Nematodes are among the most ubiquitous, abundant, and biologically diversified groups of invertebrate soil organisms. Based on their feeding behavior, nematodes can be grouped as bacterivores, fungivores, predatory, carnivores, and herbivores (plant-parasites) (Pen-Mouratov et al., 2003; Yeates and Bongers, 1999; Yeates et al., 1993). All of these nematodes generally play key roles in ecological processes like nutrient recycling, decomposition of organic matter, and suppression of diseases (Briar et al., 2007; Ferris et al., 2004; Mulder et al., 2003; Neher, 2001, 2010; Yeates, 1999; Yeates and Bongers, 1999). Nematode species abundance and occurrence, and community composition are known to be influenced by both abiotic (physical and chemical properties of soil, temperature, and moisture) and biotic (e.g. host plant occurrence and abundance) factors (Bakonyi and Nagy, 2000; Freckman and Ettema, 1993; Kandji et al., 2001; Norton, 1989; Yeates, 1999). In agricultural systems, cultural practices like tillage, crop rotation, and addition of inputs can alter physical and chemical properties of soil that in turn can modify abundances and community structures of nematodes (Porazinska et al., 1999; Timper et al., 2012). For example, incorporation of organic soil amendments and fertilizers, such as compost, can change soil properties which favor the increase in bacterivores nematodes, which shifts the nematode community, but also promotes N-mineralization in the soil that in turn can increase the crop productivity (Neher, 1999).
For plant-parasitic nematodes (hereafter PPNs), they are obligately tied to the presence and relative abundance of host plants, but like other nematodes, their relative abundances may be influenced by soil properties such as the relative amounts of sand, slit, and clay (Van Gundy, 1985). For example, high populations densities of
Over their North America distribution, pecan trees can be harmed with diminished yields from PPNs, especially three species of root-knot nematodes (RKN)
Based on the relative abundance of these PPN genera from the limited number of soil samples processed in the University of Georgia Diagnostic lab, and the potential that the sandy soils of the Coastal Plains may be more favorable for certain PPN genera, we used multivariate statistical tests, which are frequently used in community ecology studies, to determine whether the pecan nematode assemblages differed between the Piedmont and Coastal Plains ecoregions and which nematodes statistically contributed to any between ecoregion differences. Our findings could have implications for planning and implementing appropriate nematode control strategies in Georgia pecans.
Working in conjunction with Cooperative Extension agents and pecan growers, PPNs were sampled in 6 and 22 commercial pecan orchards located in four and 21 Georgia counties from Piedmont and Coastal Plain regions, respectively (Tables 1–3). Actively managed pecan orchards were selected from each county with possible symptoms (stunted growth and loss of vigor) of PPN infestations. In each orchard, 7–14 individual pecan trees were arbitrarily selected for soil sampling.
Plant-parasitic nematodes found in commercial pecan orchards in Georgia in 2017, 2018, and 2019.
County | Nematode genera and ecoregions |
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Clarke |
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Rockdale |
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Spalding |
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Walton |
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Appling |
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Berrien |
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Bibb |
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Colquitt |
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Cook |
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Crisp |
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Decatur |
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Dougherty |
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Grady |
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Houston |
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Irwin |
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Jefferson |
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Lee |
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Mitchell |
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Peach |
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Taylor |
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Tift |
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Turner |
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Ware |
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Wilcox |
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Worth |
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Survey of frequency and abundance of five major nematode genera on pecans in Georgia during 2017–2019 by county in two pecan growing ecoregions, Piedmont (P) and Coastal Plain (CP).
