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Published in Agron. J. 96:1419-1428 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

Production Paper

Evaluation of Crop Rotation and Other Cultural Practices for Management of Root-Knot and Lesion Nematodes

Robert J. Kratochvila,*, Sandra Sardanellib, Kathryne Evertsa and Elizabeth Gallagherc

a Dep. of Nat. Resour. Sci. and Landscape Architecture, Univ. of Maryland, Rm. 1112-B, H.J. Patterson Hall, College Park, MD 20742-4452
b Dep. of Entomol., Univ. of Maryland, College Park, MD 20742-4454
c Dorchester County Ext., Univ. of Maryland, P.O. Box 299, Cambridge, MD 21613-0299

* Corresponding author (rk32{at}umail.umd.edu)

Received for publication September 22, 2003.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Root-knot [Meloidogyne incognita (Kofoid & White) Chitwood] and lesion [Pratylenchus penetrans (Cobb) Chitwood & Oteifa] nematodes are important pathogens that cause yield and quality losses for most vegetable and field crops in Maryland when they exceed certain threshold levels and if control measures are not applied. Chemical nematicides are the primary control tactic, but their use is both costly and raises environmental concerns. This study was conducted to evaluate the efficacy of crop rotation and other cultural practices for management of southern root-knot nematodes (RKNs) and lesion nematodes. Three nonhost crops, a RKN-resistant soybean [Glycine max (L.) Merr.] cultivar, and poultry litter/tillage (Year 1) and fallow (Year 2) were used as summer rotation crops/management options following production of nematode-susceptible crops on two sites in Dorchester County, MD, on Downer and Hammonton sandy loam soils (coarse-loamy, siliceous, mesic Typic and Aquic Hapludults), respectively. Sorghum sudangrass [Sorghum bicolor (L.) x S. arundinaceum (Desv.) Stapf var. sudanense (Stapf) Hitchc.], grown annually as a green manure crop following a nematode-susceptible crop [potato (Solanum tuberosum L.) or cucumber (Cucumis sativus L.)], reduced the RKN population as effectively as the control treatment (soybean cultivar with no known RKN resistance and one nematicide application). Sorghum sudangrass and poultry litter/tillage/fallow were equally effective in managing the lesion nematode population. Annual inclusion of these practices was necessary to maintain the reduced population levels that were attained for these two nematode species. Finally, either summer or early-fall sampling dates were determined to be more effective than a midspring sampling date for identifying threshold levels of these two pests.

Abbreviations: RGI, root gall index (or indices) • RKN, root-knot nematode • SCN, soybean cyst nematode


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PLANT-PARASITIC NEMATODES are important pathogens that cause yield and quality losses for most vegetable and field crops if management measures are not applied. Root-knot nematodes (Meloidogyne spp.) affect more than 2000 species of plants, including forage crops, small grains, fruits, vegetables, field crops, nursery crops, turfgrasses, and weeds. Root-knot nematodes cause root galling that is often accompanied by stunting, chlorosis, and wilting of the host plants. Where high population densities of the nematode exist, crop failure can occur (Johnson et al., 1992). Lesion nematodes (Pratylenchus spp.) have approximately 400 different crop and weed species that serve as hosts (Mai et al., 1977). Their feeding causes dark lesions and an overall browning of the roots and may decrease root growth by 25% on soybean (Acosta and Malek, 1981). Crop losses have accompanied a lesion nematode infestation even when the obvious symptoms of chlorosis and stunting have not occurred (Acosta, 1999, p. 54). The wide host range for these two nematode genera makes crop rotation as a population management tactic difficult in a system that uses only susceptible crops (Potter and Olthof, 1993).

Management tactics that have demonstrated significant improvement in nematode population management include rotations that use nonhost crops, resistant cultivars, green manure crops, or nematode-suppressive plants and the use of organic soil amendments, cultivation to suppress weed host species, and fumigant and nonfumigant nematicides (Al-Rehiayani et al., 1999; Crow et al., 1996; Ferris et al., 1994; Ibrahim et al., 1993; Johnson and Motsinger, 1990; Kaplan and Noe, 1993; McSorley and Dickson, 1995; Mojtahedi et al., 1991; Weaver et al., 1995).

