ABSTRACT Strong host resistance to root-knot nematode (RKN; Meloidogyne arenaria ) introgressed from a wild diploid species to cultivated peanut was previously shown to be located on a large chromosomal region of linkage group A09. Little to no recombination in mapping populations has hindered fine mapping of the resistance genes. In order to further delineate the introgressed region, additional polymorphic markers were added to the linkage group A09 using a recombinant inbred line population developed from Gregory x Tifguard in which Gregory is the susceptible parent and Tifguard is the resistant parent harboring the alien introgression. Map distance within the introgressed region based on this population increased to 8 cM compared with zero recombination in an earlier generation. Lines with rare recombination within this introgressed region were phenotyped and it was demonstrated that one portion of the introgressed region confers moderate resistance while a smaller, distal portion confers strong resistance to RKN. Molecular markers associated with the introgressed region conferring strong resistance can be deployed in peanut breeding programs to improve selection for RKN resistance.

ABSTRACT Modern farming is dependent on continual development of improved cultivars and efficient cultural management practice. In addition, dissecting genetic components of heritable traits also relies on the development of large mapping populations. Artificial hybridization is the critical initial step in these processes. Peanut is a self-pollinating crop with a typical yield of less than three seeds per flower; therefore, significant effort is required to produce sufficient hybrid seeds for subsequent trait selection and/or establishment of mapping populations. A study was conducted to evaluate the effect of multiple factors on the success rate of artificial hybridization assessed by transmission of molecular markers unique to the paternal parent. Multiple peanut genotypes were crossed with a breeding line homozygous for both high oleic acid and nematode resistance. The impacts of operator, pollination time, flower integrity, genotype and environment on hybridization were evaluated. Data indicated that operator, pollination time and environment significantly affected the success rate of peanut hybridization. Pods from runner type parental plants that contain hybrid seeds were more likely to contain single seeds than those derived from self-pollination. Hybrid seed loss due to seed rot and peg damage reduced yield. Improving hybridization success rate by increasing humidity, decreasing temperature, personnel training and greenhouse management is recommended.

ABSTRACT The identification of molecular markers for economically significant traits should greatly improve the speed and efficiency of all peanut ( Arachis hypogaea L.) breeding programs. Development and phenotypic evaluation of recombinant inbred line (RIL) populations of peanut, along with molecular genotyping, will be essential for association of markers with traits. The primary objectives of this research were to develop 16 structured RIL populations that can be used by the peanut research community, and to begin high-resolution phenotyping of these populations. Crosses were made using a 2 by 8 (common by unique) factorial nested association mapping design. Parents were selected to attempt to maximize genetic diversity while meeting practical breeding objectives. First, two modern runner cultivars (Tifrunner and Florida-07) were selected as common parents because runner cultivars account for about 80% of the production in the U.S. Second, the eight unique parents were selected to supply diversity across market classes and botanical varieties and are donors of favorable alleles for enhancing drought tolerance and resistance to most important disease of peanut in the U.S. The eight unique parents are N08082olJCT (a Bailey derived high oleic breeding line), C76-16, NC 3033, SPT 06-06, SSD 6 (PI 576638), OLin, New Mexico Valencia A, and Florunner. The 16 populations were advanced using summer and winter nurseries. Input from multiple disciplines has resulted in a prioritized list of populations and traits that should be examined, and seed increase has begun to provide the community with material for extensive phenotyping. In-depth phenotyping and genotyping of these populations should result in markers that can be deployed by breeding programs for the development of improved cultivars.

