Crop Science 43:1109-1113 (2003)
© 2003 Crop Science Society of America
PLANT GENETIC RESOURCES
Resistance to Tomato spotted wilt virus and Root-Knot Nematode in Peanut Interspecific Breeding Lines
C. Corley Holbrook*,a,
Patricia Timpera and
Albert K. Culbreathb
a USDA-ARS, P.O. Box 748, Coastal Plain Exp. Stn., Tifton, GA 31793
b Univ. of Georgia, Coastal Plain Exp. Stn., P.O. Box 748, Tifton, GA 31793
* Corresponding author (holbrook{at}tifton.cpes.peachnet.edu)
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ABSTRACT
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The peanut root-knot nematode [Meloidogyne arenaria (Neal) Chitwood race 1] and Tomato spotted wilt virus (TSWV), genus Tospovirus, are economically significant pathogens of peanut (Arachis hypogaea L.) in the USA. Peanut cultivars are available that have resistance to either the peanut root-knot nematode (PRN) or TSWV, but no cultivars are available that have resistance to both pathogens. The objective of this research was to identify peanut breeding lines that have resistance to both pathogens. We crossed interspecific peanut germplasm with two cultivars that are susceptible to both PRN and TSWV. Progenies were examined in a greenhouse screening system to measure resistance to PRN. Subsequently, this material was evaluated for resistance to TSWV in field plots. Ten breeding lines were identified which exhibited a reduction in nematode reproduction in comparison to the nematode susceptible check ‘Georgia Green’. These breeding lines also exhibited a reduction in incidence of TSWV in comparison to the TSWV susceptible check ‘COAN’. When grown in a field with pressure from both pathogens, these breeding lines had higher yield than cultivars with resistance to only one pathogen. This is the first report of peanut germplasm with resistance to both TSWV and the peanut root-knot nematode and the first report of resistance to TSWV in interspecific peanut germplasm.
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INTRODUCTION
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SINCE 1985, TSWV has become a major problem in peanut production areas of the USA. The disease is now common in many peanut-growing areas, including Georgia, Florida, Alabama, Texas, and North Carolina, and has become the most important disease problem for many peanut growers (Culbreath et al., 1997a). The primary management tool for TSWV is cultivar selection (Culbreath et al., 1999a). ‘Southern Runner’ (Gorbet et al., 1987) was the first peanut cultivar observed to have a moderate level of resistance to TSWV (Culbreath et al., 1992). Intensive screening programs have resulted in the release of several other TSWV resistant cultivars, including Georgia Browne (Branch, 1994), Georgia Green (Branch, 1996), and Florida MDR 98 (Gorbet and Shokes, 2000a) all of which have a level of TSWV resistance similar to Southern Runner (Culbreath et al., 1994; 1996; 1997b). The multiple disease resistant cultivar C99-R (Gorbet and Shokes, 2000b) was found to have similar incidence of TSWV as Georgia Green when incidence was relatively low, but had lower incidence of TSWV when disease pressure was greater (Wells et al., 2002). The soon to be released cultivars Hull (F89/0L28-H01-7-4-1-2-b3-B) and DP-1 (F86/43-1-1-1-1-1-b2-B) and several breeding lines have higher levels of field resistance to TSWV than Georgia Green or C-99R (Culbreath et al., 1999b).
The peanut root-knot nematode is also an important pathogen in many peanut production areas of the world. In the USA, this nematode is a significant pathogen in peanut fields in Georgia (Motsinger et al., 1976), Texas (Wheeler and Starr, 1987), Alabama (Ingram and Rodriguez-Kabana, 1980), and North Carolina (Schmitt and Barker, 1988). The development of resistant cultivars will reduce yield losses and pesticide inputs. Only moderate levels of resistance have been observed in the naturally occurring germplasm of A. hypogaea (Holbrook and Noe, 1992; Holbrook et al., 1996, 2000a,b). However, very high levels of resistance to M. arenaria exist in related Arachis spp. (Baltensperger et al., 1986; Nelson et al., 1989; Holbrook and Noe, 1990). This resistance has been introgressed into A. hypogaea. Stalker et al. (1995) introgressed nematode resistance into A. hypogaea (2n = 4x = 40) from A. cardenasii Krapov. and W.C. Gregory (2n = 2x = 20). Restoring fertility of triploid F1 hybrids was accomplished by creating hexaploids and then selfing progenies until 40-chromosome progenies were identified. Garcia et al. (1996) reported that this resistance was conditioned by two dominant genes where one gene (Mag) inhibited root galling and another gene (Mae) inhibited egg production by M. arenaria. Resistance to M. arenaria also has been introgressed into A. hypogaea by means of a complex interspecific hybrid from the three nematode-resistant species, A. batizocoi Krapov. and W.C. Gregory, A. cardenasii, and A. diogoi Hoehne (Simpson, 1991). This work resulted in the release of two highly resistant germplasm lines, TxAG-6 and TxAG-7 (Simpson et al., 1993).
