© 2008 Plant Management Network.
Effect of Southern Root-knot Nematode (Meloidogyne incognita race 3) on Corn Yields in Alabama
K. L. Bowen, Professor, A. K. Hagan, Professor, and H. L. Campbell, Research Associate, Department of Entomology and Plant Pathology, Auburn University, AL 36849; and S. Nightengale, Associate Superintendent, E. V. Smith Research Center, Tallassee, AL, 36078
Bowen, K. L., Hagan, A. K., Campbell, H.L., and Nightengale, S. 2008. Effect of southern root-knot nematode (Meloidogyne incognita race 3) on corn yields in Alabama. Online. Plant Health Progress doi:10.1094/PHP-2008-0910-01-RS.
In Alabama and other southeastern states, corn is frequently planted in rotation with cotton and peanut in order to minimize soil-borne pest problems. Even in areas where peanut is not grown, corn is increasingly being planted in rotation with cotton. However, one root-knot nematode, Meloidogyne incognita race 3, causes damage to both cotton and corn. In this study, we determined levels of corn grain loss when southern root-knot nematodes are present in soil. Losses were 3.8 to 11.4% based on preceding years’ counts and 2.2 to 7.0% with current years’ counts for every 100 2nd-stage juvenile root-knot nematodes in 100 cm³ of soil. Knowledge of the percent loss in corn grain due to southern root-knot nematode populations can provide additional guidance for use of risk thresholds when growers are making management decisions.
Cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.) are important agronomic crops in Alabama, with a combined production valued at more than $300 million annually. Corn (Zea mays L.) is often recommended as a rotation crop with either cotton or peanut [e.g., (10,18)] and the value of corn production in the state has varied from $23 to $60 million over the past decade (17). Statewide acreage of these crops fluctuates from year to year as commodity and contract prices change. With corn production, it is especially true that as prices rise, corn acreage increases.
Corn in rotation with peanut contributes to the minimization of stem rot of peanut, caused by Sclerotium rolfsii Sacc. (10). In cotton, corn is a good rotational crop for minimizing populations of reniform nematode (Rotylenchulus reniformis Linford & Oliveira) (26). Historically, however, the southern root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood (races 3 and 4), has been of more concern in cotton than the reniform nematode because of the damage it causes and its widespread distribution (14). Corn can be a good host for M. incognita [e.g., (3)] which can limit its suitability as a rotation crop with cotton where root-knot nematodes are a concern (9,25).
Throughout the southeastern USA, risk or action thresholds for southern root-knot nematode populations in corn are ≥ 100 nematodes per 100 cm³ of soil (2,4,22). These thresholds for a crop to be planted are based on assays of soil collected near or at the time of maturity of the previous crop (4) in the late summer or early fall of the year preceding planting (2,22). When root-knot nematode populations are greater than the threshold value, control practices are recommended including the possible use of nematicides. However, little or no information is available on the actual yield loss when corn is affected by southern root-knot nematodes. In a rotation study recently conducted in east-central Alabama, varying populations (due to rotation sequences) of M. incognita race 3 were found. We used data from this study to determine yield losses in field corn due to root-knot nematode.
Determining Corn Grain Loss in Affected Soil
A rotation sequence was initiated in 2003 at the Plant Breeding Unit near Tallassee, AL. Rotation sequences included: continuous corn, corn alternated with 1 and 2 years of peanut, corn alternated with 1 and 2 years of cotton, 2 and 3 years of corn following 1 or more years of peanut or cotton (Table 1, Fig. 1). Following harvest of all crops in each year, the experimental area was maintained fallow through winter months.
Table 1. Crop rotation sequences in which soil populations of Meloidogyne incognita race 3 were monitored in corn in east central Alabama.
Prior to 2003, the site was cropped to cotton in 2002, sweet corn in 2001, and lupine or vetch as winter cover crops in 2000. High population levels of M. incognita had become established in the study site prior to project initiation. The North Carolina differential host range test (12) was conducted in the greenhouse on root-knot nematode isolates from the study site. Heavy galling was observed on cotton, pepper, watermelon, and tomato, but not tobacco or peanut, identifying this nematode as M. incognita race 3. The experimental design of this field study was a randomized complete block with four replications. Individual plots consisted of eight rows at 0.9-m spacing that were 9.1 m in length.
