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a Agriculture & Agri-Food Canada, Southern Crop Protection & Food Research Centre, Box 186, Delhi, ON, Canada N4B 2W9
b Agriculture & Agri-Food Canada, Southern Crop Protection & Food Research Centre, Box 6000, Vineland Station, ON, Canada L0R 2E0
reynoldsb{at}em.agr.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Abbreviations: DM, dry matter. *, ** Significant at the 0.05 and 0.01 probability levels, respectively
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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(Pratylenchus penetrans Cobb) is the most important plant-parasitic nematode in Ontario (Corbett, 1973; Potter and Olthof, 1993) and is known to invade and feed on the roots of many crops and native plants. Significant economic losses to growers due to this feeding injury have been shown on tomato (Reynolds et al., 1992); potato (Olthof, 1987); beet, lettuce, and spinach (Potter and Olthof, 1974); sweet corn (Yu and Potter, 1998); and flue-cured tobacco (Olthof et al., 1973; Miller, 1978; Johnson et al., 1982; Ball-Coelho, 1997). Chemical fumigation has been, and is still, the principal means of controlling nematodes in Ontario, particularly for high-value crops. This is despite the fact that a number of biological and cultural control techniques, such as crop rotation with nonhost plants, cultivation, summer fallowing (Barker and Koenning, 1998), soil amendments (Patrick and Toussoun, 1965; Siddiqui and Alam, 1988), solarization (Grinstein et al., 1979; Lazarovits et al., 1991), and rotation with allopathic crops (Miller and Ahrens, 1969; Siddiqui and Alam, 1987; Barker and Koenning, 1998) are known to help control root-lesion nematode populations.
Approximately 97% of the flue-cured tobacco crop land (>25000 ha in 1998) in Ontario is fumigated each year to control root-lesion nematodes and the disease black root rot (D.L. VanHooren, personal communication, 1999). At an average material cost of $484 ha-1, chemical fumigants cost Ontario tobacco growers in excess of $12 million annually and represent about 85% of all fumigants used in Ontario (P. Goodwin, personal communication, 1999). At this time, no practical alternatives to chemical fumigation exist, and it seems likely that the use of chemical fumigants will be severely restricted or even eliminated in the future (Noling and Becker, 1994). Marigold (Tagetes sp.) have been shown to be highly allopathic to various nematode species (Rickard and Dupree, 1978; Siddiqui and Alam, 1987; Gommers and Bakker, 1988), but not to soil microbial populations (Topp et al., 1998). Several reports have suggested that marigolds grown in rotation or intercropped with susceptible crops could control soil nematode populations as effectively as chemical fumigation (Daulton, 1963; Daulton and Curtis, 1963; Good et al., 1965; Miller and Ahrens, 1969; Brandle and Potter, 1990); however, no long-term testing of this hypothesis has been done.
Our objectives were to develop and evaluate a marigold rotation cropping system by determining impacts of cultural practices, such as marigold species selection, residue management, fertilization, and seeding rates, on P. penetrans control and subsequent crop growth. The new cropping system was evaluated based on the following criteria: (i) the degree of P. penetrans control provided by the rotation compared with chemical fumigation; (ii) the yield of the crop following the marigold rotation crop; (iii) the sustainability of the system in terms of soil conservation; (iv) the effects of this cropping system on other soil organisms; and (v) on a crop-specific basis, the economic viability, crop growth characteristics, and crop quality produced by the new system compared with a traditional cropping system based on chemical fumigation. This paper reports the results of trials to investigate criteria (i) and (ii). Criteria (iii) to (v) will be discussed in subsequent papers.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Marigold plots were seeded at a rate adjusted for the seed germination percentage. Seed germination percentage was determined in the spring of each year by transferring a known number of seeds to a petri dish lined with a filter paper moistened with tap water. The seeds were allowed to germinate at about 22°C for 10 d before counting, and percent germination was calculated. Treatments 3 to 11 were seeded at a rate of 100 viable seeds m-2, and treatments 12, 13, and 14 were seeded at the rates of 75, 50, and 25 viable seeds m-2, respectively, with an eight-row cone-type seeder (Fabro Ltd., Swift Current, SK) in 32 rows 18 cm apart, centered on the 6-m-wide plot, and to a depth of about 3 mm. Prior to seeding, 45 kg N ha-1 of urea was broadcast over the plots and lightly worked in. Marigold treatments 10 and 11 had an additional 45 kg N ha-1 (NH4NO3) broadcast over the plots on 26 June 1995 (site A), 28 June 1996 (site B), and 4 July 1997 (site A). Weeds were controlled by hand-hoeing in 1995 and by hand-hoeing plus the herbicides chlorthal dimethyl, 10 kg product ha-1 applied preemergent, and fluazifop-p-butyl, 2.0 L product ha-1 applied postemergent, in 1996 and 1997. A small sickle bar mower was used to cut the shoots about 20 cm above the soil surface for treatments 3 and 6 on 23, 20, and 18 August and for treatments 4, 7, 9, 10, 11, 12, 13, and 14 on 19, 20, and 22 September in 1995, 1996, and 1997, respectively. Tops were left on the soil surface with no further cutting or incorporation until the following spring. Treatments 5 and 8 were left standing over winter, flail-mowed in the spring to break up the stalks, and plowed under prior to transplanting tobacco.