Reports of the success of crop rotations using nonhost and nematode-suppressive crops are numerous. A Pacific Northwest project found that the incorporation of green manure from rapeseed (Brassica napus L.), white clover (Trifolium angustifolium L.), and sorghum sudangrass improved control of nematodes and soilborne diseases in the following potato crop (Stark, 1995; Eberlein et al., 1997, 1998). Rehiayani and Hafez (1998) reported a cultivar of sudangrass (Trudan 8) that caused suppression of northern RKN (Meloidogyne hapla Chitwood) infestation in vegetables. In another study, soil amended with all parts of sudangrass resulted in lower reproduction of M. hapla on lettuce (Lactuca sativa L.) (Viaene and Abawi, 1998). Sudangrass contains dhurrin, a cyanoglucoside compound. As the sudangrass decomposes following tillage into the soil, an enzyme degrades the dhurrin, releasing hydrogen cyanide (Adewusi, 1990). Other products such as nitriles or isothiocyanates that have nematicidal properties are also produced during the degradation of sudangrass (Donkin et al., 1995). Studies in Alabama have shown that the addition of "exotic" nonhost crops, such as switchgrass (Panicum virgatum L.) and other selected warm-season forage grasses, in rotation systems increased yields of peanut (Arachis hypogaea L.) and vegetables and managed nematode and other soilborne disease problems (McSorley et al., 1994a, 1994b; McSorley and Dickson, 1995; Rodriguez-Kabana et al., 1998). The use of radish (Raphanus sativus L.) and mustard (Brassica rapa L.) as trap crops resulted in a high level of control of sugar beet (Beta vulgaris L.) nematode (Heterode schachtti) (Gray et al., 1997; Shigaki et al., 1998). Castor bean (Ricinus communis L.) was identified as a suppressive crop for southern species of RKN (Hagan et al., 1994). Poultry litter has been used as an organic soil amendment alternative to nematicide use for suppression of RKN in South Carolina (Fortnum, 1995). This study determined that the poultry litter stimulated microbial activity in the soil that was antagonistic to the nematodes.

Maryland has two economically important species of endoparasitic nematodes, Meloidogyne incognita and Pratylenchus penetrans, that are widely distributed throughout the state (Jenkins et al., 1957; Sindermann et al., 1993). Fields cropped repeatedly to susceptible vegetable and agronomic crops on the Eastern Shore of Maryland have experienced outbreaks of RKN and root-lesion nematodes. These outbreaks have become increasingly costly to farmers who have been producing potato for the chip industry during the late 1990s (potato growers, personal communication, 1999). This contract potato production resulted in RKN-susceptible potato grown in rotation with other susceptible vegetable [cucumber, green bean (Phaseolus vulgaris L.), and sweet corn (Zea mays L.)] and field (soybean) crops under double-crop conditions following the potato harvest.

Double-crop production of these crops is particularly susceptible to the damaging effects from the nematodes because the crops are planted during late June and July and coincide with a high level of nematode activity. The University of Maryland Nematology Laboratory and Plant Clinic had noted an increase in the number of root and soil samples collected from potato fields that were containing high populations of infective-stage RKN juveniles (J2) (Sardanelli, personal communication, 2000). Affected farmers increased their utilization of chemical nematicides (Beste et al., 2001) as the primary management tactic, but their use is costly (approximately $350 to $400 ha–1). Additionally, environmental concerns have resulted in the loss of many nematicides, and for those remaining, the costs for re-registration may limit their availability for producers. This has focused attention upon the development of alternative methods for managing plant-parasitic nematodes.

The objective of this research was to determine if selected cultural practices used with susceptible crops could effectively manage the mixed parasitic nematode populations of the RKN and lesion nematodes on Maryland's Eastern Shore.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field Procedures
Dorchester County, Maryland, has approximately 1000 ha of potato for potato chip manufacture grown annually. Following a 1999 meeting of approximately 20 Dorchester County potato producers where a discussion about the widespread increase in parasitic nematode populations following potato occurred, two field sites that had potato in their crop rotation were identified for purposes of evaluating cultural management practices. These two fields had documented RKN and root-lesion nematode infestations. The two field sites are identified as (i) Farm A, comprised of a Downer sandy loam soil (coarse-loamy, siliceous, mesic Typic Hapludults), and (ii) Farm B, comprised of both a Downer sandy loam soil and a Hammonton sandy loam soil (coarse-loamy, siliceous, mesic, Aquic Hapludults). Each field site was divided into four blocks that were 45 by 120 m. The experimental design was a randomized complete block arrangement of treatments with six treatments randomly assigned to each of the blocks, creating six plots per block that were 7.5 by 120 m in size. To increase sample size and improve sampling technique because of anticipated spatial variability for nematode populations, three subplots (7.5 by 40 m) were delineated within each plot.