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County |
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PFa | ABb | MDc | PFa | ABb | MDc | PFa | ABb | MDc | PFa | ABb | MDc | PFa | ABb | MDc |
Clarke (P) | 10 | 80 | 27 | 160 | 10 | 3 | 3 | 30 | 16 | 37 | 30 | 1 | 1 | 20 | 4 | 5 |
Rockdale (P) | 10 | 10 | 7 | 7 | 20 | 2 | 2 | 80 | 3 | 6 | 100 | 20 | 69 | 70 | 1 | 1 |
Spalding (P) | 10 | 30 | 24 | 50 | –d | – | – | 40 | 2 | 2 | 100 | 23 | 111 | 30 | 1 | 1 |
Walton (P) | 30 | 33 | 21 | 124 | 23 | 4 | 10 | 100 | 8 | 31 | 87 | 27 | 97 | 27 | 2 | 3 |
Appling (CP) | 10 | 90 | 13 | 23 | 80 | 3 | 14 | 100 | 10 | 33 | 10 | 1 | 1 | – | – | – |
Berrien (CP) | 09 | 44 | 188 | 658 | 22 | 2 | 3 | 56 | 16 | 41 | 11 | 1 | 1 | – | – | – |
Bibb (CP) | 10 | 20 | 36 | 58 | 100 | 7 | 13 | 100 | 24 | 97 | 100 | 206 | 637 | 50 | 2 | 5 |
Colquitt (CP) | 07 | 71 | 41 | 83 | 100 | 4 | 9 | 100 | 44 | 89 | 71 | 13 | 25 | 14 | 2 | 2 |
Cook (CP) | 10 | – | – | – | 50 | 2 | 3 | 100 | 10 | 71 | – | – | – | 10 | 5 | 5 |
Crisp (CP) | 12 | 58 | 11 | 43 | 58 | 3 | 5 | 83 | 14 | 49 | 58 | 28 | 100 | 42 | 2 | 5 |
Decatur (CP) | 10 | 70 | 27 | 78 | 20 | 1 | 1 | 90 | 5 | 12 | 10 | 1 | 1 | 60 | 2 | 5 |
Dougherty (CP) | 10 | 50 | 5 | 11 | 50 | 3 | 5 | 100 | 24 | 45 | 20 | 2 | 2 | 20 | 1 | 1 |
Grady (CP) | 10 | – | – | – | 20 | 1 | 1 | 90 | 18 | 46 | 60 | 3 | 4 | 40 | 6 | 14 |
Houston (CP) | 14 | 43 | 14 | 34 | 14 | 2 | 2 | 93 | 54 | 293 | 14 | 16 | 30 | 36 | 3 | 9 |
Irwin (CP) | 10 | 40 | 38 | 67 | 40 | 2 | 3 | 100 | 49 | 309 | – | – | – | – | – | – |
Jefferson (CP) | 10 | 70 | 10 | 60 | 70 | 3 | 8 | 90 | 40 | 121 | 10 | 1 | 1 | 40 | 1 | 1 |
Lee (CP) | 10 | 30 | 2 | 2 | – | – | – | 100 | 11 | 26 | – | – | – | – | – | – |
Mitchell (CP) | 10 | 40 | 5 | 12 | 70 | 4 | 14 | 90 | 37 | 121 | 50 | 3 | 5 | 10 | 2 | 2 |
Peach (CP) | 20 | 15 | 35 | 84 | 95 | 10 | 29 | 85 | 18 | 72 | 95 | 27 | 187 | 50 | 2 | 4 |
Taylor (CP) | 10 | 40 | 1 | 1 | 20 | 3 | 4 | 90 | 6 | 27 | – | – | – | 20 | 3 | 3 |
Tift (CP) | 10 | 70 | 11 | 36 | 60 | 3 | 3 | 100 | 20 | 49 | – | – | – | – | – | – |
Turner (CP) | 10 | 30 | 39 | 104 | 10 | 1 | 1 | 100 | 20 | 62 | 40 | 2 | 6 | 90 | 3 | 5 |
Ware (CP) | 10 | 20 | 3 | 4 | 10 | 1 | 1 | 100 | 11 | 31 | 10 | 1 | 1 | 20 | 2 | 3 |
Wilcox (CP) | 10 | 70 | 22 | 119 | 30 | 4 | 6 | 90 | 16 | 36 | 10 | 13 | 13 | 10 | 1 | 1 |
Worth (CP) | 10 | 80 | 18 | 64 | 10 | 1 | 1 | 100 | 28 | 60 | – | – | – | 40 | 2 | 4 |
Frequency of occurrence and abundance of plant-parasitic nematodes in commercial pecan orchards from two ecoregions of Georgia, 2017–2019.