Crop production. Corn was planted at a rate of 5.8 seed/m of row in Cahaba loamy sand with < 1% organic matter on 24 March 2004, 15 April 2005, 29 March 2006, and 20 March 2007. The corn cultivar Pioneer 3167 was sown in 2003 and 2004, while the cultivar Pioneer 31G66 was planted in 2005, 2006, and 2007. Several weeks prior to planting, plot areas were either chiseled or turned then prepared for planting with a disk harrow. Alabama Agricultural Extension System recommendations were followed for fertility, weed, and insect management (8). Ammonium nitrate (34-0-0) was broadcast and soil incorporated to deliver N at 67 kg/ha prior to planting and, prior to silking, layby applications of either ammonium nitrate or ammonium sulfate (32-0-0) provided an additional 67 to 100 kg of N per ha in each year. A hose-tow irrigation system was used to provide up to 2.3 cm of water per week when rainfall was inadequate. Plots were harvested on 18 August 2004, 26 August 2005, 10 August 2006, and 14 August 2007. Corn yields are reported at 15% moisture.
The peanut cultivar Georgia Green was used in this study. Recommendations for peanut crop production from the Alabama Cooperative Extension System were followed for weed, insect and disease management (24), including in-furrow treatment with aldicarb (Temik 15G, Bayer Crop Protection, Kansas City, MO) at 1.1 kg a.i./ha. Peanuts were irrigated on the same schedule as previously described for corn.
In 2003 and 2004, the cotton cultivar Stoneville 4892 was used in rotation sequences; cotton plots were split in 2005 and 2006 into equal-sized sub-plots of the cultivars Stoneville 4892 and DPL 555. Only the cultivar DPL 555 was sown in 2007. Recommendations for cotton crop production from the Alabama Cooperative Extension System were followed for fertility, weed, insect, and disease management (6).
Nematode and yield data collection and analysis. All plots were sampled for nematode population determination within 2 weeks of harvest. Approximately 20 soil cores per plot were taken in the root zone to a depth of 10 cm with a 2.54-cm soil tube and bulked for a nematode assay (19). Root-knot nematode juveniles were separated from a 100-cm³ soil sample using a sieving and centrifugation procedure (13), enumerated, and the data presented as the number of 2nd-stage juveniles per 100 cm³ of soil. Data were tested for outliers and for fit to a normal distribution according to the Kolmogrov-Smirnov statistic (P ≤ 0.05).
Analysis of variance was done on data within each year. Means were differentiated using Fisher’s protected least significance difference (P ≤ 0.05). Non-normal data were transformed using square roots and back-transformed for presentation. Relative yields were calculated for each plot as a proportion of the maximum yield for each year, where the maximum yield was the average from plots in which corn followed peanut. Relative yield was regressed on root-knot densities and forced through an intercept of 1.0. Models describing the relationship of relative yield to root-knot nematode densities were calculated using same year data (i.e., both nematode populations and yields from the same year) and, where possible, using preceding year’s nematode populations (e.g., 2005 yield on 2004 nematode counts). This latter calculation was done because threshold values for nematode densities are based on populations determined in August through November preceding planting (2,4,22). Because nematode counts were significantly lower in sub-plots of the cotton cultivar Stoneville 4892 than DPL 555, only data from the former cultivar were used in 2005 and 2006. The coefficients of determination (R²) of resultant relative yield models were compared to determine which nematode count (preceding year or same year) explained a greater proportion of the variation in relative yield values.
Root-knot Nematode Density Affects Corn Yield
Root-knot nematode populations. Root-knot nematode counts were found to fit a normal distribution in all years except 2004. Prior to analysis, 2004 data were subjected to square-root transformation, which normalized the data. (Transformed counts are back-transformed for presentation.) Average root-knot nematode counts were lower in 2004 compared to other years and lower in 2007 than in 2005 and 2006. During the 2- to 3-week period prior to planting (and before initiation of the irrigation regime) less than 1-cm rainfall was recorded in 2004 and 2007 (Alabama Mesonet data through AWIS.com). Since normal rainfall is about 3.2 cm/week in March and early April at the study site, soil would have been dry at the time of corn planting and these conditions could have led to reduced nematode survival or egg hatch (5) and, ultimately, lower nematode populations during the 2004 and 2007 growing seasons. Otherwise, total rainfall and irrigation amounts between planting and corn harvest were generally similar in the 4-year period of this study.