Five cultural management treatments plus a control treatment comprised the six treatments. These treatments were planted/applied to their respective experimental plots following potato harvest in 2000 and cucumber harvest in 2001 (Table 1). Both Farm A and B plots were tilled with a disk and packer immediately following potato (2000) and cucumber (2001) harvest and before planting the cultural management crop treatments. This was done to bury potato and cucumber vine residues and establish a better seedbed for the management crops. No chemical weed control was used for any of the management treatments, and little weed pressure was noted in any of the plots during either year. Cash crop nematode management options are of significant economic interest to the potato farmers. Two potential cash grain crops were included as cultural management treatments in the study: (i) grain sorghum cultivar Northrup King brand KS 585 planted at a rate of 185000 seeds ha–1 and (ii) RKN-resistant soybean cultivar Manokin (Kenworthy, 1995) planted at a rate of 432000 seeds ha–1. Two crops that have been identified as nonhost species for RKN were used as green manure crops: (i) sorghum sudangrass cultivar AgriBiotech Inc. brand Green Grazer planted at a rate of 22 kg ha–1 and (ii) castor bean cultivar Mall planted at a rate of 28500 seeds ha–1. These two green manure crops were mowed after 2 to 3 mo of growth, and the biomass was tilled into the soil. The fifth treatment used in this study was an application of 4.5 Mg ha–1 of poultry litter applied following potato harvest in 2000. The poultry litter was tilled into the soil with a disk immediately following application. This treatment had no poultry litter applied to its respective plots following cucumber harvest in 2001, and the plot area in 2001 remained fallow. The control treatment was a soybean cyst nematode (Heterodera glycines)-susceptible cultivar, Pioneer brand 93B01, that had no reported RKN resistance (Pioneer, 2001). It was planted at the same rate as Manokin following potato harvest in 2000 and cucumber harvest in 2001. These control treatment plots received 100 L ha–1 of the chemical nematicide Telone II (1,3 dichloropropene) preceding cucumber planting in 2001 (Table 1). The control treatment was chosen because it was typical of the crop rotation and nematode management strategy used by the majority of Dorchester County potato farmers and is considered an industry control rather than a true control that would have been soybean cyst nematode (SCN)-susceptible soybean with no nematicide application.


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  		Table 1. Dates for management treatment and sampling activities for the root nematode management study conducted in Dorchester County, Maryland, 2000–2002.

 
Sampling of soil and roots was conducted at various times (Table 1) throughout the study to monitor plant-parasitic nematode species and to assess their population densities. Twenty soil cores (2.5-cm diam. by 20-cm depth) for nematode assay were collected from subplots at each sampling date (Table 1). Samples were thoroughly mixed, and approximately 500 cm3 of soil was placed into a zip lock bag and stored in a cooler for transport to the University of Maryland Nematology Laboratory. Root samples were collected just before harvest for each treatment crop from five randomly selected plants per subplot. Roots were placed into paper bags and stored in a cooler during transport to the Nematology Laboratory. Information about pertinent sampling activities and dates is listed in Table 1.

Cucumber is one of the cash vegetable crops included in the crop rotation for many of the potato farmers in Dorchester County. It is a crop that is susceptible to both RKN and lesion nematodes (Sardanelli et al., 1984). Cucumber was grown twice (2001 and 2002) on Farm A and once (2001) on Farm B during the study period at the industry-standard seeding rate (85500 plants ha–1 in 76-cm spaced rows). Cucumber yield was measured by pulling plants from two randomly selected 61-cm lengths of row per subplot. All cucumber at least 3 cm long was pulled from the plants and weighed.

Laboratory Procedures
Each soil sample was mixed thoroughly before removing a 250-cm3 subsample for vermiform using a modified Baermann method (Christie and Perry, 1951). Extracted nematodes were collected into a counting dish and identified and counted with a dissecting microscope at 40x. Small numbers of other plant-parasitic nematodes were identified during this procedure but were determined insignificant. Roots were rinsed with tap water and rated on the following root gall index (RGI) scale for percentage roots galled: 1 = no galling, 2 = 1 to 25, 3 = 26 to 50, 4 = 51 to 75, and 5 = 76 to 100 (Carter and Sasser, 1982).

