Locations (Missouri and Arkansas, USA) where vineyard soil/root samples were collected for plant–parasitic nematode analysis.
Relationship between vineyard age and total nematode density, with a 95% prediction ellipse, from 107 vineyard nematode samples in Missouri and Arkansas. Transformed data (Supplemental Table 1) were used for this analysis.
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A nematode survey was conducted from 30 vineyards in Missouri and Arkansas (USA) comprising 107 samples among 21 grape cultivars (Vitis aestivalis, Vitis labruscana, Vitis vinifera, Vitis hybrids, and Muscadinia rotundifolia). All the samples tested positive for nematode presence. Eleven different nematode taxa were isolated and identified, five of which have economic importance to grapevines: Xiphinema americanum, Meloidogyne spp., Pratylenchus spp., Criconemoides spp., and Tylenchulus spp. Xiphinema americanum was detected at all but two sites, with 58% of sites having populations at or above levels with expected biological and economic impact. This is primarily a concern because of the ability of X. americanum to transmit Tomato ringspot virus and other viruses. The other four nematode taxa of concern were present in fewer samples and at much lower population densities. Xiphinema index, known to vector Grapevine fanleaf virus was not identified in any of the samples collected.
Grapevines (Vitis and Muscadinia spp.) are hosts to a variety of nematodes (phylum Nematoda) around the world, but most are of unknown or insignificant concern in grape production. Although our knowledge of grapevine–nematode interactions is expanding, a comprehensive understanding of the potential impact of different nematode taxa in viticulture remains elusive. Increasingly, both grape growers and scientists believe the importance of nematodes in viticulture has likely been underestimated (Khan 2023). The major plant–parasitic nematodes that are currently known to threaten grapevines in North America are dagger nematodes (Xiphinema index and Xiphinema americanum), root-knot nematodes (Meloidogyne spp.), and lesion nematodes (Pratylenchus spp.) (Andret-Link et al. 2017; Brown et al. 1993; Garcia et al. 2019). Other notable taxa of concern are ring nematodes (Criconemella and Criconemoides spp.) and citrus nematodes (Tylenchulus spp.) (McHenry and Bettiga 2013).
The threats of plant–parasitic nematode feeding activity in vineyards are 2-fold: 1) physical injury to roots and 2) virus transmission. Physical damage from Xiphinema species can be particularly detrimental in the case of X. index, which feeds ectoparasitically on the root tips, resulting in swelling, gall formation, stunting of the root system, death of feeder root tips, and overall decline in vine vigor and growth (Brown et al. 1993; Garcia et al. 2019; Nicol et al. 1999; Raski and Lider 1959). A study in the state of Washington, USA, showed that Xiphinema spp. are deeply distributed in the vineyard soil profile (≥122 cm) and thus are very difficult to manage through traditional methods of soil treatment such as fumigation (East et al. 2019). Meloidogyne spp. are more prevalent in sandy soils, with high infestations reducing yields as a result of restrictions in water and nutrient uptake caused by root damage and production of galls (Brown et al. 1993; Nicol et al. 1999). Patches of growth-stunted vines are symptoms of feeding by Pratylenchus spp., which damage roots by feeding on root cortical tissues and forming lesions (Brown et al. 1993).
Although damage caused by root feeding can be economically significant, the viruses vectored by nematodes can be of greater concern. Of the five taxa previously listed, only the dagger nematodes (Xiphinema spp.) are currently known to vector viruses. Xiphinema index has been documented in transmitting the devastating Grapevine fanleaf virus (GFLV) (Garcia et al. 2019), whereas X. americanum has been associated with Tomato ringspot virus (ToRSV; which causes grape yellow vein disease), Tobacco ringspot virus (TRSV), Peach rosette mosaic virus (Andret-Link et al. 2017; Brown et al. 1993; Taylor and Brown 1997), and Arabis mosaic virus (ArMV) (Milkus 2001). Symptoms observed for GFLV and the yellow vein disease strain of ToRSV are in many ways similar, requiring diagnosis of the virus through laboratory testing. Common symptoms of these two viruses are poor growth and fruit set, overall vine decline with leaves displaying an oak leaf pattern, yellow mosaic coloration, and vein banding (Golino et al. 1992).
