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Published online 17 November 2005
Published in Agron J 97:1626-1635 (2005)
DOI: 10.2134/agronj2004.0290
© 2005 American Society of Agronomy
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Production Papers

Root-Knot Nematode Resistant Cowpea Cover Crops in Tomato Production Systems

P. A. Robertsa,*, W. C. Matthews, Jr.a and J. D. Ehlersb

a Dep. of Nematology, Univ. of California, Riverside, CA 92521-0415
b Dep. of Botany and Plant Sci., Univ. of California, Riverside, CA 92521-0415

* Corresponding author (philip.roberts{at}ucr.edu)

Received for publication November 29, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Root-knot nematodes, Meloidogyne spp., are serious pests of many crops worldwide. Recent limitations on the use of nematicides have enhanced the need to develop alternative management strategies, including host plant resistance. This study was conducted to determine the effectiveness of nematode-resistant cowpea [Vigna unguiculata (L.) Walp.] genotypes used as cover crops for suppressing populations of M. incognita (Kofoid and White) Chitwood and M. javanica (Treub) Chitwood and protecting susceptible tomato (Lycopersicon esculentum Mill.) grown in rotation. In six field experiments, susceptible and resistant cowpea was grown to flowering stage and the dried tops incorporated or not incorporated into the soil. These treatments were compared to wet and dry fallowing and were conducted on nematode-infested and noninfested plots. The experiments were conducted in the Coachella and San Joaquin Valleys, California. Resistance conferred by genes Rk and Rk2 reduced loss of cowpea biomass and M. incognita soil populations and partially suppressed M. javanica compared with susceptible cowpea, but not as effectively as fallow treatments. Incorporation of cowpea tops into the soil promoted tomato growth irrespective of nematode presence. On infested plots, tomato fruit yield was higher following growth and incorporation of resistant cowpea compared with growth and incorporation of susceptible cowpea or nonincorporation of cowpea and fallow treatments. We conclude that root-knot nematode-resistant cowpea is an effective cover crop for protecting susceptible vegetable crops grown under irrigation, and its beneficial effects are enhanced by incorporation of its green biomass.

Abbreviations: CVARS, Coachella Valley Agricultural Research Station • G, non-infested plots with biomass incorporation • I, infested plots without biomass incorporation • IG, infested plots with biomass incorporation • J2, second-stage juveniles of root-knot nematodes • KREC, Kearney Research and Extension Center • Pf, final (postharvest) population density (of nematodes) • Pi, initial (preplant) population density (of nematodes) • tomato Pi, population density of nematodes before planting the tomato bioassay


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE INTENSIVE crop production systems of the hot interior Coachella and San Joaquin Valleys of California have many problems (Aguiar et al., 2001). The sandy, low-organic-matter soils are easily leached of most forms of synthetic N fertilizers, and leaching can contaminate surface and groundwater. Air pollution (from dust) and loss of topsoil result from fields left bare during summer fallow periods. The combination of sandy soils, high temperatures, and intensive cultivation of nematode-susceptible crop varieties can lead to severe root-knot nematode problems, and weeds quickly build up. Cover cropping during the typical summer fallow period in Coachella or following crops harvested in early summer in the San Joaquin Valley [such as processing tomato, corn (Zea mays L.) silage, wheat (Triticum aestivum L.), and Brassica spp.] may provide an alternative system to improve soil fertility, limit erosion, and address nematode and weed problems (Aguiar et al., 2001; Ngouajio et al., 2003). Use of a leguminous cover crop can provide useful amounts of N, which is especially important to organic growers who have few inexpensive sources of this nutrient.

Root-knot nematodes are major pests of agronomic and vegetable crops worldwide and cause root galling, shoot stunting, and loss of yield (Sasser, 1980). In the Coachella and San Joaquin Valleys, where nematode-susceptible vegetables are grown intensively under irrigation, M. incognita and M. javanica are common and damage numerous crops, especially in sandy soils. During the last 50 yr, these nematode infestations have been controlled effectively and economically with fumigant nematicides, but emphasis is being placed on development and implementation of alternative nematode management, including host plant resistance, cover cropping, crop rotation, and soil amendments (Roberts, 1993; Starr et al., 2002).