ABSTRACT Peanut kernels are susceptible to colonization by some species of Aspergillus which, under conditions of drought and high temperatures, can produce aflatoxins prior to harvest. The objective of this research was to determine the mechanism by which the peanut root-knot nematode ( Meloidogyne arenaria ) increases aflatoxin contamination in peanut. Research determined 1) the role of nematode infection of roots vs. pods in increased aflatoxin contamination and 2) whether increased aflatoxin production in nematode-infected peanut is due to a greater percentage of small or immature kernels. An additional objective was to determine whether a peanut cultivar with resistance to M. arenaria would reduce the risk of preharvest aflatoxin contamination. In the greenhouse, researchers physically separated root growth from pod set and inoculated each location with M. arenaria or a water control in a 2 × 2 factorial design with 12–15 replications. Of the six trials conducted, data indicated that pod and root infection by M. arenaria was associated with elevated aflatoxin concentrations in one and three trials, respectively. This suggests that root infection by the nematode can increase aflatoxin concentrations in the peanut kernel. Another 2 × 2 factorial experiment was conducted with two peanut genotypes (Tifguard and TifGP-2) and two nematode treatments (with and without M. arenaria ) with six replications. The cultivar Tifguard is resistant to M. arenaria and TifGP-2 is susceptible. The experiment was carried out in 24 field microplots equipped with a rainout shelter. The experiment was conducted five times from 2006 to 2010. Infection of TifGP-2 by M. arenaria did not lead to greater percentages of small kernels. In only one year (2007), nematodes appeared to increase the percentage of damaged kernels, though aflatoxin concentrations were not affected by nematodes in that year. In the rainout shelter experiment, 2006 was the only year where nematode infection of peanut increased aflatoxin concentrations. In that year, there were lower aflatoxin concentrations in the nematode-resistant cultivar Tifguard than the susceptible germplasm TifGP-2 (12 vs. 136 ng/g).

In 2001, entries from the peanut core collection, a subset of the USDA peanut germplasm collection, were planted in non-replicated plots in a field with a history of Sclerotinia blight caused by Sclerotinia minor . Variability existed among entries for reaction to Sclerotinia blight. Of the 744 entries evaluated, 11% had no disease, nearly 30% had <10% disease incidence, and only 21% had 50% disease incidence or more. Most of the resistant entries had an upright growth habit and were in early and mid-maturity groups. Many of the early maturing entries were susceptible to the foliar disease pepper spot which occurred throughout the study. Entries were selected for further evaluation in replicated plots based on a nil to low (<10%) incidence of Sclerotinia blight, adaptation and/or vigor, and other desirable characteristics such as an intermediate to prostrate growth habit and pepper spot resistance. Selected entries were retested in both 2002 and 2003 (n = 62) and compared to resistant (Tamspan 90), moderately resistant (Tamrun 96), and susceptible (Okrun) reference cultivars. Most entries (55 in 2001 and 46 in 2003) had disease incidence less than Tamrun 96 and similar to Tamspan 90. In 2003 when disease incidence was highest, all 46 entries with resistant reactions similar to that of Tamspan 90 had erect plant growth habits except for entries 208 and 582 which were prostrate, and entries 273, 128, and 804 which were intermediate. Resistance to Sclerotinia blight and yield similar to Tamspan 90, plant habit, and/or reactions to pepper spot and web blotch were used to select the best entries. Entries 208, 128, 804, 582, and 273 combined resistance to Sclerotinia blight, pepper spot, and web blotch with less than erect growth habits. Entry 103 had good Sclerotinia blight resistance and yield, but an upright growth habit. Entry 92 had an upright growth habit and low yield, but good Sclerotinia blight resistance. Entries 92 and 103 had upright growth habits but were among the best entries for resistant to pepper spot and web blotch. Entries 426, 184, and 562 were upright and susceptible to pepper spot, but had resistance to web blotch and the best resistance to Sclerotinia blight. These entries appear to be useful sources of resistance to Sclerotinia blight for breeding programs and for increasing the probability of finding additional sources of resistance in clusters of germplasm identified within the entire USDA collection.

The usefulness of core collections of germplasm collections has been well established. The U.S. germplasm collection for peanuts was selectively reduced based on morphological characteristics to a mini core or “Core of the Core” collection composed of 112 of the 7432 accessions in the whole collection to make it more efficient for study. Of these samples, 108 were available from one location in the same year and were therefore exposed to one set of environmental conditions wherein genetic variability could also be examined. These samples were analyzed for total and individual amino acid content, fatty acid content, tocopherols, and folic acid content. These data provide a starting point for establishing nutrient composition within these accessions and provide an early indication of currently important characteristics in these lines which might be suited for use in random breeding initiatives.

