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| Cotton | Selected Vegetables | Peanut | Soybean | Biotechnology | |
| Deployment of Resistance | CRIS Project Review | ||||
Accelerated development of nematode resistance in cotton and vegetables, as has been accomplished so successfully in soybean and peanut by cooperating S-253 scientists (see Critical Review), is critical to crop production in the South. In this regard, some of the new molecular techniques now being implemented in soybean and peanut resistance research can be adapted for vegetables and cotton.
Cotton ---World production and use of cotton is expected to increase 10% by 2000. Loss estimates in cotton due to nematodes have increased every year since 1987. In fact, in 1995, nematodes of cotton, Meloidogyne spp. primarily, were responsible for 3.54 % average yield loss or 778,703 bales worth $311.5 million, second only to seedling diseases (Blasingame, 1996). Upland-type cotton, Gossypium hirsutum, is grown in the South and west through Texas. In some regions, more than 50% of fields are infested with root-knot nematodes. Management is currently based largely on nematicides since only a few commercial cotton cultivars with resistance to M. incognita are in use. However, high-yielding cultivars with excellent resistance to M. incognita are just now becoming available (CA). Previously, Meloidogyne resistance in cotton was limited to only a few breeding lines having tolerance to the "southern root-knot nematode". Many of the early cultivars were developed to control the M. incognita-Fusarium wilt complex, and most commercial cultivars now have tolerance or very high resistance to the fungus but no more than a low level of tolerance or resistance to the root-knot nematode, M. incognita (Starr,1993). This is a serious economic problem since M. incognita is ubiquitous and 70% of all cotton grown in the U. S. is grown in the South, including Texas. Highly resistant germplasm (Auburn 634 and NemX) is available that allows only low nematode reproduction (1,000 eggs per plant) compared to the tolerant cultivars in which reproduction is twice as great. The most susceptible cotton cultivars allow development of up to 100,000 eggs per plant.
Several states participating in S-253 have cotton breeding programs producing germplasm lines with moderate levels of resistance to M. incognita. However, in nematicide-treated plots, differences in resistance among breeding lines to M. incognita is evident. Also, some genotypes of M. incognita reproduce on "highly resistant" germplasm lines. The search for cotton that is highly resistant, i.e., reproduction is almost nil, to the root-knot nematode is now at the stage when collaboration among scientists would be very beneficial to cotton breeders, just as was seen in the development of the new soybean cultivars with multiple nematode resistance. Cotton cultivars differ by region, and it is vital that researchers in the southern U. S. move forward in the development of distinctive, adapted cultivars for the South. Coordinated nematological research is needed in developing cotton resistance so that the wide-spread use of environmentally and economically costly chemical nematicides can be reduced. Resistant varieties will be an integral part of the implementation of better IPM strategies in a crop with a history of high chemical inputs. Rotations involving resistant soybean and cotton are feasible, as are rotations of peanut and cotton, which control nematodes on both crops (Starr, 1993).
The reniform nematode, R. reniformis, is a major pathogen of cotton in a number of states (from North Carolina to Texas), with perhaps Mississippi and Louisiana having the greatest incidence at 283,000 and 206,000 ha infested, respectively (Overstreet, 1996). This nematode species is increasing in incidence and is apparently spreading rapidly in the South. Estimated losses average 15%, varying by region, but may be as high as 40%. Rotations and nematicides are the primary management tactics because resistant cultivars are in the early developmental stages. Complicating developmental research on resistant cotton varieties is the difficulty of making crosses of upland cotton (G. hirsutum) with other cotton species that are sources of resistance to the reniform nematode (Overstreet, 1996). Tolerance has been reported in some cultivars but not consistently or across geographical regions, and differences exist in fecundity and aggressiveness among various nematode populations. The extent of nematode variation will need to be assessed in order to develop cultivars with the necessary range of resistance, and a regional approach (as well as interfacing with the W-186 Project on nematode variability) to management of this increasingly widespread nematode pathogen is warranted.