Cowpea is an important grain, vegetable, and hay crop in many tropical and subtropical regions, but especially in arid savanna and Sahelian zones of West Africa (Fatokun et al., 2002; Hall et al., 2003) and is used as a cover crop in the southeastern USA to a limited extent. Cowpea has many attributes that make it an excellent candidate as a cover crop in the irrigated production systems of the southwestern USA, including excellent adaptation to sandy soils, tolerance to heat and drought (Ehlers and Hall, 1997; Hall et al., 2002), and high levels of broad-based resistance to Meloidogyne spp. (Ehlers et al., 2002).

Cowpea grown as a 70-d cover crop can fix 225 kg ha–1 of N and add substantial amounts of organic matter to the soil (Aguiar et al., 2001). Specialized forage or cover crop varieties that flower late and possess vigorous vegetative growth produce substantially more cover and fix more N than grain type cultivars because plants remaining in the vegetative growth stage produce more biomass and fix more N per day than those that transition quickly to flowering and setting pods.

Ideally, cowpea used as a cover crop should not act as a host for nematode reproduction. Although most cowpea genotypes are susceptible to M. incognita and M. javanica, cultivars with strong resistance to these species are available. Cowpea bred for cover cropping should have resistance to limit nematode multiplication and suppress nematode soil populations. Resistance to Meloidogyne spp. in cowpea was one of the early examples of nematode resistance in plants (Webber and Orton, 1902). Genetic analysis in grain-type cultivars Iron, Colossus, and Mississippi Silver revealed a single dominant resistance gene, designated Rk, that conferred resistance to M. incognita, M. javanica, and M. hapla Chitwood (Fery and Dukes, 1980). This resistance also controlled M. arenaria (Neal) Chitwood (Hare, 1959). Cowpea cultivars with gene Rk have been developed for the California blackeye dry bean industry, including California Blackeye no. 5 (CB5) and California Blackeye no. 46 (CB46) (Ehlers et al., 2002). Many other cultivars with gene Rk have been developed for dry bean or fresh production for other regions of the United States and in other countries (Hall et al., 2003). Although several Meloidogyne species are controlled by gene Rk, California isolates of M. javanica were found to be aggressive on cowpea with Rk, such as CB5 (Thomason and McKinney, 1960). In addition, some populations of M. incognita are virulent to Rk, causing extensive root galling and poor growth of cowpea cultivars with Rk (Roberts et al., 1995). The occurrence of virulent infestations prompted the identification and incorporation of additional resistance in cowpea, including gene Rk2, conferring strong, broad-based root-knot resistance (Roberts et al., 1996), and rk3, a gene with additive resistance when combined with Rk (Ehlers et al., 2000a, 2000b, 2002).

Cover crops in rotations and as green manures have shown promise for nematode control in various cropping systems. These include use of rapeseed (Brassica napus L.), sudangrass [Sorghum bicolor (L.) Moench.], and sorghum–sudangrass hybrids [S. bicolor x S. sudanense (Piper) Stapf] as green manures for suppressing M. chitwoodi Golden et al. in potato (Solanum tuberosum L.) production in the Pacific northwest (Mojtahedi et al., 1991, 1993) and lesion nematode (Pratylenchus penetrans Cobb) in Ontario, Canada (McKeown and Potter, 2001). Velvetbean [Mucuna deeringiana (Bort.) Merr.] as a rotation crop suppressed M. incognita in soybean (Glycine max L.) production (Vargas-Ayala and Rodriguez-Kabana, 2001), and millet [Pennisetum typhoides (Burm.) Stapf & Hubb] and resistant cowpea as summer cover crops were shown to suppress M. incognita in Florida vegetable double-cropping systems (McSorley et al., 1999; Wang et al., 2003).