Statistical Analyses
Root-knot and lesion nematode populations in the soil samples collected at the seven sampling dates were averaged over the three subplots comprising each of the experimental plots, and those averages were then analyzed by sample date using analysis of variance (ANOVA) procedure for randomized complete block experimental design (Gomez and Gomez, 1976). A chi square test for homogeneity of error variances for each of the sampling dates and for each of the nematode species was conducted. These tests indicated heterogeneity of error variances among the different sampling dates. All soil nematode population data were log (X + 1)-transformed as per Gomez and Gomez (1976) to normalize the data and to accommodate the zero population counts that were present. These log-transformed data were subjected to a pooled analysis of variance for measurements over time (Gomez and Gomez, 1976). Mean separation for the planned comparisons was conducted using the least significant difference (LSD) test at p = 0.10.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Root-Knot Nematode Populations
The first sampling date during May 2000 (Table 1) was determined to be too early for meaningful RKN population detection. The RKN populations on that date were either zero or so small that they were not statistically different from zero at both farms (Tables 2 and 3). No damaging effects to the potato crop caused by the nematodes were visually observed. However, by potato harvest in July and August 2000 on Farms A and B, respectively, the RKN populations had increased significantly from their levels during May 2000 (Tables 2 and 3) and had exceeded spring sampling threshold levels for a number of vegetable (>50 RKNs) or field crops (>100 RKNs) (Krusberg et al., 1993). Additionally, many of the plots on Farm B (Table 3) had even exceeded fall sampling threshold levels for vegetable and field crops (>500 RKNs) (Krusberg et al., 1993). This change in population between May 2000 and July and August 2000 indicated that the early May sampling date used for the initial population assessment had been too early to adequately detect potentially damaging levels of RKNs. This most likely occurred because the May sampling date was done before the accumulation of adequate number of degree-days for this nematode species. Nematode growth and development are directly affected by temperature (Noe, 1991). According to California Integrated Pest Management Program guidelines, RKN requires a minimum temperature of 18°C for infection and 10°C for reproduction to occur. Additionally, a minimum of 600 degree-days (DD10) is required per generation (Westerdahl, 1995). The results from the May sampling indicated that a recommendation system utilizing a degree-days system for nematode sampling in Maryland would provide better decision-making information than the current spring or fall sampling threshold-based system.


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  		Table 2. Second-stage juveniles of root-knot nematode (RKN) per 250 cm3 soil for the seven sampling dates on Farm A.

 

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  		Table 3. Second-stage juveniles of root-knot nematode (RKN) per 250 cm3 soil for the seven sampling dates on Farm B.

 
Treatment Effects Compared with Control
The September/October 2000 RKN populations on the two farms (Tables 2 and 3) measured the response to the first introduction of the management treatments that were applied following potato harvest during either July or August 2000 (Table 1). Soybean cyst nematode susceptible soybean was the control treatment minus the nematicide (not applied until spring 2001) (Table 1). A comparison between the RKN population for any of the five management treatments and the population for the control on this date indicated the efficacy of a management treatment at interrupting the RKN population dynamics. On Farm A (Table 2), a significantly lower population of RKN was observed in sorghum sudangrass plots compared with the control treatment during September/October 2000. On Farm B during October 2000, there were no significant differences in RKN populations between the control and the other five treatments (Table 3).

Before planting the cucumber crop in 2001, the control treatment plots received a commercial application of nematicide (Telone II). The nematicide was highly effective on Farm A, reducing the RKN count to zero (Table 2) as determined by the samples that were collected just before the cucumber harvest in July 2001. All other treatments on Farm A had significantly higher RKN counts at this date compared with the control.

For unknown reasons, the nematicide application was not as successful at reducing the RKN population on Farm B (Table 3) where the population present in July 2001 for the control had not significantly changed from its population observed before its application during October 2000. The only management treatment on Farm B to have a significantly lower population of RKN compared with the control in July 2001 was the poultry litter/tillage treatment (Table 3).

October 2001 RKN populations (Tables 2 and 3) measured the effect caused by the second application of the management treatments applied during July 2001 and following cucumber on both farms. The control plots on Farm A had a detectable RKN population present at this date, but this level was determined to be not statistically different from the zero population observed during July 2001 (Table 2). The only management treatment that had RKN populations that were not significantly different from the control during October 2001 on Farm A was sorghum sudangrass (Table 2). On Farm B, sorghum sudangrass had a RKN population that was significantly less than the control at the October 2001 date (Table 3).

The July 2002 RKN population on Farm A was collected just before the harvest of the second crop of cucumber that was produced on this farm during the study. On this date, the control had a zero count for RKN (Table 2). The other five treatments had significantly greater RKN populations compared with the control (Table 2), indicating that any suppressive effect on the RKN population imparted by the management treatments was not as effective at managing RKN populations as the nematicide application had been. On Farm B, the June 2002 population was collected just before harvest for the second potato crop that was produced on that farm during the study period. All RKN counts were either zero or very low at this sample date (Table 3). This was attributed to the early (March 2002) planting date for the potato crop that coincided with cooler spring temperatures that slowed the rate of infection, growth, and proliferation of the RKN, similar to what occurred with the samples collected in May 2000 (Tables 2 and 3).