Tobacco Crop Management
Rye controls 1 and 2 were plowed in early May (Table 1). Rye control 1 was row-fumigated with Vorlex Plus CP (Aventis CropScience Canada Co., London, ON) (68% 1,3-dichloropropene and related chlorinated hydrocarbons, 17% methyisothiocyanate, and 15% chloropicrin) at 67 L product ha-1 on 14 May 1996 (site A), 13 May 1997 (site B), and 10 May 1998 (sites A and B). The fumigant material was applied to the plot rows by single-point injection to a depth of about 20 cm and a hill formed over the injection trench with an additional 15 cm of soil to give an effective injection depth of about 35 cm. The fumigant hill was knocked down at transplanting. Marigold plots were flail-mowed to break up stalks and plowed on 16 May 1996 (site A), 15 May 1997 (site B), and 12 May 1998 (site A), and the land was prepared for transplanting. Tobacco seedlings produced in an unheated greenhouse were transplanted into field plots on 30 May 1996 (site A), 28 May 1997 (site B), and 19 May 1998 (sites A and B). Field plots consisted of five rows centered on the 6-m-wide plot. Rows were 1.07 m apart with a spacing of 61 cm between plants in the row. All data were collected from the middle three rows. Normal cultural practices for this region were followed, including application of 25 kg N, 19 kg P, and 124 kg K ha-1 applied in bands at transplanting plus a sidedress application of 27 kg N and 67 kg K ha-1 applied in late June for all treatments except 9 and 11, which received a sidedress application of 42 kg N and 67 kg K ha-1 in late June in 1996 and 1997.
Plant and Soil Sampling Procedures
Rye biomass production was determined by hand-cutting all the plants from an arbitrarily selected 1-m2 area not less than 1 m from the edge of each plot just prior to disking the rye down. The plants were separated into stalk and head (labeled stalk and top in the tables) components and dried at 65°C to a constant weight for dry matter (DM) determinations. A subsample was taken for chemical analyses, and the remainder was returned to the field plot. Dry plant biomass production in kg ha-1 was calculated on an area basis.
In late June, the field plot plant density was determined by counting the number of marigold plants in 4 of the 32 rows per plot and calculating the number of plants m-2 (Table 2) . Marigold plant biomass production was determined by cutting 20 plants arbitrarily selected from an area not less than 1 m from the edge of each plot just prior to the August or September mowing operations for all treatments, including the two (5 and 8) not scheduled for mowing until the following spring. Plants were separated into stalk and leaf components (labeled stalk and top in the tables) and dried at 65°C to a constant weight for DM determinations. An arbitrarily selected subsample was taken for chemical analysis, and the remainder was returned to the field plot. Dry plant biomass production in kg ha-1 was calculated based on plant count per unit area.