The objectives of this research were to determine the potential of nematode resistance in cowpea cover crops for suppressing nematode population densities in soil and their damage potential to yield of susceptible tomato following in rotation. Resistant and susceptible cowpea cover crops were compared with and without soil incorporation of the aboveground cowpea biomass and were compared with wet and dry fallow treatments. The experiments were conducted at research station sites in the Coachella and San Joaquin Valleys of California. A preliminary summary of part of this study was reported earlier (Matthews et al., 1998).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nematode Isolates and Assays
Sites 1 and 2 (Table 1) were infested with an isolate of Meloidogyne incognita race 1 collected from a vineyard at the same location [University of California-Riverside Coachella Valley Agricultural Research Station (CVARS), near Thermal, CA]. Site 3 (Table 1) was infested with an isolate of M. incognita race 3 collected from a cotton (Gossypium hirsutum L.) field near Tipton, Tulare County, CA. Greenhouse tests showed that both isolates were controlled by the Rk gene in cowpea. Site 4 (Table 1) was infested with an isolate of M. javanica collected from a cowpea field near Chino, San Bernardino County, CA. Greenhouse tests showed that this isolate was aggressive toward gene Rk, reproducing about 50% on resistant (Rk) plants when compared with susceptible (non-Rk) plants. Species and race identity of these isolates were confirmed by isozyme phenotyping and by a host differential test as described previously (Roberts et al., 1996).


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  		Table 1. Descriptions of nematode isolates and soil textures at the experimental field sites.

 
Inoculum (nematode eggs) used to infest the study sites was produced on tomato (cv. UC82) in a greenhouse and obtained from tomato roots by extraction with NaOCl (Hussey and Barker, 1973). The inoculum was introduced into Sites 1 and 2 through a drip-irrigation system (Becker et al., 1989) and into Sites 3 and 4 through injection shanks (Ball and Ferris, 1982). On each site, tomato (cv. UC82) was direct-seeded and grown for 6 wk before inoculation. Soil sampling of infested plots before planting of cowpeas indicated that both methods established uniform nematode populations.

Population densities in soil of M. incognita and M. javanica second-stage juveniles (J2) and eggs were assessed before cowpea and fallow treatments (Pi), when the cowpea canopies were cut (Pf), and immediately before planting the tomato bioassay (tomato Pi). Population assessment at each sampling time was based on a composite sample of 12 to 15 soil cores (2.5 cm diam. and 30 cm deep) taken from the center two rows of four-row plots and both rows of two-row plots. Approximately 60 cm of the ends of each row in a plot (30 cm for the 3.6-m plots) was avoided when sampling. Twelve cores were taken in shorter (3.6 to 6.0 m) plots and 15 cores in the longer (9.0 to 12.0 m) plots. Soil samples were hand-mixed and nematode J2 and eggs extracted from a 250-cm–3 subsample by gravity screening (Ayoub, 1980) through three nested sieves (850-, 150-, and 43-µm openings). Screened fractions from all three sieves were placed for 3 d in a modified Baermann funnel-mist chamber (Ayoub, 1980) for egg hatch and release of J2. At tomato harvest, 21 tomato root systems per plot were indexed for galling symptoms based on the 0 to 10 rating system of Bridge and Page (1980).

Plant Materials
Characteristics of the cowpea genotypes used in the experiments are given in Table 2. Root-knot nematode resistance traits of CB46, CB27, Iron/Clay, and IT84S-2049 were known from previous studies (Ehlers et al., 2002). Although not appropriate for use as cover crops due to low biomass production, the blackeye dry grain cultivars CB46 and CB27 were included in Exp. 1 because of their well-characterized resistance traits. IT84S-2049, developed by the International Institute of Tropical Agriculture (IITA) in Nigeria, was included in Exp. 1 and 3 because of its superior nematode resistance and moderate biomass yield potential. Cultivar Iron/Clay was included in Exp. 1 through 3 because of nematode resistance combined with high biomass yield potential. Preliminary trials were used to select additional resistant cowpea cover genotypes. Nematode resistance screening of about 500 cowpea accessions identified several with resistance to M. incognita and moderate resistance to M. javanica. Biomass yield and seed production trials were conducted on these promising accessions at CVARS. Based on selection for high biomass yield, photosensitivity, semierect growth habit, and combined resistance to root-knot nematode and Fusarium wilt (Fusarium oxysporum f. sp. tracheiphilum race 3), nematode-resistant accession IT89KD-288 was included in four experiments. Accession UCR 779 was chosen as a nematode-susceptible control because of high biomass yield in the absence of nematodes and photosensitivity.


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  		Table 2. Description of agronomic and resistance characteristics of cowpea genotypes used in the study.