Selected Vegetables --- Vegetables are important in the southern region (Tables 3-7). Southernpea (Vigna unguiculata (L.) Walp.), pepper (Capsicum annuum L.), sweetpotato (Ipomea batatas), Solanaceae, and Cucurbitaceae are all important vegetables in southern agriculture. Moreover, home gardens are very important in the diets of the rural poor, and the development of nematode-resistant vegetable cultivars will have a great impact on this sector of the population. Additionally, home gardening is the number one leisure time activity in the US. Nearly all vegetable crops are damaged by M. incognita, M. arenaria, M. javanica, and R. reniformis, among other nematodes. M. incognita is also pathogenic on black-eye dry bean, another type of cowpea, in California. Cultivar ‘Iron’ was the first cowpea shown to have resistance to M. incognita (Webber and Orton, 1902). The resistance which is controlled by a single dominant gene (designated Rk) (Hare, 1959) includes resistance to M. hapla and M. javanica (Fery and Dukes, 1980). Several root-knot nematode resistant southernpea lines and cultivars have been released through the USDA Vegetable Laboratory, Charleston, SC (Fery and Dukes, 1988). Root-knot nematodes on southernpea can be controlled by growing resistant cultivars (Roberts et al., 1995), although some M. javanica and M. incognita isolates in California can reproduce on cultivars that have the Rk gene (Roberts and Matthews, 1989; Swanson and Van Gundy, 1984). A Nigerian source of resistance (UCR 430) is available that is highly and partially resistant to isolates of M. incognita and M. javanica, respectively, that are virulent against Rk (Matthews and Roberts, 1994). Plant Introductions PI 441917, PI 441920, and PI 468104 have superior levels of resistance to M. incognita; the allele at the Rk locus conditioning this resistance may not be the Rk allele but one or more alleles conditioning a superior, dominant resistance (Fery et al., 1994). PI 468104 also is resistant to the aggressive California M. incognita isolates. Breeding line IT84S-2049 has high levels of resistance to virulent isolates of M. incognita and M. javanica, and this resistance is conditioned by a single dominant gene or is closely linked to it (Roberts et al.,1995).
Meloidogyne spp., especially M. incognita, are devastating pathogens of sweetpotato, causing deep cracks (Elliott, 1918), root galls (Poole and Schmidt, 1927), and poor quality and suppressed yield (Clark and Moyer, 1988; Dukes et al., 1985; Krusberg and Nielson, 1958). Differences in resistance and susceptibility of sweetpotato cultivars to Meloidogyne species occur (Lawrence et al., 1986; Gaspasin, 1984; Roberts and Scheuerman, 1984). The genetic inheritance of root-knot nematode resistance was difficult to elucidate because sweetpotato is a hexaploid, and resistance to M. incognita has a complex inheritance (Struble et al., 1966). Later, resistance to this species was determined to be inherited in a multifactorial, quantitative manner, controlled by several genes showing partial dominance (Misuraca, 1970). Resistance of sweetpotato to M. incognita and M. javanica has high heritability, which indicates that development of cultivars with high levels of resistance is possible (Jones and Dukes, 1980). Because of the complex genetics of the sweetpotato, mass selection techniques were utilized to combine multiple insect, nematode, and disease resistance with other horticultural qualities (Jones et al., 1976). From this program the breeding line W-51 was developed with resistance to M. incognita as well as to a resistance breaking race (Martin and Birchfield, 1973; Dukes et al., 1978; Jones et al., 1976). In fact, numerous sweetpotato germplasms and cultivars with multiple pest resistance, including resistance to Meloidogyne species, have been developed and released by the USDA, ARS sweetpotato breeding program at the U. S. Vegetable Laboratory, Charleston, SC. Although sweetpotato cultivars with high levels of resistance to root-knot nematode have been developed, these cultivars do not have all of the disease resistance and horticultural characteristics, e.g. desirable skin and flesh color, demanded by the market. In addition, because the genetics of sweetpotato are complex, all seedlings must be screened for resistance to root-knot nematode (and other diseases and pests) in order to maintain high levels of resistance in the germplasm. Combining pox (Streptomyces sp.) resistance with root-knot nematode resistance is especially important.