To date (including this study), X. index has not been detected in any vineyard in Missouri or Arkansas, USA, whereas X. americanum is widely distributed throughout the region where the nepoviruses ToRSV and ArMV have been documented (Milkus 2001; Milkus and Goodman 1999; Qiu et al. 2006). A more recent virus survey (Schoelz et al. 2021), however, consisting of 400 samples from 25 grape cultivars across Missouri, USA, did not detect any known nepoviruses (GFLV, ToRSV, TRSV, ArMV). Decline and virus-like symptoms of ‘Chardonnay’ in a Missouri, USA, vineyard in 2004 prompted research into virus sampling and testing (Lunden et al. 2010; Qiu et al. 2007). Symptoms included short internodes, stunted and crinkled leaves, leaf mosaic, vein clearing, leaf curling, decline in vine size, and reduced cluster size and set. The positive identification of GFLV in that study (Qiu et al. 2007), along with the discovery of a complex of GFLV and ToRSV yellow vein strain in combination with Grapevine rupestris stem pitting-associated virus (Lunden et al. 2010), prompted our study to determine whether the vector X. index is present in Missouri or Arkansas, USA, vineyards and to identify other nematodes that may be consequential in regional viticulture.
The objectives of our study were to conduct a widespread nematode survey across the major viticulture regions of Missouri and Arkansas, USA, encompassing diverse environments, vineyard ages, grape cultivars, and production methods, and, more specifically, to determine whether the documented presence of GFLV in the region may be a result of the heretofore-unknown presence of the nematode vector X. index.
A survey for plant–parasitic nematodes was conducted among 21 grape cultivars at 30 vineyards in Missouri and Arkansas, USA, during Oct and Nov 2008. Sampling at this time of year is expected to yield the greatest populations of nematodes present, especially Xiphinema species (McHenry and Bettiga 2013). Sites (22 in Missouri and 8 in Arkansas, USA) represented the major regions of grape production in both states as well as the diversity of cultivars grown (Fig. 1). A broad, region-wide assessment of the soils indicates that the central and southern Arkansas sites generally feature fine-sandy loams, whereas the northwestern Arkansas and Missouri sites largely encompass a variety of silt loam soils (US Department of Agriculture, Natural Resources Conservation Service 2024). Samples were collected from 21 cultivars among Vitis aestivalis, Vitis labruscana, Vitis vinifera, Vitis hybrids, and Muscadinia rotundifolia. All the V. vinifera cultivars were grafted to various rootstocks (which may or may not impart nematode resistance), whereas only ∼3% of the other cultivars were grafted (both are typical viticulture practices in the region). A total of 107 samples were collected in descending order by grape species/hybrid: Vitis hybrid (n = 59 samples), V. aestivalis (n = 21), V. vinifera (n = 12), M. rotundifolia (n = 10), and V. labruscana (n = 5). This sampling is representative of grape production in the region.
Citation: HortScience 60, 11; 10.21273/HORTSCI18858-25
One or more sampling blocks within each vineyard were delineated. These blocks mostly encompassed distinct cultivars; but, in some cases, the same cultivar was sampled from different areas (blocks) within a vineyard. Sampling blocks ranged from 1 to 8 ha, with additional variables of grafted status, irrigation status, and vineyard age recorded for each block/sample. The sampling used a walked W pattern through each block, selected to alleviate block-scale variability concerns (soil type, soil depth, fertility changes, etc.). Because vine-to-vine variability, even among neighboring vines, can be high as a result of a variety of factors, individual vines were selected for sampling based on a visual assessment of their size and health relative to surrounding vines within the block, with the most representative vines selected. Six to 10 vines within a sampling block were selected for subsample collection, and soil cores (∼15 cm in diameter × 30 cm deep) were hand-dug with a metal “sharpshooter” spade. Subsamples, consisting of both soil and roots, were collected ∼30 cm from the vine trunk, avoiding areas directly below irrigation emitters. These 6 to 10 subsamples were placed in a clean bucket and mixed thoroughly, and a representative experimental sample (>250 cm3) was collected. Samples were placed in zippered plastic bags, cooled immediately, then transported to the University of Arkansas Nematology Laboratory (Fayetteville, AR, USA) within 2 d for analysis.
Nematodes were extracted from samples by suspending in water and filtering through an 850-µm sieve over a 75-µm sieve to separate plant and soil particles from nematodes. Extracted nematodes were isolated using centrifugation–flotation (Jenkins 1964), killed by heat relaxation, fixed with 37% formalin, and mounted onto glass microscope slides (Hooper 1986). Identification and counting were performed using a Nikon Optiphot 2 compound microscope (Nikon, Tokyo, Japan) and a scanning electron microscope. Xiphinema nematodes were identified to species level, whereas all other nematodes were identified to genus (Ye 1996). The nematode population density was reported as number of nematodes per 250 cm3 of sample.