A backcross breeding program was then used to introgress the root-knot nematode resistance from TxAG-7 into peanut breeding populations (Starr et al., 1995). This work resulted in the release of COAN, the first peanut cultivar with resistance to M. arenaria (Simpson and Starr, 2001). The yield potential of COAN was found to be less than that of its recurrent parent, ‘Florunner’ (Starr et al., 1998), but Church et al. (2000) observed significantly higher yield potential in nematode resistant breeding lines that resulted from two additional backcross generations.
Peanut cultivars are not available that have resistance to both TSWV and the root-knot nematode. The objective of this research was to identify peanut breeding lines that have resistance to both pathogens.
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MATERIALS AND METHODS
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Plants from a population of segregating interspecific genetic material were crossed with the cultivars Florunner (cross 204) and MARC I (cross 209). This interspecific material was obtained from North Carolina State University and originated from a cross of A. hypogaea (PI 261942) with A. cardenasii (GKP 10017, PI 262141). Populations from two F1 plants from cross 204 and 10 F1 plants from cross 209 were advanced by single seed descent for two generations. Eight hundred ninety-four individual F4 plants were harvested.
This material was tested for resistance to M. arenaria by means of the greenhouse screening technique described by Holbrook et al. (1983) with three replications. Plants were grown in steam-pasteurized loamy sand (850 g kg-1 sand, 110 g kg-1 silt, 40 g kg-1 clay). Each pot was inoculated with 3500 eggs of M. arenaria race 1 (Gibbs isolate) which had been cultured alternately on tomato (Lycopersicon esculentum Mill. cv. Rutgers) and peanut to reduce potential contamination from M. incognita (a parasite of tomato but not peanut). Nematode inoculum was prepared by the NaOCl method (Hussey and Barker, 1973) and applied 10 d after planting.
Plants were uprooted and washed clean of soil 68 d after inoculation. The roots were placed in 1000-mL beakers containing 300 mL of 0.05% (v/v) phloxine B solution for 3 to 5 min (Daykin and Hussey, 1985). Each plant was indexed for root galls and egg masses on the basis of the following scale: 0 = no galls or no egg masses, 1 = 1 to 2, 2 = 3 to 10, 3 = 11 to 30, 4 = 31 to 100, and 5 = more than 100 galls or egg masses per root system (Taylor and Sasser, 1978).
The 894 F4:5 lines were also planted in single replicate plots at the Gibbs farm in Tift County, GA, in 1999. Plots consisted of two rows 0.8 m apart and 3 m long. Entries were planted at 13 seeds m-1. TSWV intensity was evaluated in each plot by a disease intensity rating that represents a combination of incidence and severity as described by Culbreath et al. (1997b). The number of 0.3-m portions of row containing severely stunted, chlorotic, wilted, or dead plants was counted and converted to a percentage of row length for comparison of genotypes.
These initial evaluations for resistance to the peanut root-knot nematode and TSWV indicated that this material contained genotypes that possess resistance to both pathogens. Fifteen genotypes were selected for further study. These breeding lines, along with the cultivars Georgia Green and COAN were tested for resistance to M. arenaria by the greenhouse technique described above with 10 replications. Three separate greenhouse studies using complete and incomplete sets of the same genotypes were conducted to further quantify nematode reproduction. Genotypes were planted in completely randomized block designs with eight or nine replications. Germinated seedlings were inoculated with 8100 eggs 14 d after planting. Plants were uprooted 56 d after inoculation. Entire root systems were blotted dry and weighed, and nematode eggs were extracted with 1.0% (v/v) NaOCl and counted. Data for individual plants were converted to egg per gram fresh root before analysis.
The same genotypes were also planted on 8 May 2000 and 14 May 2001 in fields with little or no M. arenaria at the Gibbs Farm [Tifton loamy sand (fine, loamy, siliceous, thermic Plinthic Kandindult)] in Tift County, GA. Each test was planted in a randomized complete block design with three replications. Plots consisted of two rows 0.8 m apart and 4.6 m long. Entries were planted at 20 seeds m-1. Plots were managed throughout the growing season by standard grower practices and were irrigated as needed. TSWV intensity was evaluated in each plot by the disease intensity rating as previously described. Plots were dug 21 Sept. 2000 and 26 Sept. 2001. The crop was allowed to dry in the field for 6 to 8 d and pods were harvested with a small plot thresher. Pods were dried with forced air (35°C) until seed moisture reached about 80 g kg-1.
These genotypes were also tested by a similar experimental design in a field heavily infested with M. arenaria at the Bowen Farm [Ocilla loamy coarse sand (loamy, siliceous, thermic Aquic Arenic Paleududults)] in Tift County, GA. Plots were planted on 19 May 2000 and dug on 30 Sept. With the exception of no nematicide usage, plots were managed throughout the growing season by standard grower practices and were irrigated as needed.