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Tobacco root P. penetrans population densities were determined by digging up the main root mass of two tobacco plants from the middle three rows per plot. The roots were washed to remove soil, allowed to drain, air-dried to remove the excess surface water, and weighed to determine the fresh root weight. An arbitrarily selected subsample of small fibrous roots from each root system was taken and transferred to a misting chamber for 14 d to extract the living nematodes into tap water (Seinhorst, 1950). The nematodes were counted and the number per gram of dry root was calculated. After extraction was complete, the root sample was oven-dried at 65°C to a constant weight for DM determinations.
To determine the impact of P. penetrans feeding on crop root growth, tobacco root length (km root m-3 soil) and weight density (kg root m-3 soil) were determined for the fumigated and nonfumigated rye controls (treatments 1 and 2) and marigold treatment 4 from four replicate plots. A 10-cm-internal-diameter auger with a low-compaction cutting head was used to obtain cores from two positions, in-row between plants and 0.2 m from the plants between rows, on 25 to 31 July 1997 (site B) and 20 to 29 July 1998 (site A). Each core was divided into 0- to 10-, 10- to 20-, and 20- to 30-cm depth increments. Roots were separated from soil by hydropneumatic elutriation (Smucker et al., 1982). Cleaned roots judged by appearance to be live were covered with water, stained with methyl violet, and preserved at 4°C with a few drops of toluene. Length of the stained, preserved samples was determined with AgVision's image analyzer system (Decagon Devices, Pullman, WA), which estimates length of root per cubic meter of soil based on the line-intersect method (Harris and Campbell, 1989). After roots were measured, they were oven-dried at 65°C to a constant weight for DM determinations, and weight densities were calculated.
Statistical Analyses
The field plot design resulted in a confounding of year and field site effects (Table 1). Statistical analysis indicated significant year x site interactions with most of the variables; therefore, results are presented for each year x site combination separately.
A subset of the rotation crop dry plant biomass data, excluding the rye controls (treatments 1 and 2; see Table 1), were analyzed as a randomized complete block using the general linear model analysis of variance procedure of CoStat 5.0 (CoHort Software, Minneapolis, MN). Where significant treatment effects were observed, the 0.05 probability level protected LSD value is given to permit the comparison of marigold rotation crop treatment means. Orthogonal contrasts are given to compare selected means and groups of means from the marigold data subset.
The complete data set for dry plant biomass (treatments 1 to 14) were analyzed. Where significant treatment effects were observed, the 0.05 probability level LSD value is given to compare treatment means and the significance level of the orthogonal contrast is given to compare the group mean of the two rye controls to the means of selected groups of marigold rotation crop treatments. This same procedure was used to analyze the tobacco yield and fresh root weight variables for 1996, 1997, and 1998.
Nematode population data were tested for skewness, a measure of the asymmetry of the data distribution, and kurtosis, a measure of the peakedness of the data distribution relative to a normal distribution, using the descriptive statistics procedures of CoHort Software. If either statistic was significant at P < 0.05, the data were transformed to log(x + y), where x = raw data and y = 1, 10, or 100, as required to produce a normal distribution in the data set. A subset of the transformed nematode population data, excluding the rye controls, was analyzed as previously described, and the results were presented as back-transformed geometric means. The complete log (x + y) transformed nematode data sets (treatments 1 to 14) also were analyzed, as previously described, and the results were presented as back-transformed geometric means.
The tobacco root length (km root m-3 soil) and weight density (kg root m-3 soil) data were transformed by ln(x) and analyzed as a randomized complete block split plot with treatment as the main plot and depth as the subplot. Where a significant treatment effect was observed, treatment geometric means were separated by protected LSD at the 0.05 level.
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Results and discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Petri dish germination tests conducted each spring indicated a germination rate of 80 to 83% for T. patula and 90 to 92% for T. erecta, which is within normal crop seed germination limits. However, examination of the plant population data presented in Table 2 indicates that plant establishment under field conditions averaged only 45% for T. patula (mean of treatments 3, 4, and 5 for 1995, 1996, and 1997) and 56% for T. erecta (mean of treatments 6, 7, and 8 for 1995, 1996, and 1997). T. erecta produced a better plant stand than T. patula in 2 of 3 yr of experimentation, but in a less favorable spring such as 1997, plant density was reduced to 32%, (mean of treatments 3, 4, and 5) for T. patula and 47% (mean of treatments 6, 7, and 8) for T. erecta. Marigold is a fine-seeded plant with a 100-seed weight of 0.30 and 0.36 g for T. patula and T. erecta, respectively, and cannot be seeded more than about 3 mm deep. At this shallow depth, the seed is very susceptible to moisture stress and wind erosion during germination and, in an extremely dry spring, sandy soils may require a light irrigation to promote germination. Small marigold seedlings typically were observed about 14 d after seeding; however, the initial growth rate until late July was quite slow, and weeds had to be controlled or they would quickly take over the field.