 
Experiment Descriptions
General Procedures
Six field experiments were conducted over 5 yr from 1997 to 2001 at the University of California-Riverside CVARS near Thermal in the Coachella Valley of southern California and the University of California Kearney Research and Extension Center (KREC) in Parlier in the San Joaquin Valley of central California. In all experiments, selected resistant and susceptible cowpea was compared with wet and (or) dry fallowing. The cowpea and fallow treatments were conducted during the summer months, with a tomato bioassay grown the following year. Tomato was shown to be a good indicator plant to bioassay for root-knot nematode in situ (Prot and Netscher, 1978). Cowpea seed was not inoculated with rhizobium before planting because prolific nodulation occurs naturally at these sites. No herbicide or fertilizer was applied when establishing the cowpea/fallow treatments nor was fertilizer applied before planting the tomato bioassays. Where weeds became a problem, especially in the wet fallow treatments, control was achieved with a combination of hand weeding and limited, directed applications of the contact herbicide glyphosate.

At CVARS, cowpea was grown as a summer crop corresponding to the usual practice of following a fall-winter/spring cash crop (double crop), with summer being the preferred period for fallowing or cover cropping. Experiments at KREC were designed to follow a summer crop the previous year (single crop). In Exp. 1 to 4 at CVARS, cowpea and fallow treatments were started from late June to early August and terminated (cut) after 70 to 84 d. The tomato (cv. UC82) bioassay was planted the following year, between late January and early March, with a June harvest. In Exp. 5 and 6 at KREC, cowpea was planted in mid-May and terminated after 83 d; the tomato bioassay was planted in mid-April of the following year and harvested in mid-August. Cowpea was grown to the vegetative to flower formation stage, corresponding to a maturity index of 1 to 2 as described previously (Aguiar et al., 2001). The tomato bioassays were harvested when at least 70% of the total fruit (by count) was pink to red (30% or less green).

Experiments 1, 2, 5, and 6 were randomized complete block designs, with four replications in Exp. 1 and six replications in Exp. 2, 5, and 6. Experiment 1 was conducted to develop damage function curves for susceptible (UCR 779) and resistant (Iron/Clay) cowpea genotypes and to assess the effects of growing resistant and susceptible genotypes on soil populations of root-knot nematode. Experiments 2, 5, and 6 were designed to measure the effects of resistant and susceptible cowpea on nematode populations in combination with cowpea biomass incorporation.

Experiments 3 and 4 were a split-plot design with infested/incorporation, infested/nonincorporation, and noninfested/incorporation treatments assigned to main plots in a randomized complete block with five replications. Cowpea, fallow, and combination cowpea/fallow treatments were randomly assigned to subplots within each main plot. This design was used to partition the effects of growing the resistant or susceptible cowpea from the effects of incorporation.

Experiment 1
Cowpea and the tomato bioassay were grown in two-row plots on a bed 1.5 m wide and 3.8 m long (45-cm buffer between plots along the row). A wet fallow treatment was included. Water was supplied through irrigation tape buried 10 to 15 cm deep with a 20-cm emitter spacing. Multiple plots of the susceptible cowpea UCR 779 and the resistant cowpea Iron/Clay were grown in each replicate block to generate enough data points (40 per genotype) for damage function curves based on plotting yield versus initial nematode population (Pi). One-half of the experiment was designed as a low-Pi regime, established by growing resistant tomato (cv. H8892) before the cowpea. The other half was a high-Pi regime, with the cowpea/fallow treatments following susceptible tomato (cv. UC82). Cowpea was hand-cut, dried in the field for 22 d after fresh weights were measured, and some plots of each cowpea entry were randomly selected and samples weighed again. From the dry weights, a dry weight factor was derived and used to calculate dry weights for the remaining plots. Subsamples of each cowpea entry were collected and placed in a drying oven to determine an absolute dry weight. After all weights were taken, the dried cowpea was removed from the field. No additional operations (discing, cultivating, bed-shaping) were performed before planting the tomato bioassay.

