Reproduced from Agronomy Journal. Published by American Society of Agronomy. All copyrights reserved. Root-Knot Nematode Resistant Cowpea Cover Crops in Tomato Production Systems P. A. Roberts,* W. C. Matthews, Jr., and J. D. Ehlers ABSTRACT growers who have few inexpensive sources of this nu- trient. Root-knot nematodes, Meloidogyne spp., are serious pests of many Root-knot nematodes are major pests of agronomic crops worldwide. Recent limitations on the use of nematicides have enhanced the need to develop alternative management strategies, in- and vegetable crops worldwide and cause root galling, cluding host plant resistance. This study was conducted to determine shoot stunting, and loss of yield (Sasser, 1980). In the the effectiveness of nematode-resistantcowpea [Vigna unguiculata (L.) Coachella and San Joaquin Valleys, where nematode-sus- Walp.] genotypes used as cover crops for suppressing populations of ceptible vegetables are grown intensively under irriga- M. incognita (Kofoid and White) Chitwood and M. javanica (Treub) tion, M. incognita and M. javanica are common and Chitwood and protecting susceptible tomato (Lycopersicon esculen- damage numerous crops, especially in sandy soils. Dur- tum Mill.) grown in rotation. In six field experiments, susceptible and ing the last 50 yr, these nematode infestations have been resistant cowpea was grown to flowering stage and the dried tops in- controlled effectively and economically with fumigant corporated or not incorporated into the soil. These treatments were nematicides, but emphasis is being placed on develop- compared to wet and dry fallowing and were conducted on nematode- ment and implementation of alternative nematode man- infested and noninfested plots. The experiments were conducted in theCoachellaandSanJoaquinValleys,California.Resistanceconferred agement, including host plant resistance, cover crop- by genes Rk and Rk2 reduced loss of cowpea biomass and M. incognita ping, crop rotation, and soil amendments (Roberts, 1993 soil populations and partially suppressed M. javanica compared with Starr et al., 2002). susceptible cowpea, but not as effectively as fallow treatments. Incorpo- Cowpea is an important grain, vegetable, and hay ration of cowpea tops into the soil promoted tomato growth irrespec- crop in many tropical and subtropical regions, but espe- tive of nematode presence. On infested plots, tomato fruit yield was cially in arid savanna and Sahelian zones of West Africa higher following growth and incorporation of resistant cowpea com- (Fatokun et al., 2002 Hall et al., 2003) and is used as pared with growth and incorporation of susceptible cowpea or nonin- a cover crop in the southeastern USA to a limited extent. corporation of cowpea and fallow treatments. We conclude that root- Cowpea has many attributes that make it an excellent knot nematode-resistant cowpea is an effective cover crop for protecting candidate as a cover crop in the irrigated production susceptible vegetable crops grown under irrigation, and its beneficial effects are enhanced by incorporation of its green biomass. 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 Tterior he intensive crop production systems of the hot in- et al., 2002). Coachella and San Joaquin Valleys of Califor- Cowpea grown as a 70-d cover crop can fix 225 kg nia have many problems (Aguiar et al., 2001). The sandy, ha 1 of N and add substantial amounts of organic matter low-organic-matter soils are easily leached of most forms to the soil (Aguiar et al., 2001). Specialized forage or of synthetic N fertilizers, and leaching can contaminate cover crop varieties that flower late and possess vigorous surface and groundwater. Air pollution (from dust) and vegetative growth produce substantially more cover and loss of topsoil result from fields left bare during summer fix more N than grain type cultivars because plants re- fallow periods. The combination of sandy soils, high tem- maining in the vegetative growth stage produce more peratures, and intensive cultivation of nematode-suscep- biomass and fix more N per day than those that transi- tible crop varieties can lead to severe root-knot nematode tion quickly to flowering and setting pods. problems, and weeds quickly build up. Cover cropping