October 2002 was the final date for assessing RKN populations on both farms. This date was late in the growing season for the SCN-susceptible soybean that had been planted to all experimental plots following either cucumber on Farm A or potato on Farm B. For this date on Farm A, the control treatment plots continued to have RKN populations that were not significantly different from the zero population that had been observed following the nematicide application in May 2001, indicating that the nematicide was continuing to manage the nematode's population effectively (Table 2). All five management treatments on Farm A had significantly greater populations compared with the control treatment at this final October 2002 assessment date. For the control treatment on Farm B during October 2002, the RKN population had increased significantly compared with its population during June 2002 (Table 3). This population was comparable to what had been observed for the control during October 2000, July 2001, and October 2001 (Table 3). Three management treatments (RKN-resistant soybean, sorghum sudangrass, and castor bean) had RKN populations that were not different from the control at this final assessment date (Table 3).

Treatment Effects over Time
Another way to measure the success of these management treatments for managing the RKN populations was to compare the nematode populations for a treatment across the seven sampling dates. Of particular interest was the change that occurred in populations between July and August 2000 and September/October 2000 and subsequently between July 2001 and October 2001, the two periods that coincided with the application of the management treatments. Any significant population reductions that occurred between those two sets of dates can be used as an indication that the treatment was effective in interrupting the RKN population dynamics.

Between July 2000 and September/October 2000 on Farm A, the control treatment (minus nematicide application) did not have any significant change in RKN population, indicating that the nematode population remained constant during production of two host crops, potato and soybean. However, the control had a significant reduction in population following the nematicide application in May 2001 that was measured by the populations present in July 2001 (Table 2). Following the nematicide application on Farm A, the RKN population did not change significantly from its level during July 2001 through October 2002 (final assessment date), indicating that effective population management of RKN had been attained by the nematicide and that the RKNs had not yet begun to repopulate at the conclusion of the study.

Only one of the management treatments on Farm A (grain sorghum) had a significant RKN population reduction between July 2000 and September/October 2000 (Table 2). This treatment was determined to be as effective as the sorghum sudangrass at reducing RKN population (Table 2). Sorghum sudangrass had a population that was significantly less than the control at the September/October 2000 date, but it had not had a significant reduction in RKN population between July 2000 and September/October 2000 as occurred for grain sorghum. Between July 2001 and October 2001 (the second time the management treatments were applied), only sorghum sudangrass significantly reduced the RKN population (Table 2).

On Farm B, significant reductions in RKN populations occurred between August 2000 and October 2000 in all six of the treatments (Table 3). This included the control treatment that had not yet had the nematicide applied to the plots. Since the nematicide applied during May 2001 did not work as effectively on Farm B as it did on Farm A, the RKN populations observed for the five management treatments during July 2001 were either not significantly different from the control (RKN-resistant soybean, sorghum sudangrass, castor bean, and grain sorghum) or significantly less than (poultry litter/tillage treatment) the control treatment (Table 3). Only two treatments had a significant decline in RKN populations between July 2001 and October 2001, sorghum sudangrass and castor bean (Table 3). The response for sorghum sudangrass on Farm B was comparable to the response observed for that treatment on Farm A.

Management Treatments and Root Gall Indices
Root gall indices are another measurement that can be used to determine the level of parasitic RKN infestation that is present. In this study, the galling indices found on potato indicated that a substantial RKN population was present on both farms near the time of potato harvest during July and August 2000 (Tables 4 and 5). The September/October 2000 RGI corresponded to the first time that the management treatments were introduced to the field sites. The RGI for the management treatments at this sample date can be used to indicate the level of host susceptibility for those treatments. Sorghum sudangrass had the lowest RGI scores at this sample date on both farms (Tables 4 and 5), indicating it was a poor host for RKNs. The RGI for July 2001 and July 2002 from Farm A were measured on cucumber. The change that occurred on Farm A from July 2001 to July 2002 provided a qualitative indication of the management potential for each treatment. Three treatments (RKN-resistant soybean, sorghum sudangrass, and poultry litter/tillage/fallow) and the control treatment had significantly lower RGI during July 2002 than during July 2001 (Table 4). This was an expected response for the control because of the nematicide application it received in May 2001. The three treatments that had similar changes for the RGI compared to the control treatment were determined to have imparted some beneficial RKN management effect. The high RGI values found during October 2002 on Farm A on SCN-susceptible soybean that was grown on all treatment plots following the cucumber harvest for that year indicated how quickly the RKN population can recover following the discontinuation of management measures.