Genomic spcience offers new research tools to explore the function of genes and their effects on plants and animals. Arachis hypogaea is a polyploid species of relatively recent origin and molecular analyses with technologies available in the 1980s and 1990s resulted in little progress in the cultivated species because of apparent lack of molecular variation. Large numbers of polymorphisms existing in wild Arachis species led to evolutionary and gene introgression studies. High throughput genomic sequencing technologies have greatly expanded the possibilities for investigating gene function, but techniques are sufficiently expensive that most federal funding has been directed toward model species and ‘major’ crops. Peanut has lagged behind many other crops, but the number of researchers working on the species in the U.S. and internationally has greatly increased during recent years. In an effort to bring researchers who work with a number of legume crops together to discuss common goals, a national strategic planning workshop was held in 2001 which led to the U.S. Legume Crops Genomics Initiative. A second workshop was held in 2004 to develop a plan with specific objectives for cross-legume genomics research and to outline milestones for accomplishments. Specifically for peanut, a genomics strategic planning workshop was organized at Atlanta in 2004 by the American Peanut Council. A broad view of genomic science was adopted and goals were set by participants to include (a) improving the utility of genetic tools for peanut genomics research, (b) improving the efficacy of technology for gene manipulation in genomics, (c) developing a framework for assembling the peanut genetic blueprint, (d) improving knowledge of gene identification and regulation, and (e) providing bioinformatic management of peanut biological information. Teams of researchers, including molecular biologists, plant breeders, pathologists, and many other disciplines need to be developed to fully utilize the potential of genomics for peanut improvement.

Identification and utilization of peanut cultivars with resistance to Cylindrocladium black rot (CBR) is a desirable approach to manage this disease. The objectives of this study were to improve greenhouse and field screening techniques for resistance to CBR, and to evaluate the reaction of selected runner-type peanut genotypes. Georgia-02C (moderately resistant to CBR) and C-99R (CBR-susceptible) were used in comparing the effectiveness of different inoculation methods in the greenhouse. Disease development was affected by both size and density of microsclerotia in soil. Use of microsclerotia at a size of ≥150 to <250 µm and a density of 1 to 5 microsclerotia/g soil provided the best separation the CBR-resistant cultivar Georgia-02C and the susceptible C-99R based on root rot severity. Genotypes with varying resistance to CBR were evaluated by growth in a naturally infested field, and by inoculating plants in the field and greenhouse. Disease incidence and severity at harvest were the most effective parameters for evaluating CBR resistance in the field and greenhouse, respectively. The cultivars Georgia-02C and Georganic had the lowest disease incidence, whereas C-99R and DP-1 had the highest disease incidence in a naturally infested field in 2005 and 2006. Incidence of CBR was moderate for Georgia-01R in both years, but was inconsistent for C34-24-85. Georgia-02C and Georganic also showed partial resistance to CBR in greenhouse tests. Inoculated plants in the field had similar reaction with Georgia-02C and Georganic showing higher CBR resistance than C-99R and DP-1 in both 2006 and 2007. The root rot severities for genotypes Georgia-02C and Georganic were lower than those for C-99R and DP-1. Incidence of CBR in the naturally infested field was significantly correlated with CBR incidence in the inoculated plants in the field (r = 0.84, P ≤ 0.01), but neither was correlated with disease ratings for greenhouse experiments. Peanut genotypes are most reliably screened by inoculating plants in the field or using uniformly infested fields. Further study is needed to improve greenhouse screening procedures.

This study assessed components of resistance for three runner-type peanut cultivars to infection by Cercospora arachidicola ( Ca ) and Cercosporidium personatum ( Cp ), the causal organisms of early leaf spot and late leaf spot, respectively. Resistance components were compared to disease resistance observed in the field. A field study monitored the progression of leaf spot incidence and severity in peanut cultivars Georgia Green, Georganic, and DP-1. Time of disease onset (TDO) and temporal epidemic rate (rate) were estimated for incidence with the logistic model, and for severity with the linear model. Early leaf spot was the predominant disease in the field. Estimates of TDO were 9 d later for DP-1 than for Georgia Green, based on incidence models, and 6 and 7 d later for Georganic and DP-1 than for Georgia Green, respectively, based on severity models. Incidence progression rate was highest for Georganic in 2002 and Georgia Green in 2003, while severity progression rate was highest for Georgia Green across years. A detached leaf assay was used to determine components of resistance for these genotypes to infections by Ca and Cp . Infection frequency 30 d after inoculation, lesion diameter, and percent necrotic area were greatest for Georgia Green for both pathogens. Besides a 2-d longer latent period for resistant genotypes, no Ca reproduction differences were detected. For Cp , latent period was shorter for Georganic than DP-1, and sporulation per unit lesion area was greatest for Georganic. Enhanced field resistance to early and late leaf spots reported for DP-1 and Georganic is in part due to lower infection frequencies, smaller lesions, and for DP-1, longer latent periods.