Sweetpotato is a host of the reniform nematode (Peacock, 1956), and is seriously damaged by it (Martin, 1960). Nematicides may greatly increase yields in reniform nematode-infested fields (Birchfield and Martin, 1965). All cultivars tested supported reproduction by the nematode, but ‘Goldrush’ was less suitable. That cultivar was also one of the most sensitive to nematode injury. Reniform nematode feeds on the small feeder roots and causes severe reduction in yield and quality, and chlorosis on some occasions. Cracking may also be associated with reniform damage and was greatly reduced using nematicides (Clark et al., 1980). All sweetpotato cultivars tested in North Carolina are susceptible to this nematode species.
M. incognita is pathogenic on pepper (Capsicum annuum), and causes yield losses world-wide (Di Vito et al., 1985; Zamora et al., 1994). This species suppresses yield of hot peppers in New Mexico (Thomas and Cardenas, 1985) and sweet peppers in Italy (Di Vito et al., 1985, 1993). Nematicides have been used to control nematodes in many high value vegetable crops, including peppers (Thomas, 1994). Resistance to M. incognita has been identified in Capsicum species (Di Vito et al., 1993; Fery and Dukes, 1986; Hendy et al., 1985; Martin and Crawford, 1958; Zamora et al., 1994), but the mechanisms of resistance are not known. Differences in penetration and development of root-knot nematodes were observed on two C. annuum lines, PM 217 and PM 687, that differ in genes Me1, Me2, Me3, and Me4 for resistance to various Meloidogyne species (Hendy et al., 1985). Mechanisms of resistance to the dominant N gene (Hare, 1957) have not been identified. Molecular markers linked to root-knot nematode resistance have not been identified for resistance in cowpea and pepper, but other markers are tightly linked with resistance to M. arenaria in peanut (Burow et al., 1995) and to the Mi gene conferring resistance in tomato to M. incognita, M. javanica, and M. arenaria (Williamson et al., 1994a,b).
Root-knot nematodes, especially M. incognita, M. arenaria, and M. javanica, cause serious damage to cucumber, melon, watermelon, pumpkin and squash. Where available, resistant cultivars provide the most economical and environmentally benign means of control. Differences in field resistance to Meloidogyne spp. among 21 melon cultivars and breeding lines have been reported in Florida, but good resistance in muskmelon or other cucurbits is not available. A high level of resistance to the four major root-knot nematode species is present in the African horned cucumber (Cucumis metuliferus), but attempts to develop interspecific hybrids with cucumber cultivars have failed. Resistance to M. javanica and M. arenaria races 1 and 2 occur in Cucumis sativus var. hardwickii line 90430, and cucumbers resistant to these species or races currently are being developed at North Carolina State University. Resistance to M. incognita, M. javanica and M. arenaria has also been identified in the West Indian gherkin (C. anguria) and other wild Cucumis spp. (Walters, 1996).
Rotations in vegetable production systems for management of Meloidogyne spp. may involve tropical plants such as velvetbean, horsebean, sesame, castorbean, crotalaria, partridge pea, and American jointvetch. Some success has been achieved using such plants to reduce densities of Meloidogyne species (McSorley et al., 1994; McSorley and Dickson, 1995). Response of other plant-parasitic nematodes to these crops must be evaluated when planning rotations (McSorley and Dickson, 1995).
The reniform nematode limits yield and quality of cantaloupe by restricting size and sucrose percentage, respectively (Heald, 1980). Cucumber and pumpkin are also hosts of this parasite, and watermelon has been listed both as a host and as a nonhost, which supports reports of significant variation among biotypes of the reniform nematode. Mangement of reniform nematode is difficult due to the extensive crop and weed host range, prolific reproduction, and ability to remain in the soil for long periods without a host. Crop rotations using nonhosts, such as corn, grasses, and sugarcane, and resistant cultivars of soybean and cotton have been used to manage this species in cotton. Crop rotation could be useful in cucumber production if nonhosts or resistant cultivars could be introduced into the cropping system. Planting resistant cultivars would be the most economical method of control, making feasible control available to farmers world-wide, but resistance to R. reniformis has not yet been identified in cucurbit species. Complicating the development of management options are the possibility of reniform nematode races (Sipes, 1996) and the necessity of correct species identification. Currently, there are 11 described species of Rotylenchulus and the host ranges of these species vary considerably in their breadth and plant species affected (Vovlas and Troccoli, 1996).