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  		Table 4. Root gall index (RGI) caused by root-knot nematode (RKN) on Farm A.

 

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  		Table 5. Root gall index (RGI) caused by root-knot nematode on Farm B.

 
On Farm B, a high level of galling was observed on cucumber roots for all treatments (Table 5) during July 2001. The RGI indicated no galling present on any of the management crop treatments during October 2001. The failure to detect RGI during June 2002 on Farm B (Table 5) corresponded to the very low populations for RKN detected on this sampling date (Table 3). This was further evidence that summer and early-fall sampling is better for RKN population detection than a midspring sampling date. Samples for RGI index determination were not collected in October 2002 on Farm B.

Lesion Nematode Populations
Management Effects Compared with Control
The May 2000 initial sampling date used on both farms for lesion nematode was too early to detect potentially damaging threshold levels (Krusberg et al., 1993) of lesion nematode (Tables 6 and 7). This was the same as had occurred for RKN detection, and it became strikingly apparent by the significant increase in lesion population that occurred between May 2000 and July–August 2000 for both Farm A and Farm B (Tables 6 and 7). The lesion nematode populations on both farms during July–August 2000 (Tables 6 and 7) were near the threshold level for soybean (>300 lesion nematodes) and greater than the threshold (>200 lesion nematodes) for many vegetable crops that could possibly be produced following potato.


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  		Table 6. Number of lesion nematodes per 250 cm3 soil for the seven sampling dates on Farm A.

 

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  		Table 7. Number of lesion nematodes per 250 cm3 soil for the seven sampling dates on Farm B.

 
For the control treatment before any nematicide application, the lesion nematode populations increased significantly between July 2000 and September/October 2000 on Farm A (Table 6) while no significant population change occurred on Farm B between August 2000 and October 2000 (Table 7). The nematicide application for the control was applied during May 2001. It did not cause a significant change in the lesion nematode populations for the control on either Farm A or Farm B between September/October 2000 (before nematicide application) and July 2001 following its application (Tables 6 and 7). This was different from the response to the nematicide that was achieved during the same time period for RKN on Farm A (Table 2) where a significant decline in population occurred. However, on Farm B, the lesion nematode population response was the same no change in population that had occurred for RKN during the same time period (Table 3).

This response was confusing because any population reduction caused by Telone II occurs immediately after fumigation and is considered no longer effective 3 wk postapplication. In this study, it took until October 2001 for significant reductions in lesion nematode populations for the control treatment to be observed on the two farms (Tables 6 and 7). This delayed response to the nematicide application has two possible explanations. First, since the nematicide had been effective at controlling RKN on Farm A in July 2001 (Table 2) and since no significant reduction in lesion nematode populations was achieved in July 2001 on Farm A, the Telone II was possibly not effective against lesion nematodes. Or, it may have just taken longer for the lesion nematode populations on both Farm A and B to respond to the nematicide, similar to the response observed for RKNs on Farm B (Table 3). In either case, any significant reduction in lesion nematode populations that a management treatment had compared with the control for any sample date was considered to be of interest. Also of note was the fact that any lesion nematode population suppression realized from the nematicide application made during May 2001 was no longer observed by October 2002 on either farm Tables 6 and 7).

The lesion nematode population present during September/October 2000 represented the response to the different management treatments following their first introduction to the two sites. On Farm A, three management treatments (sorghum sudangrass, grain sorghum, and poultry litter/tillage/fallow) had significantly lower lesion nematode populations during September/October 2000 than were observed for the control (Table 6). On Farm B, sorghum sudangrass and the poultry litter/tillage/fallow treatments had populations that were significantly less than those present for the control during October 2000 (Table 7).

The lesion nematode populations that were present on both farms during July 2001 followed production of the susceptible crop, cucumber. All treatments on Farm A (Table 6) and all but one treatment on Farm B (sorghum sudangrass that was significantly greater than the control) had lesion nematode populations that were equivalent to the control treatment. Since the lesion nematode population for the control on Farm A had not changed significantly between September/October 2000 and July 2001 (pre– and post–nematicide application), it caused confusion for determining any population management effect caused by the treatments. The treatments were managing the nematode populations as effectively as the control, but the control treatment had not had a significant change in lesion nematode population between dates that were pre– (Sept.–Oct. 2000) and post–nematicide (July 2001) application on both farms (Tables 6 and 7).