The total nematode population per sample was ascertained by adding the population density of each nematode taxon found in each sample. Frequency tables and descriptive statistics were calculated and created in Excel (Microsoft 365; Microsoft Corporation, Redmond, WA, USA). Data transformations were determined using the Box-Cox transformation procedure to meet assumptions of normal distribution, independence, and variance homogeneity (Supplemental Table 1). Transformed data were used to conduct an analysis of variance using the general linear model in SAS (SAS version 9.4; SAS Institute, Cary, NC, USA) to determine whether nematode population densities were affected by the independent variables in the survey: grape species/hybrid, cultivar, grafted status, irrigation status, and vineyard age. Relationships between vineyard age and nematode density were determined by creating scatterplots with 95% prediction ellipses using the SGPLOT procedure in SAS (SAS version 9.4; SAS Institute, Cary, NC, USA). Mean separations were performed using Tukey’s honestly significant difference test at P ≤ 0.05.
Eleven different taxa of plant parasitic nematodes were identified in this survey (Table 1), including five of economic concern for grapevines: X. americanum, Meloidogyne spp., Pratylenchus spp., Criconemoides spp., and Tylenchulus spp. Frequency of nematodes among samples was greatest for X. americanum, found in 98% of the 107 samples and at all but 2 of the 30 vineyards [one planted with ‘Chambourcin’ (Vitis hybrid) and the other with ‘Muscat Canelli’ (V. vinifera)]. Nematode frequency followed with Helicotylenchus spp. (80% of samples), Criconemoides spp. (47%), Paratylenchus spp. (43%), Pratylenchus spp. (23%), Tylenchorynchus spp. (16%), Tylenchulus spp. (12%), Meloidogyne spp. (8%), Hemicycliophora spp. (3%), Paratrichodorus spp. (2%), and Aorolaimus spp. (1%). Xiphinema index was not identified in any of the vineyard sites sampled. Nematode frequency was separated further by grape species/hybrid (Table 1). For example, Meloidogyne spp. and Tylenchulus spp. nematodes were not found in any M. rotundifolia vineyards, whereas 100% of samples from M. rotundifolia, V. aestivalis, and V. labruscana were positive for X. americanum nematodes.
The most abundant nematode throughout the survey was Helicotylenchus spp., with a mean population density of 139 per 250-cm3 sample, and with a maximum count of 2800 individuals in a sample (Table 2). Criconemoides spp., Paratylenchus spp., and X. americanum were also abundant, with mean numbers of 89, 84, and 48, and maximum counts of 3800, 2064, and 492 individuals, respectively, per 250-cm3 sample. McHenry and Bettiga (2013) state that nematode populations from a single grapevine may range from 0 to 10 million individuals; hence, sampling results are often inconsistent. In terms of nematodes of particular concern for grapevines, X. americanum was most abundant in number, followed by Meloidogyne spp. and Pratylenchus spp. Pratylenchus spp. were present among all grapevine species/hybrid, although in very low numbers on V. vinifera. The presence of X. americanum at nearly all sites raises concern for the spread of ToRSV and other nepoviruses, especially among sensitive cultivars.
The economic threshold (ET) for nematodes in agriculture is defined as the nematode population at which a crop’s potential loss in value is equal to the cost of nematode control (Ferris 1978). Although ET levels for nematodes have been developed for many crops, especially annuals [e.g., corn, soybean, cotton, peanut (Mehl 2024)], true nematode ET levels are largely undeveloped in viticulture worldwide. Some university extension publications [e.g., Dickerson et al. 2000 (for South Carolina)] propose detrimental nematode population levels for grapes; however, most of the data do not appear to be based on published peer-reviewed or easily accessible research, nor are they true ET figures based on the cost of control. Rather, they tend to propose levels of expected vine damage in relation to defined nematode population levels. McHenry and Bettiga (2013) proposed low–medium–high nematode population numbers (per kilogram of sample) for important nematode taxa in California viticulture, and outlined the potential degrees of grapevine damage and crop loss associated with the various population levels; however, they did not define ET levels specifically. Because the work by McHenry and Bettiga (2013) is one of the most-cited resources on critical nematode population levels in viticulture, we used their data as a benchmark for comparison with our data.
