Introduction: A remarkable number of accomplishments were made under the three objectives of the preceding S-253 Southern Regional Project. Objectives 1 and 2 are approximately 90% completed for soybean and 75% completed for peanut. Objective 3 is approximately 20% completed for both crops. The research on nematode identification and variation performed under the Southern Regional Nematology Project immediately preceding S-253 laid the foundation for the present research on new sources of resistance and incorporating that nematode resistance into adapted cultivars. As progress on developing nematode resistance in soybean and peanut occurred, it became evident that nematode resistant cultivars of other widely planted southern crops, such as cotton and vegetables, would be needed, supplemented by other nematode management options. The development of resistance is critical at this time because of the ubiquitous distribution of the major plant-parasitic nematodes, the relative lack of adapted nematode-resistant cultivars in many crops, the on-going federal and state reviews of nematicides, the potential for extending the durability of resistant cultivars through biological and cultural management strategies, and the availability of new technologies which will make certain transgenic crosses feasible and reduce the development time of new cultivars. Cotton and vegetables are increasing in acreage and economic importance and should be included in research to provide new nematode resistance in southern crops.

Accomplishments

Objective 1: Identify and evaluate new sources of resistance genes in soybean, peanuts, and cotton to plant-parasitic nematodes:

Soybean: Evaluation of soybean germplasm and genotypes for resistance to races of Heterodera glycines (soybean cyst nematode), Meloidogyne spp. and races (root-knot nematode), and Rotylenchulus reniformis (reniform nematode) has been a major focus of research among Regional Project S-253 participants. After years of negotiating, scientists in Illinois obtained over 700 Chinese soybean lines that were land races from regions not represented in earlier collections; these lines were added to the national soybean germplasm collection. Most of these lines (primarily breeding lines) and cultivars were from regions not represented in earlier collections. These lines have been tested for resistance to H. glycines but few had good resistance. A large number of additional lines have been obtained from southern China, but they have not been tested. More than half of over 400 soybean genotypes screened against SCN in Arkansas, Georgia, Mississippi, and/or North Carolina exhibited resistance to race 3, whereas fewer were resistant to races 1, 6, and 9, and very few were resistant to races 2, 4, and 5. Of all soybean genotypes evaluated in various states, only Hartwig proved to have high resistance to all H. glycines races, especially race 2 of this parasite. Cultivars TN5-92, Cordell, and Pioneer 9521 also exhibited considerable resistance to race 2 in greenhouse and field tests in North Carolina. Unfortunately, the Hartwig cultivar was found to be highly susceptible to both races of M. arenaria. Resistance to the reniform nematode is often linked to resistance to H. glycines race 3. However, cultivars that had plant introduction PI 88788 in their background showed little or no resistance to this pathogen. A number of soybean genotypes (Forrest, Pioneer 9521, Hartwig, TN5-92, PI 437654) were highly resistant to the reniform nematode in greenhouse tests. About one-third of the soybean PI’s and selected cultivars evaluated against this nematode exhibited resistance.

The stock of soybean germplasm with resistance to the southern root-knot nematode, M. incognita, has been expanded greatly, but sources of resistance to M. javanica and M. arenaria still are few. A number of the recently-developed, high yielding soybean cultivars are susceptible to most root-knot nematode species. In Georgia, DNA markers (restriction fragment length polymorphisms) are being used to identify quantitative trait loci (QTL) conditioning resistance to M. arenaria, M. javanica, and M. incognita on the molecular map of soybean. The economic importance of resistance to root-knot nematode was documented in a field test involving race 1 of M. arenaria; yields of susceptible cultivars ranged from 21.5 to 45.3 bu/A, whereas, yields of the resistant cultivars ranged from 47 to 61 bu/A. A recently described root-knot nematode in Tennessee, M. trifoliophila, may pose a new threat to soybean in that current cultivars resistant to other Meloidogyne species exhibit moderate disease development when inoculated with this new pathogen. The sources of nematode resistance, as characterized in S-253, have great potential for facilitating more effective management of nematodes throughout the southern and other geographic areas of the United States. Clearly, reliance on multiple-nematode species resistance in soybean will become more important as nematicides continue to be removed from the market, including their use on crops rotated with soybean.

Peanut: Efforts in North Carolina and Texas have focused on the introgression of resistance from wild Arachis spp. into the cultivated peanut. Because A. hypogaea is an allotetraploid, whereas most wild species are diploids, introgression of nematode resistance from the wild species into cultivated peanut is not a trivial process. The Texas effort has focused on the development of resistance to M. arenaria and has followed a diploid route to introgress resistance into A. hypogaea. Three wild species were used to develop a nematode-resistant complex hybrid, TxAG-6, that is cross-compatible with A. hypogaea. All three wild species [A. batizocoi, A. cardenasii, and A. diogoi (= A. chacoensis)] are resistant to M. arenaria and A. cardenasii also is resistant to M. hapla. Three molecular markers, using randomly amplified polymorphic DNA (RAPD), have been used to identify a single resistance gene derived from A. cardenasii. This gene is the only resistance gene specifically identified and is the only resistance gene present in several advanced generation lines examined to date. Because of the multiple sources of resistance used to generate TxAg-6, it is likely that additional resistance genes are present in early generation breeding lines.