On Farm A, the poultry litter/tillage/fallow treatment that was a fallow-only treatment during the period July–October 2001 had a significantly lower population of lesion nematodes compared with the control (Table 6) at the October 2001 sampling time. Two other treatments on Farm A, sorghum sudangrass and grain sorghum, had lesion nematode populations that were not significantly different from the control (Table 6). On Farm B, four of the management treatments were observed to have lesion nematode populations that were not different from the control (Table 7), with the exception being grain sorghum, which had a significantly greater lesion nematode population compared with the control (Table 7).

The lesion nematode populations did not change significantly for the control treatment between October 2001 and June/July 2002 on either farm (Tables 6 and 7). This indicated that the management effect supplied by the nematicide application during spring 2001 was still providing population control if it in fact had worked. On Farm A, only the poultry litter/tillage/fallow treatment had a lesion nematode population comparable to the control treatment during July 2002 (Table 6). On Farm B, all five of the management treatments had lesion nematode populations that were statistically equivalent to the control (Table 7) at the June 2002 sampling date. The poultry litter/tillage/fallow treatment was the only management treatment that worked effectively on both farms at providing lesion nematode control that was equivalent to the control treatment (Tables 6 and 7).

By October 2002, the lesion nematode population for all the treatments, including the control treatment on Farm A, had increased significantly over what it had been during July 2000, which was before the first introduction of the management treatments (Table 6). On Farm B, all treatments had lesion nematode populations that were either statistically equivalent to or had exceeded the populations measured in August 2000, a date that was just before the application of any of the management treatments (Table 7).

Treatment Effects over Time
The effectiveness of the various management treatments at interrupting the population cycles of lesion nematodes can also be measured by observing the changes in the lesion nematode populations by treatment over time. Since there is a question regarding the effectiveness of the nematicide application as part of the control, this was actually a better mechanism for measuring the effectiveness of the management treatments.

The only treatment on Farm A to have a significant decline in lesion nematode population that coincided with the first application of the management treatments during July 2000 and measured by the September/October 2000 counts was the poultry litter/tillage/fallow treatment (Table 6). None of the treatments on Farm B had a significant decline in population coinciding with the first application of the treatments during August 2000 and measured by the October 2000 counts (Table 7).

The July 2001 sample date on both farms was within a few days of harvest for the susceptible crop, cucumber. Any changes in lesion nematode population that occurred between the September/October 2000 sampling date and the July 2001 date indicated a possible residual management effect imparted by the management treatment. All management treatments but one for both Farm A and B did not change in population between those two dates and, for the most part, were not significantly different from the levels that had been present with the potato crop (July and August 2000) (Tables 6 and 7). On Farm A, the only significant change was an increase for the poultry litter/tillage/fallow treatment (Table 6). On Farm B, the sorghum sudangrass treatment had a significant increase in lesion nematode population between those two dates (Table 7). Since the population increased on Farm A for poultry litter/tillage/fallow, a treatment that had seen a population reduction between July 2000 and September/October 2000, any control benefit that was conferred by the first introduction of that treatment appeared to have little residual activity when the susceptible cucumber crop was produced.

Following the cucumber crop that was harvested on both farms during July 2001, the management treatments were applied for the second and final time to the two sites. The lesion nematode population changes that occurred during the period between July 2001 and October 2001 determined the management effect these treatments supplied with their second application. Significant declines for lesion nematode populations were observed for all the management treatments on both farms with the exceptions of grain sorghum and poultry litter/tillage treatments on Farm B (Tables 6 and 7). Since these populations represented the populations present following the last time the management treatments were applied, they likely indicated a cumulative beneficial effect imparted by their use for the control of lesion nematodes.

To better measure the cumulative effect that the management treatments may have supplied, the change in lesion nematode population that occurred between September/October 2000 and October 2001 is considered. On Farm A, all treatments except sorghum sudangrass had significantly lower lesion nematode populations during October 2001 than were measured during September/October 2000, indicating they all were supplying some management effect. However, the poultry litter/tillage/fallow was determined to be the best treatment on Farm A at effectively managing the lesion nematode populations because its population during October 2001 was also significantly less than the control population on the same date (Table 6). The sorghum sudangrass treatment had lesion nematode populations that were the same during both September/October 2000 and October 2001.

The management treatments on Farm B that had lesion nematode populations during October 2001 that were significantly less than those during October 2000 were the control treatment, the RKN-resistant soybean, and castor bean. The other three treatments had populations of the nematode that were not significantly different for those two dates (Table 7). On this farm, the sorghum sudangrass and poultry litter/tillage/fallow treatments offered the best management of lesion nematode populations because they both had populations that were significantly lower than the control treatment in October 2000. In addition, neither treatment had significant changes in the lesion nematode populations that were measured just before cucumber harvest during July 2001. The nematode populations following the final application of the management treatments (measured by the October 2001 sample) had not changed significantly from October 2000.