Lower nematode densities in 2004 than in other years could also be due to a lower reproduction rate for M. incognita on ‘Pioneer 3167’ (planted only in 2004) than on ‘Pioneer 31G66.’ Unfortunately, published reports do not provide direct comparisons of these two corn cultivars relative to their susceptibility to M. incognita race 3. However, in one study in Georgia, the reproduction ratio for root-knot nematodes on ‘Pioneer 3167’ (Pf/Pi = 27) was 85% less than the average (= 179.5) of 24 cultivars tested (3); while in 2 years of testing in Alabama, the reproduction ratio of M. incognita on ‘Pioneer 31G66’ was 73.5% and 47.5% of the average over 15 cultivars (11). Thus, relative to an apparently random collection of cultivars, root-knot nematode reproduction appears to be lower on ‘Pioneer 3167’ than on ‘Pioneer 31G66.’ The two corn cultivars used in the current study were selected arbitrarily, based on their agronomic adaptation to the region and on seed availability. Both cultivars are considered full-season cultivars with good to excellent performance and neither is modified for Bt toxin expression. ‘Pioneer 3167’ is not modified for glyphosate resistance, while ‘Pioneer 31G66’ is modified with glyphosate resistance.
Population densities of root-knot nematodes varied among rotation sequences and were lowest in plots of corn following peanut (Fig. 2). Lower root-knot nematode populations were expected in plots of corn following peanut, since M. incognita race 3 does not reproduce on peanut (7). Highest root-knot nematode populations were generally observed when corn was cropped behind cotton. Although M. incognita race 3 has been shown previously to reproduce well on corn [e.g., (3)], these results indicate that the nematode reproduces nearly as well on corn as it does on cotton in our conditions.
No other plant parasitic nematodes were detected in corn plots of this study; free living nematodes were not assayed. In addition, no other diseases were observed. Thus, it appears that the southern root-knot nematode was the primary pathogenic agent to be affected by crop rotations and to account for differences in corn yield.
Corn yields. Corn yields were found to fit a normal distribution (P > 0.15) in all years except 2004. Prior to analysis, 2004 yield data were subjected to square-root transformation, which normalized the data. (Yield data for 2004 are back-transformed for presentation.) Average corn grain yields were higher in 2004 (> 10,000 kg/ha) than in subsequent years in which similar yields were observed, averaging between 6,000 and 7,700 kg/ha (Fig. 3). These yields are reflective of prevailing state yield averages for these years (17), but might also be due to higher yield potential for the cultivar Pioneer 3167 which was planted only in 2004.
In 3 of 4 years, grain yield of corn following peanut was significantly greater than when corn followed cotton. Grain yield of corn following corn was significantly less than that of corn following peanut in 2 of 4 years (Fig. 3). In 2007, yields were lower in plots of monoculture corn or corn following cotton compared with corn after peanut, but this was not a significant difference. Thus, in all study years, lower corn yields were consistently observed from plots with rotations involving cotton or continuous corn than from plots with a rotation to peanut.
Nitrogen from peanut residues could be contributing to the higher corn grain yields observed in this study when corn followed peanut in rotation. However, previous research has shown that nitrogen from peanut residue did not increase the biomass of rye as a winter cover crop immediately following the peanut crop (1). In addition, neither leaf N nor seed yield increased in cotton due to residual nitrogen from peanut grown in the preceding season (15). Thus, it appears that residual nitrogen from peanut is likely too low to contribute to increased corn yield as observed in the current study.