Additional work in Texas has confirmed the parasitism of peanut by populations of M. javanica from Egypt, India and Texas. These studies indicate that resistance to M. javanica is present in early generation breeding lines TxAG-7 and TP-223 that are resistant to M. arenaria. Data from several lines derived from the fourth backcross generation, however, suggest that genes conditioning resistance to M. javanica differ from those that condition resistance to M. arenaria.

Objective 2: Characterize and facilitate development of resistant cultivars:

Soybean: Collaborative research involving participants of S-253 and soybean breeders/geneticists in the southern United States has facilitated the release of a number of promising soybean cultivars and germplasm lines with resistance to H. glycines, root-knot, and/or reniform nematodes. Progress has been made in finding a level of tolerance to the ectoparasitic nematode, Hoplolaimus columbus (lance) in cultivars evaluated in South Carolina and Georgia. In addition, the genetic basis and general mechanism for resistance to Meloidogyne spp. in selected soybean genotypes have been investigated. Differential root-cellular responses are associated with genotypes resistant to M. arenaria as compared to susceptible lines. Inbred populations of H. glycines were used to identify resistance markers in soybean and to facilitate the development of a genetic map for this nematode. Collaborative research via this project has contributed to the development and release of a large number of new soybean cultivars, especially in Georgia, Arkansas and Illinois. These cultivars include: Uark-5896 (approved for release) and R93-6214 (will be released in 1996 as a germplasm line) from Arkansas; and Cook, Doles, HyPerforma 798, Haskell, and Benning from Georgia. In addition, advanced lines with resistance to multiple races of SCN will become sources of resistance that will be crossed with high-yielding private cultivars. This has the potential to provide soybean growers with resistant cultivars that produce higher yield than were previously possible.

Peanut: A back-crossing program has been used to introgress the resistance from TxAG-6 into high yielding cultivars of runner, spanish, and virginia market-type peanut. Seven backcross generations have been completed and field testing of advanced generation lines for yield potential and other important agronomic traits began in 1996. These advanced generation lines have retained a level of resistance to M. arenaria that provides greater than 90% inhibition of the reproduction of the nematodes.

Efforts in North Carolina have focused on development of resistance to M. arenaria in Virginia market-type peanut, using A. cardenasii as the source of resistance. High levels of resistance have been introgressed into several breeding lines using a hexaploid route. These resistant lines are being screened for yield potential in field tests. Two RAPD markers linked to the resistance genes have been identified. In lines with high levels of resistance, resistance also segregates in a 3:1 ratio indicative of a single dominant gene. Segregation patterns are more complex in lines with moderate levels of resistance. This program complements the Texas program as it has used a different approach of introgression and may have identified different resistance genes from those in the Texas program. Available evidence indicates that resistance to M. arenaria in A. cardenasii is conditioned by multiple, dominant, major genes.

Several different sources of resistance to multiple Meloidogyne species have been identified and introgressed into numerous peanut breeding lines. Host resistance for management of root-knot nematodes attacking peanut will probably be available to growers within five years. The availability of multiple genes for resistance to the major root-knot nematodes attacking peanut should be an important asset in that it will allow development of gene deployment systems to enhance the durability of the resistance being developed. The identification of resistance to M. javanica also is likely to be of future importance. Even though M. javanica populations parasitic on peanut have been identified from only two locations in the United States, the species is widely distributed, and other populations not yet tested are likely to be parasitic on peanut. As resistance to M. arenaria is deployed it is probable that populations of M. javanica parasitic on peanut will become more widespread due to their increased competitiveness. Similar shifts in root-knot nematode species due to introduction of specific resistance genes have been documented for tobacco, a crop often grown in rotation with cotton, peanut, and soybean. Such shifts are similar to those that have been documented for races of SCN in response to use of specific resistance genes in soybean.

Objective 3: Integrate resistant cultivars into sustainable cropping systems:

Soybean: Integration of nematode resistant cultivars into cropping systems should enhance the durability of host-resistance genes. Greenhouse and field experiments indicated that the durability of cultivar Hartwig’s general SCN resistance may depend on the use of rotation/cropping systems. In addition to host resistance, other management tactics evaluated included nonhost rotation crops, cover crops, green manure crops, biological controls, and other cultural practices such as minimal or no-till, and time of planting. Crop rotation with grain sorghum, sweet clover, Green Graze (a hybrid between sudangrass and sorghum), barley, sunflower and wheat enhanced soybean yields as well as reducing relative levels of SCN in Arkansas, Florida, North Carolina, South Carolina and/or Tennessee. Wheat was found to produce exudates that apparently disrupt the hatching and migration of SCN toward the soybean roots. In contrast to the positive effects of crop rotation on soybean yields, no-till practices in a long-term study actually lowered yields (due to severe weed pressure).

The use of cover crops and green manure crops noticeably improved the tilth of the soil but did not significantly increase organic matter; therefore, the water-holding capacity was not improved. A Group III soybean cultivar planted in April had fewer cysts and eggs of SCN than did the same cultivars planted in May or June, or Group IV, V, VI, or VII cultivars planted in April, May or June. However, yield of the Group III cultivars planted in April was the lowest of any of the cultivar-planting date combinations. Therefore, planting of a short-season cultivar early is not an economical practice for cyst nematode-infested fields.

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