Peanut --- Root-knot nematodes are important and widely distributed pathogens of peanut (Arachis hypogaea L.) throughout the peanut production areas of the southern United States. These pathogens are reported from all peanut producing states with 30 to 40% of the fields in some states being infested (Ingram and Rodriguez-Kabana, 1980; Wheeler and Starr, 1987). Meloidogyne arenaria is the dominant species in most of the southern region, with M. hapla being dominant in the more northern production areas of North Carolina, Oklahoma, and Virginia. Meloidogyne javanica is not usually pathogenic to peanut, however, pathogenic populations do occur in Egypt (Tomaszewski et al., 1994) and India (Patel et al., 1988). Two populations of M. javanica pathogenic to peanut have been identified in the United States, one from Georgia (Minton et al., 1969) and one from Texas (Tomaszewski et al., 1994). Additionally, an undescribed Meloidogyne species also attacks peanut in north Texas (Abdelmomen et al., 1996).
Prior to the mid-1980's, no confirmed resistance to root-knot nematode species in peanut or related Arachis species was known, despite the screening of several thousand genotypes of A. hypogaea. In Florida, resistance to M. hapla was reported from A. glabrata, a wild species that is incompatible with A. hypogaea (Baltensperger et al., 1986). In Texas, resistance to M. arenaria was observed in 21 Arachis species and 2 interspecific hybrids (Nelson et al., 1990). They also reported resistance to M. hapla in two Arachis species and one interspecific hybrid. Resistance is apparently conferred by multiple major genes with different resistance mechanisms in the different wild species (Nelson et al., 1990; Starr and Simpson, 1991). Resistance to M. arenaria in several wild Arachis spp. was confirmed in Georgia (Holbrook and Noe, 1990). Based on observations of resistance to M. arenaria and M. hapla in several wild Arachis species, a systematic search of the A. hypogaea germplasm collection for useful sources of resistance was initiated (Holbrook and Noe, 1992). Several peanut accessions with moderate levels of resistance have been identified. None had the high levels of nematode resistance observed in the wild species or the interspecific hybrids. Resistance to Egyptian populations of M. javanica pathogenic to peanut has been identified in an interspecific hybrid (TxAG-7) that is also resistant to M. arenaria (Tomaszewski et al., 1994).
Currently, three research groups are working to develop resistance to root-knot nematodes that has been identified in different Arachis germplasm sources. Efforts in Georgia are focused on a continued systematic search of the available germplasm collection of A. hypogaea for useful sources of resistance (Stephenson et al., 1995). Resistant A. hypogaea germplasm suppressed population densities of M. arenaria up to 60% (Noe et al., 1992). Yield potential of three moderately resistant accessions was equal to that of currently grown susceptible cultivars in the absence of nematode pressure and was superior to that of the susceptible cultivars in the presence of damaging levels of M. arenaria (Holbrook et al., 1995). Efforts in Texas have focused on introgression of resistance to M. arenaria, M. javanica, and an undescribed Meloidogyne species from three wild species into cultivated peanut via a complex hybrid using a diploid introgression pathway (Simpson, 1991) and backcrossing to several popular cultivars. A single dominant gene conferring resistance has been identified in several advanced generation breeding lines (Choi et al., 1996). Moreover, three DNA fragments linked to this resistance gene, which is apparently derived from A. cardenasii, have been identified (Burow et al., 1996). RAPD markers linked to the resistance gene were developed, and lack of co-segregation of resistance to M. arenaria and to M. javanica indicated that resistance to these two nematode species is conditioned by different genes (Starr et al., 1996). Efforts in North Carolina are focused on the introgression of resistance from A. cardenasii into virginia market-type peanut using a hexaploid introgression pathway (Stalker et al., 1994). Two dominant genes conferring resistance have been identified, along with a DNA fragment linked to both genes (Garcia et al., 1996). There is a strong possibility the genes for resistance identified independently in Texas and North Carolina are different from each other because different introgression pathways were used by the two programs and differences in expression of resistance exist. Arachis cardenasii is believed to have more than two dominant genes for resistance to M. arenaria (Starr and Simpson, 1991).