Effects of Crop Management on Marigold Growth
Mowing time had no signficant effect on dry plant biomass production in 1995 (Table 3)
. Mowing time effects were observed for both T. patula and T. erecta in 1996 and for T. patula in 1997 (Tables 4 and 5) . For both species, stalk yield increased as mowing time was delayed from late August to late September in 1996, indicating continued growth in September. The reduction from August to September 1997 in top yield, which is the leaf portion of the plant for the marigold crops, was caused by a foliage disease that affected mainly the lower leaves in September and, as a result, reduced leaf biomass production (Table 5). Pratylenchus penetrans density was affected by mowing time for the T. patula fall count in 1996 only (Table 4), but the nematode counts were small, well below the economic threshold, and this result is not considered important.
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Additional N applied to marigold treatments 10 and 11 in late June or early July 35 to 45 d after seeding, which is the time of most rapid growth for marigold in this area, had no effect on dry plant biomass production or P. penetrans control in the rotation crop year (Tables 3, 4, and 5) or on tobacco yield or P. penetrans population densities in the following crop year (data not shown).
Seeding at 100, 75, 50, and 25 viable seeds m-2 did not produce the desired marigold plant densities because of greatly reduced and inconsistent germination under field conditions, but did produce a sufficient range in values (Table 2) to permit some evaluation of the effects of plant density. Plant density affected marigold biomass production in 1995 only, but these data are inconsistent. The lowest seeding rate in 1995, the most favorable production year observed in the entire trial, produced the highest yield of dry plant biomass. This result may have been due to increased tillering and branching of the top at a field plant density of 29 plants m-2 (data not shown). Marigold dry plant biomass production (treatments 3 to 11) varied more widely than did rye, but with the exception of T. erecta in 1996, was equal to or greater than rye. The results overall (Tables 2, 3, 4, and 5) suggest that a field density of about 45 marigold plants m-2, which can be achieved with a seeding rate of about 100 seeds m-2, will equal the biomass production of rye.
Only for the 1996 fall P. penetrans population count was any difference observed with marigold plant density (Table 4), but the nematode population counts were <50 kg-1 of soil, and this result is not considered important. A plant density of about 20 plants m-2 reduced P. penetrans population densities to less than 100 kg-1 of soil in the rotation crop year (Tables 3, 4, and 5) and in the following field crop year (data not shown).
Initial spring P. penetrans population densities for 1995 on site A and for 1996 on site B were the same, which is the expected result given that treatments had not yet been applied, and indicates a uniform spatial distribution of P. penetrans populations across the experimental area. By midseason of rotation year 1 and continuing through to the fall, we observed that P. penetrans populations in all marigold plots were reduced to levels lower than the rye plots. In 1997 on site A (Table 5), the spring P. penetrans populations were different between plots cropped to rye in 1995 and plots cropped to marigold in 1995 even though tobacco, an excellent P. penetrans host plant, was grown on all plots in 1996. Therefore, it appears that the nematicidal effects of the 1995 marigold rotation persisted for more than 1 yr even when the land was cropped to a suitable host plant in the year following the rotation crop.
Effects of Prior Rotation on Field Crop Yield and P. penetrans Population Densities
Chemical fumigation prior to transplanting reduced spring root-lesion P. penetrans population densities to levels below the economic threshold of 500 nematodes kg-1 soil (Olthof et al., 1973) in 1996, 1997, and 1998 (Tables 6, 7, 8, and 9)
. Crop yield following the rye rotation was improved by fumigation in every year. In 1996 and 1997, by the end of the growing season, P. penetrans population densities in the fumigated rye plots were equal to the nonfumigated rye check plots (Tables 6 and 7). This repopulation of nematodes after row fumigation is consistent with reports by Johnson et al. (1982). The degree of nematode repopulation observed in 1996 and 1997 was not observed in 1998 on either site A or B (Tables 8 and 9). This slower repopulation may have been due to the extremely dry spring and early summer conditions in 1998.