Ideally, cowpea used as a cover crop should not act during the typical summer fallow period in Coachella or as a host for nematode reproduction. Although most following crops harvested in early summer in the San cowpea genotypes are susceptible to M. incognita and Joaquin Valley [such as processing tomato, corn (Zea M. javanica, cultivars with strong resistance to these mays L.) silage, wheat (Triticum aestivum L.), and Bras- species are available. Cowpea bred for cover cropping sica spp.] may provide an alternative system to improve should have resistance to limit nematode multiplication soil fertility, limit erosion, and address nematode and and suppress nematode soil populations. Resistance to weed problems (Aguiar et al., 2001 Ngouajio et al., Meloidogyne spp. in cowpea was one of the early exam- 2003). Use of a leguminous cover crop can provide use- ples of nematode resistance in plants (Webber and Or- ful amounts of N, which is especially important to organic ton, 1902). Genetic analysis in grain-type cultivars Iron, Colossus, and Mississippi Silver revealed a single domi- P.A. Roberts and W.C. Matthews, Jr., Dep. of Nematology, and J.D. Ehlers, Dep. of Botany and Plant Sci., Univ. of California, Riverside, Abbreviations: CVARS, Coachella Valley Agricultural Research Sta- CA 92521-0415. Received 29 Nov. 2004. *Corresponding author (philip. tion G, non-infested plots with biomass incorporation I, infested roberts@ucr.edu). plots without biomass incorporation IG, infested plots with biomass incorporation J2,second-stagejuvenilesofroot-knotnematodes KREC, Published in Agron. J. 97:1626���1635 (2005). Integrated Pest Management Kearney Research and Extension Center Pf, final (postharvest) popu- lation density (of nematodes) Pi, initial (preplant) population density doi:10.2134/agronj2004.0290 �� American Society of Agronomy (of nematodes) tomato Pi, population density of nematodes before planting the tomato bioassay. 677 S. Segoe Rd., Madison, WI 53711 USA 1626 Published online November 17, 2005

Reproduced from Agronomy Journal. Published by American Society of Agronomy. All copyrights reserved. ROBERTS ET AL.: NEMATODE-RESISTANT COWPEA COVER CROPPING 1627 nant resistance gene, designated Rk, that conferred re- fornia. A preliminary summary of part of this study was reported earlier (Matthews et al., 1998). sistance to M. incognita, M. javanica, and M. hapla Chitwood (Fery and Dukes, 1980). This resistance also controlled M. arenaria (Neal) Chitwood (Hare, 1959). MATERIALS AND METHODS Cowpea cultivars with gene Rk have been developed Nematode Isolates and Assays for the California blackeye dry bean industry, including Sites 1 and 2 (Table 1) were infested with an isolate of California Blackeye no. 5 (CB5) and California Black- Meloidogyne incognita race 1 collected from a vineyard at the eye no. 46 (CB46) (Ehlers et al., 2002). Many other cul- same location [University of California-Riverside Coachella tivars with gene Rk have been developed for dry bean Valley Agricultural Research Station (CVARS), near Ther- or fresh production for other regions of the United mal, CA]. Site 3 (Table 1) was infested with an isolate of M. States and in other countries (Hall et al., 2003). Al- incognita race 3 collected from a cotton (Gossypium hirsutum though several Meloidogyne species are controlled by L.) field near Tipton, Tulare County, CA. Greenhouse tests gene Rk, California isolates of M. javanica were found showed that both isolates were controlled by the Rk gene in tobe aggressiveoncowpea with Rk,suchas CB5(Thoma- cowpea. Site 4 (Table 1) was infested with an isolate of M. sonand McKinney, 1960). In addition, some populations javanica collected from a cowpea field near Chino, San Bernar- of M. incognita are virulent to Rk, causing extensive dino County, CA. Greenhouse tests showed that this isolate was aggressive toward gene Rk, reproducing about 50% on root galling and poor growth of cowpea cultivars with resistant (Rk) plants when compared with susceptible (non- Rk (Roberts et al., 1995). The occurrence of virulent in- Rk) plants. Species and race identity of these isolates were festations prompted the identification and incorpora- confirmed by isozyme phenotyping and by a host differential tion of additional