Soybean --- The persistence and adaptability of plant-parasitic nematodes requires constant monitoring and updating of management decisions. The development of resistant cultivars, some with resistance to several species and races of nematodes, has provided important management options for the producer. Moreover, nematode management through crop rotation is enhanced by rotating with resistant cultivars, nonhosts, and susceptible soybean cultivars in combination (Schmitt and Noel, 1984). The four most damaging diseases on soybean for the period 1990-1994, in order of importance were H. glycines (soybean cyst nematode), charcoal rot, Meloidogyne spp. and ectoparasitic nematodes together, and root and stem rots (Wrather et al., 1995). However, while soybean cyst nematode caused the greatest losses over the years, crop loss was cut by two-thirds, from 1.034 million metric tons in 1974 to 0.377 million metric tons in 1994. Root-knot nematode damage has similarly declined over those two decades. These declines are attributed to high yielding disease resistant cultivars and advances in disease management technologies. Soybean production increased from 467 million bushels in 1974 to 595 million bushels in 1994, due, in part, to nematode resistant cultivars developed by this project (see Critical Review). Moreover, while soybean acreage declined during this time from 21.3 million acres to 17.6 million acres, the mean yield per acre has risen sharply from 22.0 to 33.8 bushels/acre, due to more effective management of H. glycines through use of resistant cultivars (Bradley and Duffy, 1982) and, possibly, to cropping on better soils than in the past. However, soybean cyst nematodes are still the primary pest of soybean in the U. S., in part because of the abandonment of crop rotations (Noel, 1992). The selection pressure imposed by the continued cropping of resistant cultivars often leads to shifts in the ratios of species and subspecies of nematode populations that exist in a particular site (Young, 1992a, b). To increase the useful life of resistant crop cultivars, it is critical to identify and monitor shifts in field populations of nematodes and alter cropping sequences of nematode-resistant cultivars accordingly. In addition, changing sources of resistance enhances resistant cultivar durability (Young, 1992b). Optimally, resistant cultivars could be integrated with rotational or cover crops that are non-hosts or crops that produce nematode-antagonistic allelochemicals (Halbrendt, 1996). For example, wheat is a potential winter crop which suppresses H. glycines, possibly by producing a hatch or invasion retardant (Baird and Bernard, 1984), and rye produces substances on decomposition that are toxic to M. incognita and P. penetrans (Patrick et al., 1965). Research is continuing on the potential of fungal parasites of cyst nematode eggs and females as biocontrol agents (Kim and Riggs, 1992). The use of multiple biological control agents is also being explored. The fungal parasite of soybean cyst nematode eggs and females, ARF18, isolated in Arkansas, reduces numbers of eggs or females by up to 90%. Nematode trapping fungi capture a high percentage of juveniles and may, in combination with ARF18, represent a first step in using combinations of organisms.
Biotechnology --- Efficient and reliable nematode identification is critical for selecting and developing appropriate resistant plant cultivars. Although biotechnology is not used extensively at present for routine nematode identification, significant progress is being made in the development of protein, antibody, and DNA analyses to distinguish nematodes at the species and subspecies levels ( Barker and Davis, 1996; Baum et al., 1994 a, b; Caswell-Chen et al., 1993; Curran and Robinson, 1993; Burrows, 1990; and Hyman, 1990). These emerging methods of nematode identification can be interfaced with traditional morphological and biological means of nematode identification for maximum benefit. The ability to clone and identify naturally-occurring plant resistance genes and specifically transfer these to desired crop species may overcome some obstacles now encountered in the development of cultivars resistant to nematodes. This potential may be particularly relevant to many of the vegetable crops included in this project for which no known source of nematode resistance has been identified. The most intensively studied nematode-resistance genes in plants are the Hs1pro-1 gene in sugar beet conferring resistance to the sugar beet cyst nematode, Heterodera schactii (Cai et al., 1997), and the single, dominant Mi gene in tomato that confers resistance to M. incognita, M. arenaria, and M. javanica, but not to M. hapla (Williamson et al., 1994a, b). The report of the Hs1pro-1 gene is the first of a cloned resistance gene to an animal pest, although more than a dozen plant resistance genes that make plants resistant to bacteria, fungi, and viruses have been reported previously. The new gene originated in a wild beet plant and encodes a 282-amino acid protein with leucine-rich repeats. The structure of the protein indicates that the site of action may be at the membrane level where it mediates protein-protein interactions between pathogen and host. The Mi gene in tomato has an allele for acid phosphatase, Aps-1, which has served as a reliable phenotypic marker for root-knot nematode resistance (Rick and Fobes, 1974) and has been used as a DNA marker to "walk" to the Mi gene using overlapping clones. RFLP and RAPD markers in tomato that are even more tightly-linked to the Mi gene have been obtained that can be used to isolate the resistance gene (Williamson et al., 1994b).