Coastal Plain region | Piedmont region | |||||||||||||||
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Mean percentages of 6 soil samples | Mean percentages of 4 soil samples | |||||||||||||||
Nematode genus | PFa | ABNb | STD | MDc | Sand | Silt | Clay | pH | PFa | ABNb | STD | MDc | Sand | Silt | Clay | pH |
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2 | 35 | 9.3 | 138 | 87.5 | 5.92 | 6.56 | 6.6 | 3 | 4 | 0.7 | 5 | 65.5 | 17.25 | 17.28 | 5.45 |
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28 | 3 | 1.6 | 14 | 87.5 | 5.92 | 6.56 | 6.6 | 33 | 2 | 1.0 | 5 | 65.5 | 17.25 | 17.28 | 5.45 |
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7 | 4 | 1.5 | 15 | 87.5 | 5.92 | 6.56 | 6.6 | 3 | 2 | 0.4 | 2 | 65.5 | 17.25 | 17.28 | 5.45 |
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1 | 1 | 0.1 | 1 | 87.5 | 5.92 | 6.56 | 6.6 | 13 | 4 | 1.5 | 7 | 65.5 | 17.25 | 17.28 | 5.45 |
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1 | 6 | 2.9 | 42 | 87.5 | 5.92 | 6.56 | 6.6 | 3 | 4 | 0.7 | 5 | 65.5 | 17.25 | 17.28 | 5.45 |
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93 | 23 | 35.2 | 309 | 87.5 | 5.92 | 6.56 | 6.6 | 75 | 7 | 7.2 | 37 | 65.5 | 17.25 | 17.28 | 5.45 |
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44 | 25 | 47.6 | 658 | 87.5 | 5.92 | 6.56 | 6.6 | 37 | 23 | 27.0 | 160 | 65.5 | 17.25 | 17.28 | 5.45 |
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1 | 1 | 0.1 | 1 | 87.5 | 5.92 | 6.56 | 6.6 | 15 | 3 | 1.3 | 6 | 65.5 | 17.25 | 17.28 | 5.45 |
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30 | 44 | 58.6 | 637 | 87.5 | 5.92 | 6.56 | 6.6 | 82 | 23 | 27.7 | 111 | 65.5 | 17.25 | 17.28 | 5.45 |
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46 | 5 | 4.2 | 29 | 87.5 | 5.92 | 6.56 | 6.6 | 17 | 3 | 1.6 | 10 | 65.5 | 17.25 | 17.28 | 5.45 |
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4 | 23 | 11.1 | 164 | 87.5 | 5.92 | 6.56 | 6.6 | 8 | 2 | 0.9 | 6 | 65.5 | 17.25 | 17.28 | 5.45 |
Each tree was then sampled by removing 10 random soil cores (15 cm deep x 2.5 cm diam) under canopy drip line around tree stem (Khanal et al., 2016). Soil cores from each tree were combined into one composite sample and 7–14 such composite samples were collected for each orchard. A total of 282 composite samples collected from all the selected pecan orchards and each composite sample was placed in a plastic bag and transported back to the Extension Nematology Laboratory, Athens, Georgia in coolers to assay nematode populations.
Plant-parasitic nematodes were collected from a 100 cm3 soil sub-sample taken from each of 282 composite samples using centrifugal sugar floatation technique (Jenkins, 1964). Nematodes from each sample were identified to genus level using diagnostic keys by Mai et al. (1996) and counted at 40X magnification using an inverted compound microscope. Then their frequency of occurrence, abundance, and maximum population densities were calculated using standard formulas (see Tables 2 and 3 captions).
Soil pH and texture (proportions of sand, silt, and clay) analyses were conducted with four samples from two representative pecan orchards (Clarke and Walton counties – Piedmont ecoregion) and with six samples from four pecan orchards (Dougherty, Houston, Peach, and Tift counties – Coastal Plain ecoregion). A total of 10 soil samples were analyzed at the University of Georgia soil test laboratory in Athens, Georgia using standard methods (Gee and Bauder, 1986; Thomas, 1996).