Relationship of corn yield to nematode populations. In all four years of the study, relative yield of corn was inversely proportional to root-knot nematode densities as determined immediately prior to harvest of the same year. Models describing relative yield, based on root-knot counts in the same year, were significant in 3 of 4 years (Table 2). The regression coefficient of the 2004 model, indicating losses of 7.2% per 100 root-knot juveniles in 100 cm³ of soil, was substantially greater than coefficients from later years’ models which estimated losses at 2.2 to 3.0%. The 2007 model was not significant, although the regression coefficient of this model (and thus the loss estimate) was similar to that of the 2006 model.
Table 2. Statistics from regression models of relative plot yields on counts of 2nd-stage juveniles (J2) of Meloidogyne incognita race 3, Tallasee, AL.
x Corn cultivar Pioneer 3167 was planted in 2004, while ‘Pioneer 31G66’ was planted in 2005, 2006, and 2007.
y This value is a percent and represents the change in relative yield per single root-knot nematode detected per 100-cm³ soil sample. Negative values reflect yield loss.
z Significance set at P = 0.05.
The three models based on preceding year’s root-knot nematode counts also showed inverse relationships between relative yield and nematode populations (Fig. 4). Two of these three models were significant. Preceding year’s nematode counts explained more of the variation in 2005 and 2006 relative yields than did same year nematode counts, as indicated by the higher R² values of models with preceding year’s counts. This observation follows recommendations that nematodes be sampled in late summer of the season preceding spring planting (2,4,22).
The 2005 and 2006 models indicate 11.4 and 3.8% loss, respectively, in corn grain yield for every 100 root-knot nematodes in 100 cm³ of soil sampled in late summer of the preceding year. The disparity in these loss estimates may be due to a number of factors, including the fact that the 2004 corn cultivar differed from that in other years. In fact, when the nematode counts from ‘Pioneer 3167’ plots are removed from calculations, the resultant 2005 model indicates 7.2% loss per 100 nematodes which is closer to the loss estimate of 2006. Environmental differences between years could also have affected the degree of loss experienced in any year. The regression coefficient of the 2007 model also differed substantially from the 2005 and 2006 models (0.4% loss per 100 nematodes), but this model was not significant. However, there were two obvious outlying observations in the 2007 model (nematode counts from 2006 > 800) and removal of these two observations changed the 2007 loss estimate to 2.1% per 100 nematodes. While this loss estimate is closer to that of the 2006 model, significance of the recalculated 2007 model did not improve.
The effects of plant parasitic nematodes on corn yields have not been well quantified in recent years. Sumner and Minton (21) noted that populations of several nematodes [M. incognita, Pratylenchus spp., Criconemella ornata (Raski) Crozzoli & Lamberti, and Paratrichodorus minor (Colbran) Siddiqi] contributed to grain yield losses due to Rhizoctonia solani Kühn on corn, but did not quantify this effect. Todd and Oakley (23) determined that early season populations of Pratylenchus spp. (lesion nematodes) caused a 1% loss in corn seed yield per 1000 nematodes per gram of root tissue. Loss estimates determined in the current study (between 2 and 7% per 100 nematodes) were greater than those for Pratylenchus, and this would be expected given corn can tolerate greater populations of lesion nematodes than root-knot nematodes (2,4,20).
In this study, loss estimates were calculated using nematode densities as assayed near crop maturity as well as in the late summer several months prior to planting corn. Nematode counts made in late summer prior to planting accounted for more of the variability in relative corn yields than did counts done near crop maturity. In addition, "risk thresholds" for nematodes on corn are based on densities determined at the end of the preceding crop season (2,4,22) and allows time and planning for nematode management. Generally, when the threshold is exceeded, the cost of a nematicide application may be justifiable (22). The specific information on losses provided through this study can provide further guidance in decisions regarding nematicide use in corn. For example, when corn grain yield potential is anticipated at 7500 kg/ha and corn prices are $314/tonne, a 4% loss at the threshold level of 100 root-knot nematodes would equal 300 kg/ha or about $47/ha loss. Since terbufos at 1.5 kg a.i./ha (highest label rate of application) costs about $47 (16), this is approximately the break-even point for terbufos use in corn (8) at these prices. Greater losses, higher yield potential, or higher corn prices would justify terbufos use, while lower prices or lower yield potentials would not.
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