DNA markers that are tightly linked to nematode resistance genes in plants also represent a powerful tool for plant breeders to utilize "marker-assisted selection" to more efficiently develop improved crop cultivars (Tanksley et al., 1989). Molecular markers and positional cloning are now being used to isolate and tag resistance genes to root-knot and cyst nematodes in soybean (Boerma and Hussey, 1992; Concibido et al., 1994; Tamulonis et al., 1997a, b, c). Resistance to cyst nematodes in soybean is conditioned by several genes that are collectively considered as quantitative trait loci (Concibido et al., 1994). RFLP analyses of oligogenic resistance to three root-knot nematode species that was derived from soybean plant introductions, and DNA marker analyses for resistance genes in peanut derived from the wild species, Arachis cardenasii, are now being conducted by members of this project.
In contrast to mechanisms associated with naturally-occurring plant resistance genes, the fundamental molecular mechanisms of compatible nematode-plant interactions are also being investigated and may be applied to develop novel mechanisms of resistance in transgenic plants that target the molecular events involved in compatible nematode-plant interactions. The esophageal gland secretions from the stylets of cyst and root-knot nematodes appear to be critical in feeding site establishment (Goverse et al., 1994; Hussey et al., 1994). Transgenic plants that can inactivate the function of essential nematode secretions may have a novel form of resistance. Efforts are underway to develop transgenic plants that express genes that encode antibodies ("plantibodies") that bind to nematode secretions to neutralize their activity (Baum et al., 1996). Further analyses of the nematode gene products that are essential for plant parasitism may suggest other potential targets to interrupt susceptible nematode-plant interactions.
Deployment of resistance --- Deployment of resistant cultivars with other sustainable agricultural practices ensures the durability of resistance and enhances the value of rotations, cover cropping, intercropping, fall planting, double cropping, and biological control. Such practices have the advantage of carry-over herbicide and nutrient effects, better soil tilth, less selection pressure on pests and pathogens of monocultured crops, and less soil compaction and erosion. In the case of biological control, alternative crops promote suppressive soils by stimulating antagonistic microflora and biological control agents ( Kloepper et al., 1991, 1996). Species of Meloidogyne and R. reniformis have wide host ranges and reproduce on many potential rotation crops, while H. glycines has a restricted host range and can therefore be managed through crop and cultivar selection. Rotations of sorghum and soybean, for example, in alternate years provided greater yields in soils having cyst, stunt, lesion, and spiral nematodes than could be obtained with insecticide or nematicide application (Trevathan and Robbins, 1995). Examples of rotations for nematode control include those for root-knot and cyst nematodes, as well as other genera. Rotation of resistant and non-host vegetable and cover/green manure crops have been used to control root-knot nematodes in vegetable crops. In Florida, castor bean, velvet bean, American jointvetch, and sorghum-sudangrass effectively suppressed populations of Meloidogyne spp. compared to rotations with okra or weed-fallow (McSorley et al., 1994). Two- or three-year rotations of bermudagrass reduced numbers of M. incognita race 1 in the soil and increased yields of the succeeding susceptible crops of okra and squash (Johnson et al., 1995). Populations of root-knot nematodes were reduced following resistant sweetpotato compared to susceptible sweetpotato (Dukes et al., 1985). Some rotations are better than others at reducing nematode populations, either because plant species differ in host suitability or they produce allelochemicals which are considered to be secondary metabolites that are released into the rhizosphere (Halbrendt, 1996). An example of passive suppression would be the crop rotation systems for suppressing root-knot nematode in peanut (Rodriguez-Kabana and Ivey, 1986; Rodriguez-Kabana et al., 1987; Rodriguez-Kabana et al., 1991). Alternatively, active suppression