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Although no significant differences from mowing time were observed, allowing the marigold crop to stand over winter would be preferred to fall tillage because of greater protection from soil erosion and prevention of early release of N from the marigold residue before the following crop is in place to trap it and prevent NO3 movement to groundwater. Correlations between marigold rotation crop total dry plant biomass production (treatments 3–14) in 1995 and subsequent tobacco yield in 1996 were significant, but the correlation coefficients were only 0.31* and 0.37** for marigold stem vs. tobacco leaf and marigold leaf vs. tobacco leaf, respectively. These r-values are low, significant for the 1996 tobacco crop only, and suggest that the main effects of the marigold rotation crop were from P. penetrans control. Thus, a marigold plant density of about 20 plants m-2 was sufficient to control P. penetrans population densities more effectively than chemical fumigation, and, although dry marigold biomass production at this plant density was less than rye, subsequent tobacco yield was not reduced.
When two tobacco crops were grown following a single marigold rotation crop, yields were not significantly reduced and P. penetrans populations were equal to those in plots that received chemical fumigation (Table 9). A significant difference in spring P. penetrans population densities existed between fumigation and T. erecta treatments 6 and 7, but the nematode counts were <15 kg-1 of soil. This apparent persistence of P. penetrans population control for 2 yr following a single marigold crop may have important applications in certain biannual or perennial crops affected by root-lesion nematodes, such as medicinal herbs, raspberries, apples, etc., and where growers do not have enough land for a 2-yr rotation.
Marigold seed costs were $170 kg-1 (1996) for T. patula and $226 kg-1 (1997) for T. erecta. Based on mean field plant establishment rates observed in these trials of 45 and 56% for T. patula and T. erecta, respectively, and a plant density of 20 plants m-2, which is sufficient for P. penetrans control equal to or better than chemical fumigation, the cost of seed for a T. patula rotation crop was $221 ha-1 and $294 ha-1 for T. erecta.
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Conclusions |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Root-lesion nematode control provided by the marigold rotation grown at a plant density of about 20 plants ha-1 was equal to chemical fumigation in midseason and superior in the fall and subsequent 2 yr. A seeding rate of 1.3 kg viable seeds ha-1 for either marigold species produced this plant density. A single marigold crop reduced P. penetrans population densities to levels below crop economic thresholds within 75 d after seeding, and this control persisted for the rest of the rotation year and for the next 2 yr.
Flue-cured tobacco yield following the marigold rotation crop was equal or superior to the Ontario industry standard practice of using a fall-seeded rye rotation crop for winter cover, disking down the mature straw and grain the following summer, allowing the rye to self-seed for winter ground cover, and row-applying chemical fumigants prior to transplanting tobacco the following spring. Dry marigold biomass production at a density of about 45 plants ha-1, which can be obtained by seeding at a rate of 3.0 kg viable seed ha-1, was equal to rye. Tobacco yield was not significantly correlated to rotation crop biomass production within the range of values observed in this trial, and it appears that the reduction in P. penetrans population densities occurring at 20 plants ha-1 was responsible for the increase in tobacco yield.
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ACKNOWLEDGMENTS |
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Received for publication May 17, 1999.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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This article has been cited by other articles:
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  B. Ball-Coelho, A. J. Bruin, R. C. Roy, and E. Riga Forage Pearl Millet and Marigold as Rotation Crops for Biological Control of Root-Lesion Nematodes in Potato Agron. J., March 1, 2003; 95(2): 282 - 292. [Abstract] [Full Text] [PDF]
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B. R. Ball-Coelho, L.B. Reynolds, A. J. Back, and J. W. Potter Residue Decomposition and Soil Nitrogen are Affected by Mowing and Fertilization of Marigold Agron. J., January 1, 2001; 93(1): 207 - 215. [Abstract] [Full Text]
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