Tomato spotted wilt, caused by Tomato spotted wilt tospovirus (TSWV) is a major problem in peanut ( Arachis hypogaea L.) producing areas of the southeastern U.S. The integrated program used to manage spotted wilt relies heavily on cultivars with field resistance to TSWV, and finding new sources and greater levels of resistance to TSWV is highly desirable. Field tests were conducted in 2003 and 2004 in Marianna, FL and Tifton, GA to compare three peanut breeding lines, F NC94022-1-2-1-1-b3-B, C 11-2-39, and C 11-186 to that of standard moderately resistant cultivar Georgia Green for field response to TSWV. F NC94022-1-2-1-1-b3-B was of particular interest because it was developed from a cross between lines of A. hypogaea subsp. hypogaea var. hirsuta Köhler and A. hypogaea subsp. hypogaea var. hypogaea . In all tests, final spotted wilt ratings for breeding lines F NC94022-1-2-1-1-b3-B, C 11-2-39, and C 11-186 were lower and pod yields were higher than for Georgia Green. In three tests, final spotted wilt intensity ratings did not differ among F NC94022-1-2-1-1-b3-B, C 11-2-39, and C 11-186. At Marianna in 2004, spotted wilt intensity ratings were lower and pod yields were higher in F NC94022-1-2-1-1-b3-B than in any other entry. The high level of field resistance to TSWV in F NC94022-1-2-1-1-b3-B is presumably derived from its hirsuta type parent, PI 576638.

Peanut breeders are constantly in search of new sources of genes that confer tolerance or resistance to biotic and abiotic stresses to improve the production and quality. The objective of this study was to evaluate peanut lines generated from interspecific crosses for amounts of wild species introgression, including genes for resistance to peanut root-knot nematodes, tomato spotted wilt virus and leaf spot diseases. Nine diploid Arachis species were crossed with peanut breeding lines and 130 different interspecific hybrid lines were developed. These lines were evaluated for the amount of introgression using RFLP analyses, plant morphology, and disease resistant phenotypes. Based on RFLPs, 41 lines showed measurable introgression and 12 hexaploid-derived lines were polymorphic for at least four probes. Greenhouse and field evaluations indicated that resistance was not present in the lines tested for tomato spotted wilt virus, early leaf spot, or Cylindrocladium black rot. However, resistance approaching that of the wild species was found for the peanut root-knot nematode ( Meloidogyne arenaria ) among lines derived from crosses with Arachis diogoi, A. correntina, A. batizocoi , and A. cardenasii. Introgression lines were resistant (disease ratings of 1.5 to 4.5 and lesion numbers 8 to 63) compared to Southern Runner (ratings of 5.5 to 6 and lesion numbers of nearly 500) for late leaf spot ( Cercosporidium personation ) in field evaluations performed in Gainesville, FL over 2 yr. The greatest resistance was found among lines from crosses with A. batizocoi, A. duranensis, A. stenosperma, A. magma , and A. diogoi. Results indicate that it should be possible to identify molecular markers to tag resistance genes for use in conventional breeding programs and stack these genes in highly productive peanut cultivars.