occurs in another peanut rotation in which velvetbean may release nematicidal exudates that suppress root-knot nematodes (Rodriguez-Kabana et al., 1992). All rotational crops must pass the tests of economic and logistical considerations before implementation by growers. In a rotation experiment in fields infested with the lance nematode, Hoplolaimus columbus, projected net incomes ranged from a net loss to a net profit depending on crop sequence, and profitability of various rotations changed as nematode densities increased (Noe et al., 1991). In soybean, a crop with almost total reliance on nematode-resistant cultivars, planting early maturing cultivars in mid-to-late June and rotation with a non-host kept H. glycines populations at a manageable level (Schmitt, 1991). Rotations of soybean, corn and cotton for control of the soybean cyst nematode came into wider use in Arkansas through educational programs that demonstrated the lack of economic return when using nematicides to control the cyst nematode on soybean. Growers were able to understand the importance of prolonging the effective life of cyst nematode-resistant varieties by alternating susceptible or tolerant cultivars and crops. The benefits of rotations on soils and the interruption of pest cycles are also important (Slack et al., 1981).
CRIS Project Review --- A complete and thorough search of current CRIS projects was done to identify research objectives that were similar to those in this project revision. A number of projects funded to members and participants of the current S-253 project had similar subsets of the revised S-253 objectives; this is to be expected. Several other projects were identified that had objectives that were similar to (but do not necessarily duplicate) those of this project revision. These projects included: complete cloning of the Mi gene in tomato; early events in the response of resistant tomato to root-knot nematode; effects of cover crops on yield and nematode infestation of tomato; winter cover crops in vegetable production systems; double-cropping root-knot resistant tomato/cucumber in combination with green manure; effects of long-term crop rotations and winter cover crops on nematode populations; management of Meloidogyne with multiple resistance in vegetable crop rotations; construction of a DNA marker linkage map of soybean for soybean cyst nematode; management and related biology of nematode parasites of field and vegetable crops; management of nematodes to reduce crop loss and nematicide use on irrigated crops in Georgia; and evaluate cotton cultivars for resistance to reniform (1) and root-knot nematodes (2).
The following plant breeding projects were also funded: tomato, eggplant and pepper for resistance to the root-knot nematode (1); peanut for root-knot resistance; cotton for root-knot and/or reniform nematode resistance (5); identify molecular markers associated with resistance to root-knot nematode in peanut (1) and cotton (1); biochemical markers for nematode resistance and tolerance (1); and mechanisms of resistance to root-knot nematode in peanut (1).
NE-171 (Northeastern Regional Technical Committee): Biological and cultural management of plant-parasitic nematodes. Objectives: 1. Identify and evaluate biological agents for plant-parasitic nematodes; 2. Assess the impact of rotational crops and organic matter management on plant-parasitic nematodes and their microbial biocontrol agents; 3. Integrate tactics developed in Objectives 1 and 2 with those currently available to attain acceptable nematode management.
W-186 (Western Regional Technical Committee): Genetic variability in cyst and root-knot nematodes. Objective 1. To characterize variability and gene frequencies in plant-parasitic nematodes by phenotypic assessment of host range, response to resistance, response to environmental conditions, biological processes and morphology; 2. To elucidate variability and gene frequencies in plant-parasitic nematodes by molecular, histochemical, morphological, and anatomical markers to identify variability; 3. To determine nematode fitness and adaptability relative to environment, host plant, and host plant resistance; and 4. To design and develop management strategies for cyst and root-knot nematodes relative to genetic variability.
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