Experiment 2
Cowpea and the tomato bioassay were grown in a single row on 80-cm-wide beds, with two beds per plot. Plots were 9 m long for the cowpea, fallow, and cowpea/fallow treatments, and the center 6 m of each plot was used to collect tomato fruit yield and root systems for the bioassay. Water was provided by buried tape as described for Exp. 1. The dry fallow/cowpea treatment plots were not watered for 21 d while all other treatments were watered to maintain soil moisture. The dry fallow/cowpea plots were then reconnected to the irrigation tape, and all cowpea plots were planted. Cowpea was hand-cut, dried in the field for 36 d, and weighed. The dried canopies from each plot were placed onto the beds for incorporation using a rotivator. All treatments including fallow received the same mechanical operations (discing, rotivating). To stimulate breakdown of the cowpea biomass, beds were rotivated twice more before bed-shaping and planting the tomato bioassay.

Experiments 3 and 4
Plot dimensions were identical to those in Exp. 2. Water was provided by buried tape as described in Exp. 1 and 2. In Exp. 3, cowpea was hand-cut and dried in the field for 14 d after measuring fresh weights, and some plots of each cowpea entry were randomly selected and samples weighed again to determine a dry weight factor to calculate dry weights for remaining plots. Subsamples of each cowpea entry were collected to determine a final dry weight. In Exp. 4, cowpea was hand-cut, dried in the field for 27 d, and weighed. For nonincorporation treatments, cowpea biomass was removed from the field. For incorporation treatments, any cowpea canopies used for weight determination were placed back onto the beds before incorporation. All plots, including nonincorporation treatments, were rotivated three times before bed-shaping and planting the tomato bioassays. In Exp. 4, the dry fallow period was 28 d in the dry fallow/cowpea treatment.

Experiments 5 and 6
Cowpea and the tomato bioassay were grown in single rows on 80-cm-wide beds. Plot size was 12.2 m long and four beds wide for the cowpea and fallow treatments, and tomato fruit yield and root systems for the bioassay were taken from the center 6 m of the two middle beds. Cowpea was furrow-irrigated, hand-cut, and dried for 35 d. Tomato was irrigated with surface drip tape (20-cm emitter spacing) until fruit set, at which time the plants were furrow-irrigated until harvest.

Statistical Analysis
Nematode Pi and Pf, tomato Pi, cowpea yield, tomato fruit yield, and root gall index data were analyzed with the SAS (SAS Inst., 1996) ANOVA for Exp. 1, 2, 5, and 6 and with SAS GLM program for Exp. 3 and 4. For Exp. 3 and 4, where significant main plot x subplot interactions occurred, least square means comparisons were used. A log10 (x + 1) transformation was applied to nematode data (Pi, Pf, tomato Pi) before statistical analyses, to equalize variances among treatment means, and least significant difference values are not indicated because they are not relevant to the untransformed data presented in the tables.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Effects of Nematode Pi on Biomass of Resistant and Susceptible Cowpea
The Pi of M. incognita in Exp. 1 ranged from 0 to 2430 and 0 to 3236 J2/250 cm–3 of soil in 40 plots each of resistant Iron/Clay and susceptible UCR 779, respectively. Regressions of cowpea biomass dry weight (kg ha–1) on M. incognita Pi showed an overall lack of biomass suppression of resistant Iron/Clay over the range of initial nematode densities (R2 = 0.036) compared with a large suppression of susceptible UCR 779 biomass dry weight over a similar range of nematode population densities (R2 = 0.630) (Fig. 1) . At the highest Pi levels (>500 J2/250 cm–3 of soil; log10 = 2.7), biomass weight of UCR 779 was less than 25% of that in noninfested plots whereas Iron/Clay was greater than 95% of noninfested plots (Fig. 1). Biomass of susceptible and resistant cowpea genotypes differed even under low-nematode-Pi levels due to differences in genotype biomass yield potential, as indicated under the low-nematode-Pi regime in Exp. 1 (Table 3). However, across all sites with high-nematode-Pi levels, the susceptible UCR 779 produced significantly less (P < 0.05) biomass than the resistant genotypes due to nematode infection (Tables 3–7). For example, UCR 779 dry biomass yield was only 1528 kg ha–1 under the high-Pi regime compared with 5106 kg ha–1 under low nematode Pi while yield of resistant genotypes under low or high nematode Pi was 3981 to 10531 kg ha–1 (Table 3). In Exp. 3 and 4, biomass yield of susceptible UCR 779 was four- to fivefold higher (P < 0.05) in noninfested plots than in infested plots, whereas the resistant genotypes were not different between infested and noninfested plots (Tables 5 and 6).