Data were subjected to analysis of variance and genotypic means were compared by Fisher's protected least significant difference (LSD). Unless otherwise stated, differences referred to in the text were significant at P 
0.05.
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RESULTS AND DISCUSSION
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Twelve of the 15 selected breeding lines showed a reduction in galling and egg mass ratings compared to the nematode susceptible check, Georgia Green (Table 1). However, all of these genotypes showed greater galling and egg mass ratings in comparison to the nematode resistant check, COAN.
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Table 1. Root galling and Meloidogyne arenaria egg-mass ratings on selected peanut genotypes when tested in the greenhouse- trial 1.
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Nematode eggs per gram of fresh root was measured for all breeding lines in the 4th greenhouse trial, and for incomplete sets of these breeding lines in the 2nd and 3rd greenhouse trials (Table 2). Ten breeding lines showed consistent reductions in eggs per gram fresh root in comparison to Georgia Green when tested in at least two separate greenhouse trials. Two of the lines that did not support lower nematode reproduction were among the three lines that did not show reduced galling and egg mass ratings (Table 1). On the basis of nematode eggs per gram of fresh root, none of the breeding lines appear to have a level of resistance as high as the level of resistance in COAN (Table 2).
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Table 2. Meloidogyne arenaria reproduction on selected peanut genotypes when tested in the greenhouse trials 2, 3, and 4.
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Only moderate levels of resistance to M. arenaria have been reported in A. hypogaea (Holbrook et al., 2000a; b), whereas very high levels of resistance have been observed in wild species (Baltensperger et al., 1986; Nelson et al., 1989; Holbrook and Noe, 1990) and interspecific material (Simpson et al., 1993; Stalker et al., 2002). Resistance to M. arenaria derived from A. cardenasii has been reported to be controlled by simple dominant inheritance of one or two genes with major effects (Garcia et al., 1996; Choi et al., 1999). The breeding lines in the present study have moderate levels of nematode resistance, and all lines are more susceptible than COAN. Nevertheless, the resistance in these breeding lines is likely to have originated from A. cardenasii because the other parent in the original interspecific cross is highly susceptible to M. arenaria (Garcia et al., 1996). They also observed progeny from interspecific crosses with A. cardenasii that exhibited moderate levels of resistance to M. arenaria. These observations indicate that other genes may be involved in the inheritance of nematode resistance in interspecific peanut material.
TSWV pressure was much greater in 2000 than in 2001, and there was a genotype x year interaction (Table 3). In 2000, all breeding lines exhibited a significant reduction in incidence of TSWV in comparison to the TSWV susceptible check, COAN. Twelve of these lines exhibited a reduction in incidence of TSWV in comparison to the moderately resistant TSWV check, Georgia Green. Fewer differences were observed in 2001. However, all breeding lines had less TSWV incidence in comparison to COAN in both years.
The origin of the genes for resistance to TSWV in these breeding lines is not clear. Florunner and MARC I are both highly susceptible to TSWV (Culbreath et al., 1994); therefore, it is likely that the gene(s) for TSWV resistance came from the interspecific parent. PI 261942 is an accession included in the U.S. peanut core collection (Holbrook et al., 1993). The core collection was examined for sources of resistance to TSWV, and this accession was found to be highly susceptible (Anderson et al., 1996). The gene(s) for TSWV resistance likely came from the A. cardenasii accession. However, transgressive segregation for resistance to TSWV is not uncommon, particularly in the heavy selection pressure that is prevalent in peanut breeding nurseries in the southeastern USA.
The difference in TSWV pressure between years was also reflected in lower pod yield for each genotype in 2000; however, the genotype x year interaction effects were not significant (Table 4). All breeding lines exhibited higher yield in comparison to COAN. No difference in yield were observed between the breeding lines and Georgia Green when averaged over both years.
This is the first report of peanut germplasm with resistance to both TSWV and the peanut root-knot nematode. In the peanut production region of the southeastern USA, peanut in fields with the peanut root-knot nematode also experience pressure from TSWV. In such a situation, the yield of currently available virus resistant cultivars will be reduced by nematode pressure, and the yield of currently available nematode resistant cultivars will be reduced by TSWV. When grown in a field with pressure from both pathogens, 13 of these breeding lines had higher yield than Georgia Green and COAN (Table 5).
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Table 5. Mean yield and final intensity of TSWV of selected peanut genotypes when grown at a location heavily infested with M. arenaria.
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ACKNOWLEDGMENTS
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We thank Dr. H. Thomas Stalker at North Carolina State University for providing seeds of the original interspecific genetic material. The contributions and technical support of Shannon Atkinson, David Clements, Vickie Griffin, Dannie Mauldin, Adam K. Montford, Betty Tyler, and William Wilson are greatefully acknowledged. This work was supported in part by the Georgia Peanut Commission.
Received for publication June 20, 2002.
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ABSTRACT
INTRODUCTION