An extensive working collection of Arachis germplasm is maintained by the USDA at the Southern Regional Plant Introduction Sta. in Griffin, GA. Much of this collection is maintained also under long-term seed storage at the Nat. Seed Storage Lab. in Ft. Collins, CO. The working collection consists of 9027 accessions of A. hypogaea and 684 accessions of Arachis species. About half of the A. hypogaea accessions are unimproved landraces collected in the crop's centers of diversity in South America. The other half is comprised of germplasm obtained from countries outside of South America. The U.S. germplasm collection of peanut was the first major germplasm collection to have a working core collection. Research has verified that this core collection can be used to improve the efficiency of germplasm utilization. This has stimulated a great amount of germplasm evaluation work and has resulted in the identification of numerous sources of resistance to several economically significant pathogens. Considerable efforts in the U.S. also have been devoted to the use of wild species of Arachis for sources of resistance to pathogens. Programs are ongoing to introgress high levels of resistance or immunity to early ( Cercospora arachidicola Hori) and late ( Cercosporidium personatum Berk. & M.A. Curtis) leaf spots, nematodes, and viruses. Genetic resources have been particularly useful in adding disease resistance to peanut cultivars. This has had a significant economic impact on U.S. peanut farmers. The largest impacts have been from the development of cultivars with resistance to Sclerotinia blight ( Sclerotinia minor Jagger), the peanut root-knot nematode [ Meloidogyne arenaria (Neal) Chitwood race 1], and tomato spotted wilt Tospovirus. Use of these resistant cultivars has an estimated economic impact of more that $200 million annually for U.S. peanut producers.

The U.S. maintains a large (> 8000 accessions) and genetically diverse collection of peanut ( Arachis hypogaea L.) germplasm. It is costly to screen all accessions within this collection for traits that could be useful in cultivar development. The objective of this research was to identify countries of origin that are rich sources of resistance to important peanut diseases. This would allow peanut breeders to focus their efforts on smaller subsets of the germplasm collection. Accessions in the peanut core collection were evaluated for resistance to late ( Cercosporidium personatum Berk. & M. A. Curtis) and early ( Cercospora arachidicola Hori) leaf spot, tomato spotted wilt Tospovirus (TSWV), the peanut root-knot nematode [ Meloidogyne arenaria (Neal) Chitwood race 1], and Cylindrocladium black rot (CBR)[ Cylindrocladium crotalarie (Loos) Bell & Sobers]. These data then were examined to determine if genes for resistance clustered geographically. Several geographical areas that appear to be rich sources for disease-resistant genes were identified. China had a relatively large number of accessions with resistance to the peanut root-knot nematode. Peru appeared to be a rich source of material with resistance to CBR. Resistance to late leaf spot was more frequent than expected in accessions from Bolivia and Ecuador. Bolivia was also a valuable source of resistance to early leaf spot. Early leaf spot resistance also was more prevalent than expected in accessions from India, Nigeria, and Sudan. India, Israel, and Sudan were valuable origins for material with resistance to TSWV. Accessions with multiple disease resistance were most common in India, Mozambique, and Senegal. This information should enable plant breeders to utilize more efficiently the genes for disease resistance that are available in the U.S. germplasm collection.

The genetic base of peanut ( Arachis hypogaea L.) in the U.S.A. has at times been extremely narrow, particularly in specific production areas where a single cultivar may be grown in near-monoculture. Because peanut is not a native North American species, all U.S. cultivars necessarily trace their ancestry to plant introductions (PIs), but most of the genetic base of current cultivars rests on selections from farmer-stock peanuts of obscure origin. The objectives of this study were to (a) summarize and document the use of introduced genetic resources in cultivar development and (b) estimate the resulting economic impact. Different PIs were used as parents in early breeding programs. B.B. Higgins used Gambian line Basse as a parent of the GA 207 cross that gave rise to selections used in Georgia, Florida, and North Carolina as the basis for further improvement. PI 121067 was one of seven parents used by W.C. Gregory to initiate the program in North Carolina. A different set of PIs including PI 121070, PI 161317, PI 168661, and A. monticola Krapov. & Rigoni were used in the Texas and Oklahoma programs. Recycling of lines as parents and exchange of germplasm among breeding programs proliferated these PIs in the pedigrees of cultivars released since 1960. Over the past 20 yr, there have been concerted efforts to incorporate additional germplasm into U.S. breeding populations, usually with the purpose of improving resistance to diseases or pests, but also with the objective of broadening the genetic base. These efforts have had a significant economic impact on U.S. peanut farmers, the largest from the development of cultivars with resistance to Sclerotinia blight ( Sclerotinia minor Jagger), root-knot nematodes ( Meloidogyne spp.), and tomato spotted wilt virus. Use of these resistant cultivars has an economic impact of more than $200 million annually for U.S. peanut producers. In the runner and virginia market types, the average PI ancestry of all cultivars was 17.9%. There are several examples of successful cultivars with up to 25% ancestry from a single PI, including Georgia Green and NC-V11. In the spanish market type, most successful cultivars have derived 50% or more of their ancestry from PIs. Several recent or impending releases incorporate PI germplasm but have not yet been proven in the U.S. seed market.