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  		Fig. 1. Cowpea biomass yield in relation to preplant Meloidogyne incognita initial population densities (Pi) in soil on plots planted with (A) resistant Iron/Clay and (B) susceptible UCR 779 in Experiment 1 in the Coachella Valley.

 

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  		Table 3. Meloidogyne incognita race 1 initial (Pi) and final (Pf) population densities in soil, root galling, and yield of cowpea biomass and tomato fruit in a cowpea cover (no incorporation) and tomato rotation sequence (Experiment 1).{dagger}

 

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  		Table 7. Meloidogyne incognita race 3 and M. javanica initial (Pi) and final (Pf) population densities in soil, root galling, and yield of cowpea biomass and tomato fruit in a cowpea cover (with incorporation) and tomato rotation sequence (Experiments 5 and 6).

 

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  		Table 4. Meloidogyne incognita race 1 initial (Pi) and final (Pf) population densities in soil, root galling, and yield of cowpea biomass and tomato fruit in a cowpea cover (with incorporation) and tomato rotation sequence (Experiment 2).{dagger}

 

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  		Table 5. Meloidogyne incognita race 1 initial (Pi) and final (Pf) population densities in soil, root galling, and yield of cowpea biomass and tomato fruit in a cowpea cover (with or without incorporation and infestation as main plots) and tomato rotation sequence (Experiment 3).

 

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  		Table 6. Meloidogyne incognita race 1 initial (Pi) and final (Pf) population densities in soil, root galling, and yield of cowpea biomass and tomato fruit in a cowpea cover (with or without incorporation and infestation as main plots) and tomato rotation sequence (Experiment 4).

 
Nematode Populations after Cowpea Cover and Fallowing
Preplant (Pi) population densities of M. incognita race 1 in Exp. 1–4 were not different between treatments in each experiment (Tables 3–6). These uniform infestation levels allowed comparisons of nematode populations in soil following susceptible and resistant cowpea cover crop genotypes and dry and wet fallowing (Tables 3–6). In Exp. 1, the site was divided to provide two regimes of nematode Pi, a low-Pi regime of 6 to 50 J2/250 cm–3 of soil and a high-Pi regime of 603 to 977 J2/250 cm–3 of soil (Table 3). The nematode Pf values were much higher (P < 0.05) following susceptible cowpea UCR 779 than all other treatments of resistant cowpea and dry or wet fallowing (Tables 3–6). Among these other treatments, Pf values were lower (P < 0.05) following dry fallowing in Exp. 2 and 4 than resistant cowpea cover treatments (Tables 4 and 6). Wet fallowing treatment resulted in nematode Pf similar to that under resistant cowpea cover cropping in Exp. 1, 3, and 4 although in Exp. 4, resistant cowpea (IT89KD-288) resulted in higher (P < 0.05) nematode Pf than wet or dry fallowing. The combination treatment of dry fallow plus resistant cowpea cover in Exp. 2 and 4 did not result in lower Pf values than either treatment alone.

These differences in nematode Pf were also found for M. incognita race 3 in Exp. 5 and for M. javanica in Exp. 6 (Table 7); Pf values were much higher after incorporated susceptible cowpea UCR 779 than incorporated resistant cowpea IT89KD-288 and wet fallowing. Wet fallowing decreased nematode Pf compared with resistant cowpea (P < 0.05) in Exp. 6 but not in Exp. 5 (Table 7).

Nematode Populations before Tomato Cropping
Nematode population levels before planting tomato (tomato Pi) showed similar overall differences between cowpea and fallow treatments in each experiment as Pf levels assessed the previous season following cowpea or fallow treatments. Nematode population decline between the cowpea and tomato growing seasons resulted in an average reduction in mean numbers of J2/250 cm–3 of soil following susceptible cowpea of 86% for M. incognita race 1 (Exp. 1–4) in Coachella and >99% for M. incognita race 3 (Exp. 5) and M. javanica (Exp. 6) at the San Joaquin Valley sites. More variable reduction of the lower populations following resistant cowpea and fallow treatments occurred in each experiment, in part due to the greater sampling variance to mean relationships for small population densities (Tables 3, 4, 5, 7, and 8). In both Exp. 3 and 4, nematode Pi before tomato planting following cowpea incorporation compared with nonincorporation as main plot effects were not different (Tables 5 and 6).