We used a trio of multivariate analyses to evaluate the potential for ecoregion associated pecan PPN community differences. First, we used Non-Metric Multi-Dimensional Scaling (NMS or NMDS) (Kruskal, 1964; McCune and Grace, 2002), an ordination analysis that more accurately represents the structure within and between biological communities that PCA and PCoA (McCune et al., 2002), to visualize the patterns of PPN abundance and occurrence in soil samples collected from the Piedmont and Coastal Plain. NMS analysis was conducted using two data matrices, a primary and a secondary matrix. The primary matrix consisted of count data (relative abundance) from all 11 nematode from each pecan tree composite sample. Because nematode relative abundance varied by several orders of magnitude between genera and some genera were not present in a composite sample (a zero), we
To test for statistically significant differences between the Piedmont and Coastal Plant pecan PPN communities, we retained the primary matrix (
Because MRPP only tests for group membership, e.g. whether the pecan PPN communities differed between Piedmont and Coastal Plain ecoregions, we applied Indicator Species Analysis (ISA) (Dufrêne and Legendre, 1997) to identify which PPN genera, if any, were statistically associated with either ecoregion. Indicator values range from 0 to 100, with a 100 indicating a perfect indicator species (one that is mutually exclusive to a group and always occurs in the highest relative abundance within that group) and zero (a species that is not affiliated with any group either through occurrence or relative abundance) (see Severns and Sykes, 2020 for a description of the analysis). Probability values of ecoregion association were attained through 5,000 randomizations in PC-ORD 7.
The distribution of 11 PPN genera in six and 22 pecan orchards varied among the four and 21 counties that located in Piedmont and Coastal Plain ecoregions, respectively (Table 1). Also, the percent frequencies of occurrence, abundances, and maximum population density of five major PPNs including
The distribution of 11 PPN genera that were associated with pecans in both geographic regions of Georgia varied between Coastal Plain and Piedmont ecoregions (Table 3). Also, the differences in frequencies of occurrences of PPNs and their communities are correlated with the properties of soil from both ecoregions (Table 3). For example, the percent frequencies of occurrence of ring (93%) and root-knot (44%) nematodes were comparatively higher in Coastal Plain, which is dominated with sandy soil containing average of 87.52% sand, 5.92% silt, and 6.56% clay with a pH of 6.6 (averages of six samples) than those in the Piedmont ecoregion (75 and 37%, respectively), which is dominated with sandy-clay-loam soil containing average 65.47% sand, 17.25% silt, and 17.28% clay and a mean pH of 5.5 (averages of four samples). In contrast, the percent frequencies of occurrence of dagger (33%) nematodes was higher in the Piedmont than in the Coastal Plain ecoregion (28%) (Table 3). The maximum population densities of nine nematodes were comparatively higher in Coastal Plain with sandy soil than those in the Piedmont ecoregion with sandy-clay-loam soil (Table 3).
NMDS ordination of the Georgia pecan PPN community yielded a three-dimensional solution that explained 91.9% of the variation in the soil sample data. The final stress of the 3-d solution was 13.36, a relatively stable NMDS ordination for ecological communities (McCune and Grace, 2002). Composite soil samples from the Piedmont ecoregion occurred primarily on one side of the ordination (left), while samples from the Coastal Plain occurred primarily on the opposite side (Fig. 1). In the Piedmont, the centroids (asterisks in Fig. 1) of lesion and sheath nematodes occurred in the center of the Piedmont samples (dark gray pyramids), suggesting these PPNs were proportionately dominant in the Piedmont. The centroids for stubby root, dagger, and stunt were positioned within the Coastal Plain samples (dark gray pyramids), while the centroids for ring, RKN (root-knot nematode), cyst, and spiral were not as obviously associated with either ecoregion (Fig. 1).
3-D NMS ordination of PPN soil samples from 282 Pecan trees in the Piedmont and Coastal Plain ecoregions of Georgia, USA (Final stress of the 3-D NMS solution = 13.36, proportion of variance explained by all three axes combined = 0.911). Dark gray pyramids (▲) represent pecan soil samples collected from the Piedmont ecoregion (
MRPP indicated that the pecan PPN communities statistically differed between the two ecoregions (MRPP results:
Indicator Species Analysis (see Severns and Sykes, 2020 for an analysis description) results showing which PPN nematodes were statistically associated (*) with pecan soil samples from the Piedmont (60 soil samples) or Coastal Plain (222 soil samples) ecoregions of Georgia, USA.