Peanuts become contaminated with aflatoxins when subjected to prolonged periods of heat and drought stress. The effect of drought tolerance on aflatoxin contamination is not known. The objectives of this research were to evaluate preharvest aflatoxin contamination in peanut genotypes known to have drought tolerance and to determine the correlation of drought tolerance characteristics with aflatoxin contamination. Twenty genotypes with different levels of drought tolerance were grown in Yuma, AZ (a desert environment) and under rain-protected shelters in Tifton, GA. Two drought-tolerant genotypes (PI 145681 and Tifton 8) and an intolerant genotype (PI 196754) were selected for further examination in a second experiment with two planting dates in 1997 at Tifton. Drought and heat stress conditions were imposed for the 40 d preceding harvest. The drought-intolerant genotype had greater preharvest aflatoxin contamination than Florunner (the check cultivar) in the tests conducted in 1997. Both drought-tolerant genotypes had less preharvest aflatoxin contamination than Florunner in these tests. Significant positive correlations were observed between aflatoxin contamination and leaf temperature and between aflatoxin contamination and visual stress ratings. Leaf temperature and visual stress ratings are less variable and less expensive to measure than aflatoxin contamination. Leaf temperature and visual stress ratings maybe useful in indirectly selecting for reduced aflatoxin contamination in breeding populations.

The peanut root-knot nematode ( Meloidogyne arenaria , race 1) is a world-wide pest of peanut ( Arachis hypogaea L.). Several moderately resistant genotypes have been identified in the cultivated peanut species. Our objective was to determine the expression of resistance for six of these genotypes. We examined four potential expressions of resistance—(a) fewer second-stage juveniles (J2) penetrate the roots, (b) fewer J2 establish functional feeding sites, (c) slower maturation, and (d) reduced fecundity (eggs per female). Seedlings of the susceptible cultivar Florunner and the resistant genotypes were inoculated with J2 of M. arenaria , and transplanted 3 d later to synchronize nematode development. Penetration was assessed at 3 and 10 d; development at 10 (or 12), 17, 22, and 27 d; and fecundity at 60 d after inoculation. The experiments were conducted in a greenhouse or growth chamber. The number of J2 within the roots was similar in resistant and susceptible peanut after 3 d; however, numbers were lower in two of the resistant genotypes than in Florunner after 10 d. A greater percentage of J2 failed to develop in all of the resistant genotypes (72 to 79%) than in Florunner (50%) after 17 d. Of the J2 that did begin to develop, the rate of maturation and fecundity was similar in resistant and susceptible genotypes. A lack of development indicates that the J2 failed to establish a feeding site. Therefore, the primary expression of resistance in the six peanut genotypes appears to be a reduction in the percentage of J2 that establish a functional feeding site. The decline in J2 after infection may be related to the failure to establish a feeding site.

Epidemics of tomato spotted wilt, caused by tomato spotted wilt Tospovirus (TSWV), were monitored in field plots of runner-type peanut ( Arachis hypogaea L.) cultivars Georgia Green and Georgia Runner and numerous breeding lines from four different breeding programs as part of efforts toward characterizing breeding lines with potential for release as cultivars. Breeding lines were divided into early, medium and late maturity groups. The tests were conducted near Attapulgus, GA and Marianna, FL in 1997 and in Tifton, GA and Marianna, FL in 1998. Epidemics in some early and medium maturing breeding lines, including some genotypes with high oleic acid oil chemistry, were comparable to those in Georgia Green, the cultivar most frequently used in the southeastern U.S. for suppression of spotted wilt epidemics. No early maturing breeding lines had lower spotted wilt final intensity ratings or higher yields than Georgia Green. However, spotted wilt intensity ratings in some late maturing lines and a smaller number of medium maturing lines were significantly lower than those of Georgia Green. Several of those lines also produced greater pod yields than Georgia Green. Results from these experiments indicated that there is potential for improving management of spotted wilt though development of cultivars that suppress spotted wilt epidemics more than currently available cultivars. There was no indication that differences in spotted wilt ratings corresponded to differences in numbers of thrips adults or larvae.