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  		Table 8. Levels of significance based on probabilities that least squares means (lsm) for (in) = least squares means for (jn) for tomato yield in a cowpea cover and tomato rotation sequence in Experiment 3 (upper right) and Experiment 4 (lower left).{dagger}

 
Tomato Infection and Yield after Cowpea and Fallow Treatments
Tomato fruit yields were higher (P < 0.05) following resistant cowpea and fallow treatments than following susceptible cowpea in infested plots in all experiments except Exp. 5 and some treatments in Exp. 4 in which the tomato yields were not different at P ≤ 0.05. These differences were found in treatments both with and without incorporation of cowpea biomass. In Exp. 3 (Tables 5 and 8) and 4 (Tables 6 and 8), tomato yield was compared in main-plot treatments of infested without incorporation, infested with incorporation, and noninfested with incorporation. Tomato yield was similar per cowpea or fallow treatment between infested plots with or without incorporation in Exp. 3 but higher (P < 0.05) with than without incorporation for the resistant cowpea and resistant cowpea/dry fallow treatments in Exp. 4 and higher (P < 0.05) for cowpea and wet fallow but not dry fallow treatments in Exp. 3 and 4 (Tables 5, 6, and 8). In noninfested main plots in Exp. 4, incorporated cowpea IT89KD-288 resulted in higher (P < 0.05) tomato yield than wet or dry fallow or incorporated cowpea UCR 779 treatments (Tables 6 and 7).

Tomato yields were similar following resistant cowpea compared with wet and dry fallow treatments in Exp. 2, 5, and 6. Root gall indices on tomato in these resistant cowpea and fallow treatments were consistently lower than those in the susceptible cowpea cover crop treatment in each experiment. However, wet fallowing compared with resistant cowpea IT89KD-288 with incorporation resulted in lower (P < 0.05) M. javanica root galling on tomato in Exp. 6. The dry fallow treatment was most effective in suppressing the nematode root galling on tomato in each experiment it was tested (Exp. 2–4). This effect resulted in higher tomato yields than following resistant cowpea with or without incorporation on infested plots in Exp. 3 (Table 5) but not in Exp. 2 and 4 where tomato yield was enhanced by resistant cowpea with biomass incorporation (Tables 4, 6, and 7).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Our experiments were aimed at determining the potential of cowpea cover cropping in promoting the yield of warm-season vegetables produced on root-knot nematode infested soils in the interior desert valleys of California. While the general benefits to soil health and crop management of legume cover cropping and its biomass incorporation are well-documented (Magdoff and Van Es, 2000; Powers and McSorley, 2000), the effectiveness of root-knot resistant cowpea cover crops in promoting subsequent crop yields in root-knot infested soils is not known. In this study, we have demonstrated that cowpea genotypes selected as high biomass producers, including UCR 779 and IT89KD-288 as well as some genotypes currently in use such as Iron/Clay, are beneficial under California conditions in promoting yield of tomato grown after their soil incorporation as a cover crop. Our results show the importance of nematode resistance in the cowpea cover crop to protect against root-knot nematode infection in both the cover crop and the vegetable crop grown after it.

The field experiments showed that nematode infection of the susceptible cowpea cover crop had two important negative consequences for the cropping system. First, the susceptible genotype was intolerant of infection especially at high initial nematode densities in soil (Pi). The damage functions developed in Exp. 1 showed this effect with a strong negative correlation between susceptible cowpea UCR 779 biomass and M. incognita Pi. This result was supported by the consistently low biomass weights for UCR 779 compared with resistant cowpea on infested plots in all experiments and with UCR 779 on noninfested plots in Exp. 3 and 4. These results also confirmed other reports of M. incognita impact on cowpea growth and yield (Duncan and Ferris, 1983; Roberts et al., 1995). The biomass reductions of susceptible cowpea were typically 50% or greater and diminished the contribution of the incorporated green matter to soil fertility. Second, susceptible cowpea supported nematode multiplication, resulting in high residual population densities in all experiments, as shown by the nematode Pf and tomato Pi values. These population densities caused heavy nematode infection of tomato plants and suppressed tomato yields, often by 50% or more compared with treatments where nematodes were absent or controlled by resistance or fallowing. These levels of yield suppression are consistent with other studies on root-knot susceptible tomato (Roberts and May, 1986). Resistant tomato genotypes are widely available and effective in limiting root-knot damage potential (Roberts, 1992) and could be used as a management strategy where susceptible cowpea cover crops are grown.