Between October 2001 and July 2002, all five of the management treatments on Farm A had significant increases in lesion nematode populations (Table 6). The lesion nematode populations responded differently between October 2001 and June 2002 (following potato) for the five management treatments on Farm B (Table 7) where the only treatment that had a significant increase in lesion nematode population was the RKN-resistant soybean (Table 7). There were two differences in the way the sites were managed before June/July 2002 that may explain the differences observed between the two farms. First, cucumber was the crop grown on Farm A during this time period while potato was produced on Farm B. Second, the sample date was 11 June 2002 for the potato crop on Farm B, and it was nearly a month later, 8 July 2002, for the cucumber crop on Farm A. The difference in the two dates was accompanied by additional degree-days that would complement the reproduction and proliferation of the lesion nematode on Farm A. Both these factors may have influenced the lesion nematode population levels measured on the two farms.

Management Treatments and Cucumber Yield
Yield for cucumber under the different management treatments is reported in Table 8. The poultry litter/tillage/fallow treatment for Farm A in 2001 was the only treatment where significant yield improvement compared with the control treatment (nematicide application before production of the cucumber crop in 2001) was observed. Since the other management treatments did not include additional fertilizer to equalize them with the poultry litter treatment, the yield response for this treatment is attributed to the additional nutrients obtained from the poultry litter. No significant yield response was observed for this treatment on Farm B during 2001. For the most part, the cucumber yield in this study did not reflect the significant differences observed for RKN and lesion nematode populations among the management treatments. This was attributed to the timely irrigation on both farms for the cucumber crop that never allowed moisture stress to the crop.


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  		Table 8. Cucumber fruit yield for the six treatments evaluated for their root-knot nematode (RKN) and lesion nematode population management on two Dorchester County farms, 2000–2002.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
It is evident from this research that sampling date for assessing the threshold levels of RKNs and lesion nematodes can greatly influence the population estimates. Currently, Maryland recommends either spring or fall sampling for assessing RKN and lesion nematode threshold levels for vegetable, corn, and soybean production (Krusberg et al., 1993) with fall sampling preferred. The extremely low parasitic nematode populations found in May 2000 in this study (Tables 2, 3, 6, and 7) supported this fall sampling recommendation. This date was determined to be too early to satisfactorily measure populations of the two species, probably because inadequate degree-days had been accumulated from the date of planting potato for either infection or reproduction by the nematode to occur. Since this failure to detect threshold populations can affect the recommendation for pest population management (Krusberg et al., 1993), a sampling system relying upon degree-days needs to be adapted for Maryland.

The management practice used in regular rotation with the susceptible host crops that induced the best RKN suppression was sorghum sudangrass. On Farm A, RKN populations were significantly less than the control treatment during October 2000 and equivalent to the control on October 2001 (Table 2). In addition, sorghum sudangrass was the only management treatment to significantly decrease the RKN population between July 2001 and October 2001. On Farm B, sorghum sudangrass significantly reduced the RKN population to a level less than the control following its second application cycle (October 2001) (Table 3). However, once sorghum sudangrass was removed from the crop rotation (October 2002 on both farms), the RKN populations began to return to the levels present during July and August 2000, dates that were before the first application of the management treatments (Tables 2 and 3).

Two of the management treatments reduced lesion nematode populations. On both Farm A and Farm B, sorghum sudangrass and poultry litter/tillage/fallow were able to significantly reduce the lesion nematode populations below the levels that were present for the control treatment during October 2000 (Tables 6 and 7). On Farm A, the poultry litter/tillage/fallow treatment continued to have a significantly lower population on October 2001 than the control while the sorghum sudangrass treatment was equivalent to the control treatment. Both treatments had the same lesion nematode population as the control during October 2001 on Farm B (Table 7).

The use of sorghum sudangrass in a regular rotation with susceptible vegetable and agronomic crops can be a suitable management practice for RKNs and lesion nematodes. In addition, the late-summer and early-fall production of this high-biomass–producing crop may also help effectively manage residual N. Research regarding N uptake when sorghum sudangrass is used as a late summer and early fall cover crop should be conducted to determine that potential.


   ACKNOWLEDGMENTS
 
The authors express their gratitude to the two farmer-cooperators, Mr. David Andrews and Mr. Russell Stevens, who supplied the field sites and the equipment and labor to plant and manage the plots. This project was funded by a grant from the Northeast Sustainable Agriculture Research and Education program.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Mention of trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by University of Maryland and does not imply its approval to the exclusion of other products or vendors that may also be suitable.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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