Tomato spotted wilt virus (TSWV) is among the greatest yield-reducing viruses affecting peanut ( Arachis hypogae a L.). The best known method of control of TSWV is through the use of resistant cultivars. Unfortunately, only a few peanut genotypes with moderate levels of resistance are known. The objectives of this study were to identify additional sources of resistance to TSWV and to determine whether plant descriptor information is associated with reaction to TSWV in the field. Peanut plant introductions from a core collection were evaluated for resistance to TSWV in field trials from 1991 to 1993. Great variability was found among PIs for reaction to natural TSWV epidemics at Attapulgus, GA. Accessions which exhibited potential resistance in 1991 or 1992 were reevaluated in subsequent years. Disease pressure was high in 1993 and 27 accessions exhibited significantly greater resistance than Florunner, and one (PI 262049) had lower disease incidence than Southern Runner. Only minor relationships existed between TSWV incidence and plant descriptor traits (growth habit and maturity) using the stepwise multiple regression procedure. Peanut accessions with resistance to TSWV were found among all maturity levels and growth habits. These accessions provide additional parents and may provide additional genes for resistance that may be useful in developing resistant cultivars.

Peanuts ( Arachis hypogaea L.) are often subjected to drought during some period in the growing season. A large root system may improve the plant's ability to continue growth during a drought. During greenhouse and field screening trials for resistance to the peanut root-knot nematode [ Meloidogyne arenaria (Neal) Chitwood, race 1], 16 peanut genotypes were observed to have very large root systems. Using these 16 genotypes plus cultivars Florunner, Southern Runner, and germplasm line Tifton 8 as checks, several studies were conducted to evaluate these genotypes for drought avoidance characteristics. In the first study, root and shoot development were observed at 15-d intervals on plants grown from seed in sand-filled pots. In a second 2-yr study, selections were grown in the field under portable rain-exclusion shelters that created controlled periods of stress. In addition, the genotypes were also planted and observed in unsheltered naturally drought stressed field plots. In the sand-filled pot study, plant inventory (PI) numbers 315628, 268885, 318740, 269106, and 314893 developed the largest root systems. In the field drought stress studies, low visual stress ratings were recorded for Southern Runner, Tifton 8, PI 295722, and PI 315628. Low canopy temperatures characterized PI 315628, Tifton 8, and PIs 295722, 259637, and 268885. When averaged over three tests, sheltered (1991 and 1992) and unsheltered (1991), Tifton 8, PI 318740, Florunner, PI 315622, and PI 315628 produced the highest yields. Two of these higher yielding genotypes (Tifton 8 and PI 315628) had low stress and temperature ratings and PI 315628 also had the largest root system measured in this study.

Peanut ( Arachis hypogaea L.) is susceptible to aflatoxin contamination when pods are developing under drought conditions in the field. The development of cultivars which resist preharvest aflatoxin contamination would be advantageous, but has been limited by the lack of genes for resistance. Several genotypes have been suggested as potential sources of resistance. Conflicting results have been reported on how useful this resistance may be, and some of these sources have never been specifically examined for resistance to preharvest aflatoxin contamination. The objective of this study was to evaluate aflatoxin contamination under drought stressed conditions in potentially resistant peanut genotypes. Twelve peanut genotypes were planted in a randomized complete-block design in field plots in Yuma, AZ in 1991 and 1992. Ten of these genotypes were also planted in a randomized complete-block design in field plots in Tifton, GA in 1992. All plots were inoculated with Aspergillus inoculum and were subjected to 40 to 50 d of drought stress immediately prior to harvest. After harvest, aflatoxin contamination (ppb) of seed was measured. None of the genotypes included in this study were more resistant (P≤0.05) to preharvest aflatoxin contamination than Florunner. The results of this study indicate that it would be desirable to identify higher levels of resistance to preharvest aflatoxin contamination in peanut.