The nematode-resistant cowpea genotypes in our experiments produced high biomass yields compared with the susceptible cowpea in infested plots. This was confirmed by the nonsignificant damage function relationship of resistant Iron/Clay, which had similar biomass yield across a wide range of M. incognita Pi (Fig. 1). Both IT89KD-288 and Iron/Clay had high biomass production potential under these growing conditions, regardless of nematode infestation. These genotypes suppressed nematode multiplication, and the low residual population densities after cover cropping caused less measurable reduction in tomato yield compared with susceptible cowpea. Other resistant genotypes (CB46, IT84S-2049, CB27) included in Exp.1 produced similar suppressive effects on the M. incognita populations although as dry grain types, their biomass production potential was low.

Nematode resistance is conferred by gene Rk in Iron/Clay and by gene Rk2 (or an equivalent allele) in IT89KD-288. Gene Rk2 confers a stronger broader-based resistance to M. incognita and M. javanica than Rk (Roberts et al., 1996). Thus, IT89KD-288 or other genotypes with Rk2 would be preferable as cover crop genotypes. Neither gene confers an absolute resistance as is found for some nematode R genes, such as Mi in tomato, and our results confirmed that some nematode reproduction occurs on resistant cowpea. In fact, M. javanica reproduced significantly even on cowpea with Rk2, and this was reflected by the Pf, tomato Pi, and tomato root gall indices in Exp.6 (Table 7). The comparisons between infested and noninfested main plots in Exp. 3 and 4 further indicated that Rk- and Rk2-based resistance in cowpea cover crops do not provide full protection of the following susceptible crop on heavily infested sites. However, incorporation of green biomass of resistant cowpea cover provided additional N and increased soil fertility, resulting in the highest tomato yield of any treatment in both infested and noninfested plots in the experiment (Exp. 4) that provided the best indicator of tomato production.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The consistent results across experiments indicated that cowpea is a convenient, desirable legume for use as a cover crop in irrigated tomato production systems; it produces excellent biomass that is easy to incorporate in soil and breaks down rapidly without deleterious effects on tomato planting. Root-knot nematode-resistant cowpea genotypes protected both cowpea biomass yield potential and yield of susceptible tomato after cover cropping. Although the protective value of the resistance was not complete, recommendations can be made to include resistant cowpea as an effective cover crop on root-knot nematode-infested fields. As a component of an integrated pest management strategy, incorporated cover crops of resistant cowpea can suppress nematode populations, improve soil health, and enhance yield of tomato and other nematode-susceptible crops grown in rotation. Resistant cowpea could be combined with resistant vegetable main crops to ensure that the protection is adequate and to offset the need to treat the soil with fumigant or other nematicides. Since resistance genes differ in their specificity for recognition by nematode populations, for example genes Rk in cowpea and Mi in tomato, the risk of selecting virulent populations should not be enhanced by having resistant cover crops and resistant vegetable crops in rotation. Dry and wet fallowing reduced nematode populations and benefited tomato yield compared with susceptible cowpea cover. Thus, fallowing would provide an additional component for nematode management in the cowpea cover crop system, either as an alternative to cover cropping in some years or as an extension of the cycle before or after the cover crop period.


   ACKNOWLEDGMENTS
 
This research was supported in part by grants from the Blackeye Council of the California Dry Bean Research Advisory Board and by the Bean/Cowpea CRSP, USAID Grant no. DAN-G-SS-86-00008-00. The opinions and recommendations are those of the authors and not necessarily those of USAID. We thank Walt Graves for guidance early in the study and Vince Samons for excellent technical field assistance.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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