain after the removal process are killed and do not provide a resource for plant-parasitic nematode survival. Population levels of potentially-harmful nematode species may be reduced by crop-rotation with appropriate non-host or resistant plant species. The length of rotation will depend upon the survival capabilities of the target nematode with a shorter time period for the citrus nematode (1 year) and longer for dagger nematodes (4 or more years). If a fallow period is implemented, good weed management must be achieved since many weeds are hosts to nematodes. Cover crops and organic amendments may be used to build soil carbon levels and to provide resources for the beneficial organisms of the soil food web. The selection of a rotation or cover crop should be made with knowledge of the host range of the target plant-parasitic nematode.

Virus Vector Considerations

Besides their direct damage to plants, some nematodes are vectors of plant viruses. In that case, very low numbers of surviving viruliferous nematodes can cause serious damage by transmitting virus particles to new vines. The viruses transmitted by dagger nematodes are classified as Nepoviruses and it is important to attempt elimination of their vectors prior to establishing a new vineyard. Two dagger nematode species are particularly important in this regard in vineyards; X. index transmits grapevine fanleaf virus (GFLV) and the X. americanum species complex transmits tomato ringspot virus (ToRSV). In the past, elimination of these vectors has been attempted through the use of high rates of soil fumigants. Where such practices are no longer available, a very long non-host crop rotation may be necessary. Virus infected root remnants are slow to decay and can support a residual innoculative vector population, often for longer than 10 years. Some rootstocks are available with resistance to X. index, although that does not necessarily confer resistance to the virus because particles may be transmitted during exploratory stylet thrusts by the nematode. However, at least one rootstock, O39-16, which is resistant to X. index, confers a tolerance to the virus and vines remain productive for long periods even though they are infected.

Rootstocks

In recent years, grape rootstocks have been developed and released that have resistance to several species of plant-parasitic nematodes. Besides their nematode-resistance characteristics, it is important to select rootstocks that have appropriate vigor and horticultural characteristics suitable for the local conditions. Summary of available data on the host status of grape rootstocks to nematodes is presented in Table 1.

Soil Health

A healthy soil has physical, chemical, and biological characteristics favorable for sustaining plant growth while providing organic matter decomposition, nutrient cycling, soil fertility, and water purification. Favorable physical characteristics include good soil structure and particle aggregation, and the absence of restrictive layers. Favorable chemical characteristics include a balance of mineral nutrients at levels necessary for sustained plant growth but below levels toxic to resident organisms. Favorable biological characteristics usually include an abundance and diversity of soil organisms supported by high organic carbon and high microbial biomass. As soil conditions change diurnally, seasonally, and with depth, different organisms among a diverse assemblage become functionally dominant in different regions of the soil profile. Under such circumstances, plant-parasitic nematodes are seldom observed at damaging levels and their populations may be regulated by several factors: bottom up effects of innate resistance of plants to the resident nematodes, and top-down effects of predation by organisms feeding on the plant-parasitic nematodes. When natural systems are converted to conventional agricultural practices, soil carbon levels decline, biological activity is reduced, and predator organisms are decimated due to lack or resources. The decline of the converted system occurs rapidly. It appears to be more difficult, and requires much more time, to rebuild the system than to destroy it.

Cultural Practices

Manures and other soil amendments can improve vine vigor and frequently reduce the effect of nematode infestation. To reduce stress on vines, take measures to prevent soil compaction and stratification, to improve soil tilth and drainage, and to control other pests. Proper irrigation, improving soil-water holding capacity, avoiding over-cropping, and fertilizer application also reduce stress on vines and help lessen the effect of plant-parasitic nematodes.

Chemicals

Application of fumigant nematicides can be an effective way to reduce plant-parastic nematode populations. Soil fumigants have a broad spectrum effect; they kill not only pest species but beneficial organisms as well. The resurgence of pest species after soil fumigation may occur more rapidly than that of some of the beneficial organisms that might otherwise regulate the pest species. The advantage of a postplant systemic material is that small applications can move to the zone where nematodes are feeding, avoiding high dosage rates necessary to diffuse throughout the soil and root zone. Repeated applications of nematicides over time may be necessary to realize yield benefits. Once a previously-damaged plant is protected from nematode damage, some time will be required for the plant to rebuild its root system so that productive vine growth is supported.

Always read and carefully follow all label information when applying soil fumigants.