Nematode genus | Indicator value (0–100) | Ecoregion |
|
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Ring, |
73.8 | Coastal Plain | 0.0002 |
Spiral, |
48.1 | Piedmont | 0.0002 |
Stubby-root, |
36.1 | Coastal Plain | 0.0028 |
Root-knot, |
24.4 | Coastal Plain | 0.69 |
Dagger, |
15.3 | Piedmont | 0.85 |
Sheath, |
14.8 | Piedmont | 0.0002 |
Lesion, |
13.0 | Piedmont | 0.0002 |
Lance, |
5.8 | Coastal Plain | 0.28 |
Pin, |
3.6 | Coastal Plain | 0.64 |
Stunt, |
3.3 | Coastal Plain | 0.83 |
Cyst, |
2.0 | Coastal Plain | 0.71 |
The first systematic survey of pecan PPNs in Georgia indicated that 11 genera (
The MRPP analysis indicated that the pecan PPN communities had strong statistical differences between the ecoregions. While a relatively new statistical test of association, ISA is rarely used in plant disease studies but it has successfully identified known PPNs causing plant disease from field samples (Severns et al., 2020) and revealed multiple causal agents in an emerging plant disease (Rivedal et al., 2020; Severns and Sykes, 2020). Indicator Species Analysis in the present study clearly identified several PPN genera that had strong statistical associations with one of the two ecoregions. We also found that the maximum population densities of 9 out of 11 PPN genera were comparatively higher in the Coastal Plain than those occurring in the Piedmont. These ecoregion differences in the pecan PPN community and ecoregion-specific association with individual PPN genera may be due to soil texture characteristics that are known to influence PPN distribution and abundance (Norton and Hoffmann, 1974; Norton et al., 1971). The well-drained and coarse, sandy soils in the Coastal Plain (Markewich et al., 1990) and comparatively ill-drained in the Piedmont (Markewich et al., 1990) may be conducive and unfavorable for movement/reproduction/development of PPNs, respectively (Anonymous, 2018; Jones et al., 1969; Kim et al., 2017; Koenning et al., 1996; Pang et al., 2011; Sasser, 1954; Seshadari, 1964). Other soil factors (e.g., nutrients or organic matter) may also affect PPN populations (Noe and Barker, 1985; Norton et al., 1971), but those soil traits were not measured in our study. We recognize that there were fewer replicate sites for the Piedmont ecoregion compared with the Coastal Plain, yet the six replicate sites in the Piedmont are considered appropriate for valid analyses and conclusions. Nonetheless, future studies may expand upon our study and utilize more sites and replicates from each ecoregion.
Our results agree with the findings of previous researchers, who reported greater PPN population densities of nine genera (cyst, dagger, lance, pin, ring, root-knot, spiral, stubby-root, and stunt nematodes) in the rhizospheres of different crops in sandy compared to clay soils (Brodie, 1976; Dropkin, 1980; Koenning et al., 1996; Lewis and Smith, 1976; Martin et al., 1994; Olabiyi et al., 2009; Wallace et al., 1993). In contrast, the population density of root-lesion nematode,
Although we found 11 PPN genera in our pecan study, only
In the present study,
Next to RKNs, spiral nematodes (
In the rhizosphere of Georgia pecans, stubby-root and dagger nematodes were present in 39 and 29% soil samples with the highest population densities of 29 and 14 nematodes/100 cm3 soil, respectively. Both of these nematodes are considered economically important because they may vector various types of plant virus diseases. According to Brown et al. (1995), nepoviruses are transmitted by species in the genera
Although both stunt (
To the best of our knowledge, this is the first study that revealed associations between soil properties and PPN community structures in pecan orchards located in Piedmont and Coastal Plain ecoregions of Georgia. We demonstrated that 11 PPN genera were common to both ecoregions but their communities strongly differed in composition between the ecoregions. These ecoregion-associated differences appear to be related to soil texture and drainage. Further studies are needed to discover the influence of chemical properties of soil on the abundance and diversity of PPN communities of pecans and other crops in the region that in turn may help in planning IPM programs for overall pecan husbandry in Georgia.