resistance in cowpea, including gene test as described previously (Roberts et al., 1996). Rk2, conferring strong, broad-based root-knot resis- Inoculum (nematode eggs) used to infest the study sites tance (Roberts et al., 1996), and rk3, a gene with additive was produced on tomato (cv. UC82) in a greenhouse and ob- resistance when combined with Rk (Ehlers et al., 2000a, tained from tomato roots by extraction with NaOCl (Hussey 2000b, 2002). and Barker, 1973). The inoculum was introduced into Sites 1 Cover crops in rotations and as green manures have and 2 through a drip-irrigation system (Becker et al., 1989) and shown promise for nematode control in various crop- into Sites 3 and 4 through injection shanks (Ball and Ferris, 1982). On each site, tomato (cv. UC82) was direct-seeded and ping systems. These include use of rapeseed (Brassica grown for 6 wk before inoculation. Soil sampling of infested napus L.), sudangrass [Sorghum bicolor (L.) Moench.], plots before planting of cowpeas indicated that both methods and sorghum���sudangrass hybrids [S. bicolor S. suda- established uniform nematode populations. nense (Piper) Stapf] as green manures for suppressing Population densities in soil of M. incognita and M. javanica M. chitwoodi Golden et al. in potato (Solanum tubero- second-stage juveniles (J2) and eggs were assessed before sum L.) production in the Pacific northwest (Mojtahedi cowpea and fallow treatments (Pi), when the cowpea canopies et al., 1991, 1993) and lesion nematode (Pratylenchus were cut (Pf), and immediately before planting the tomato penetrans Cobb) in Ontario, Canada (McKeown and Pot- bioassay (tomato Pi). Population assessment at each sampling ter, 2001). Velvetbean [Mucuna deeringiana (Bort.)Merr.] time was based on a composite sample of 12 to 15 soil cores as a rotation crop suppressed M. incognita in soybean (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. Approxi- (Glycine max L.) production (Vargas-Ayala and Rodri- mately 60 cm of the ends of each row in a plot (30 cm for the guez-Kabana, 2001), and millet [Pennisetum typhoides 3.6-m plots) was avoided when sampling. Twelve cores were (Burm.) Stapf & Hubb] and resistant cowpea as summer taken in shorter (3.6 to 6.0 m) plots and 15 cores in the longer cover crops were shown to suppress M. incognita in (9.0 to 12.0 m) plots. Soil samples were hand-mixed and nema- Florida vegetable double-cropping systems (McSorley tode J2 and eggs extracted from a 250-cm 3 subsample by grav- et al., 1999 Wang et al., 2003). ity screening (Ayoub, 1980) through three nested sieves (850-, The objectives of this research were to determine the 150-, and 43- m openings). Screened fractions from all three potential of nematode resistance in cowpea cover crops sieves were placed for 3 d in a modified Baermann funnel- for suppressing nematode population densities in soil mist chamber (Ayoub, 1980) for egg hatch and release of J2. and their damage potential to yield of susceptible to- At tomato harvest, 21 tomato root systems per plot were in- dexed for galling symptoms based on the 0 to 10 rating system mato following in rotation. Resistant and susceptible of Bridge and Page (1980). cowpea cover crops were compared with and without soil incorporation of the aboveground cowpea biomass Plant Materials and were compared with wet and dry fallow treatments. The experiments were conducted at research station Characteristics of the cowpea genotypes used in the experi- ments are given in Table 2. Root-knot nematode resistance sites in the Coachella and San Joaquin Valleys of Cali- Table 1. Descriptions of nematode isolates and soil textures at the experimental field sites. Soil texture (0���30 cm) Texture Site Experiment Years Location��� Root-knot nematode isolate % sand���% silt���% clay classification 1 1 and 2 1 and 3 CVARS Meloidogyne incognita race 1 64���32���4 sandy loam 2 3 and 4 2 and 4 CVARS Meloidogyne incognita race 1 77���20���3 loamy sand 3 5 4 KREC Meloidogyne incognita race 3 62���31���7 sandy loam 4 6 4 KREC Meloidogyne javanica 71���25���4 sandy loam ��� CVARS, University of California-Riverside Coachella Valley Agricultural Research Station, Thermal, CA KREC, University of California Kearney Research and Extension Center, Parlier, CA.