Nematode density in our study is reported on a by-volume basis. Factors such as soil moisture, soil bulk density, and the inherent variability among soils sampled make direct volume-to-mass comparisons of our samples (250 cm3) with the 1-kg figures in McHenry and Bettiga (2013) imprecise; however, for the purpose of developing the best possible understanding of the nematode threshold status in the study region, populations in our samples were multiplied by four to approximate 1 kg. Nematode population numbers at the low end of the medium population density range in McHenry and Bettiga (2013) were then designated as critical threshold levels for the purpose of assessing our data.
For X. americana, 57.9% of samples had population levels at or above threshold, whereas concerning levels of the citrus nematodes (Tylenchulus spp.) were not found in any samples (Table 2). Meloidogyne spp. were low in frequency among sites and density within samples, with only a handful of sites above threshold levels. Muscadinia rotundifolia was the only grapevine species/hybrid where no Meloidogyne spp. were found, which may indicate some level of resistance. A survey of M. rotundifolia vineyards in Georgia and North Carolina, USA (Jagdale et al. 2019), also found very low numbers of Meloidogyne spp. nematodes (9% of samples) compared with Helicotylenchus spp. (90%) and Xiphinema spp. (58%).
Table 3 elucidates statistical differences (P ≤ 0.05) among nematode taxa in response to the independent variables grape species/hybrid, cultivar, grafted status, irrigation status, and vineyard age. The independent variables did not impart consistent trends or effects across all nematode taxa, but of note are that X. americanum, Meloidogyne spp., and Pratylenchus spp. populations varied significantly by cultivar; Tylenchulus spp., Criconemoides spp., and Helicotylenchus spp. populations depended on vineyard age; Helicotylenchus spp. was the only nematode taxon potentially influenced by grafting; and irrigated vineyards in general had greater nematode populations, but effects of irrigation (or not) on specific nematode taxa were not discerned.
Among the independent variables in this study, we chose vineyard age as an example for correlation analysis to determine whether there might be a relationship between vineyard age and nematode population density. Nonlinear regression analysis was used to fit the data to an exponential population decay model based on the scatterplot in Fig. 2. Although the relationship between the predictor (vineyard age) and the response (nematode density) was statistically significant (P < 0.0001), the model only explained a small portion of the variability. This suggests that vineyard age had a real but nevertheless weak influence on nematode density in our study, and that other factors (or combinations of factors) may account for most of the variability. Certainly, additional factors external to our study also influence nematode presence in vineyards.
Citation: HortScience 60, 11; 10.21273/HORTSCI18858-25
Our survey underscores the diversity of plant–parasitic nematodes in Missouri and Arkansas, USA, vineyards and confirms the presence of five nematode taxa of economic importance to grapevines. The most concerning of these is X. americanum, which is known to vector several viruses. Fortunately, X. index, the worrisome vector of the nepovirus GFLV, was not identified in any samples. The lack of X. index in any of the samples collected raises questions as to how GFLV was established in Missouri vineyards (Kovacs and Qiu 2002; Qiu et al. 2006); we are not aware of any surveys having been conducted for GFLV in Arkansas, USA. Although GFLV may have been introduced on infected planting material, further research into other potential vectors of GFLV is needed to determine how this nepovirus is spread. Although this survey was conducted in 2008, we are not aware of any subsequent viticulture-based nematode surveys conducted or published from the region; therefore, our results remain relevant as a critical foundation for viticulture management and for additional needed research. More research on grapevine–nematode dynamics in the Missouri–Arkansas, USA, region is needed as the industry continues to expand, especially in terms of alleviating risks while developing mitigation and management strategies.
Locations (Missouri and Arkansas, USA) where vineyard soil/root samples were collected for plant–parasitic nematode analysis.
Relationship between vineyard age and total nematode density, with a 95% prediction ellipse, from 107 vineyard nematode samples in Missouri and Arkansas. Transformed data (Supplemental Table 1) were used for this analysis.
Contributor Notes
We thank the Missouri Life Sciences Trust Fund and the Missouri Wine and Grape Board for supporting this project.
A.L.T. is the corresponding author. E-mail: thomasal@missouri.edu.
Locations (Missouri and Arkansas, USA) where vineyard soil/root samples were collected for plant–parasitic nematode analysis.
Relationship between vineyard age and total nematode density, with a 95% prediction ellipse, from 107 vineyard nematode samples in Missouri and Arkansas. Transformed data (Supplemental Table 1) were used for this analysis.
