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Crop Science 43:1305-1312 (2003)
© 2003 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Inheritance of Resistance to Tobacco Cyst Nematode in Flue-Cured Tobacco

Barbara J. Crowdera, Carol A. Wilkinson*,a, Charles S. Johnsonb and Jon D. Eisenbachc

a Dep. of Crop and Soil Environmental Science, Southern Piedmont Agricultural Research and Extension Center, 2375 Darvills Road, Blackstone, VA 23824
b Dep. of Plant Pathology, Physiology, & Weed Science, Southern Piedmont Agricultural Research and Extension Center, 2375 Darvills Road, Blackstone, VA 23824
c Dep. of Plant Pathology, Physiology, & Weed Science, Virginia Polytechnic Institute and State Univ., 103 Price Hall, Blacksburg, VA 24061

* Corresponding author (wilki{at}vt.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The tobacco cyst nematode [Globodera tabacum solanacearum (Miller and Gray) Behrens] is an important pathogen affecting flue-cured tobacco (Nicotiana tabacum L.) in Virginia and North Carolina and type 32 tobacco in Maryland. The objective of this study was to determine the mode of inheritance of resistance to G. t. solanacearum in the flue-cured tobacco cultivars Coker 371 Gold and Kutsaga 110. Each cultivar was crossed to the susceptible cultivar K 326 and F1 progeny were backcrossed to each parent. Plants from each parent and F1, F2, BC1Ps, and BC1Pr progeny were evaluated for G. t. solanacearum resistance in the greenhouse. Six-week-old transplants were inoculated with 6000 G. t. solanacearum eggs from crushed cysts. Eight weeks after inoculation, a 1-g sample of fibrous root was stained and vermiform, swollen, pyriform, and adult nematodes were counted. The number of cysts and eggs per 400 000 mm3 of soil were counted from each transplant. Generation means analyses were performed. Additive and dominance gene action play an important role in resistance to G. t. solanacearum in Coker 371 Gold and Kutsaga 110. F2 progeny data from the Coker 371 Gold cross fit a 3:1 (resistant:susceptible) segregation ratio and BC1Ps generation data fit a 1:1 segregation ratio, indicating that resistance to G. t. solanacearum is conferred by a single dominant gene. A continuous range of variation was observed among the F2 progeny for the K 326 x Kutsaga 110 cross, indicating resistance in Kutsaga 110 is quantitative. Globodera tabacum solanacearum resistance in Coker 371 Gold and Kutsaga 110 may be derived from different sources.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
THE TOBACCO CYST NEMATODE, Globodera tabacum solanacearum (Miller and Gray, 1972; Behrens, 1975; Stone, 1983), is a serious soil-borne pest of flue-cured tobacco in the Southern Piedmont region of Virginia and some areas of North Carolina and type 32 tobacco in Maryland. Three subspecies of Globodera tabacum (Lownsbery and Lownsbery, 1954; Behrens, 1975) have been described: G. t. tabacum (Lownsbery and Lownsbery, 1954; Behrens, 1975; Stone, 1983), G. t. virginiae (Miller and Gray, 1968; Behrens, 1975; Stone 1983), and G. t. solanacearum (Stone, 1983). Globodera tabacum tabacum parasitizes Connecticut shade tobacco and occurs in New York, Massachusetts, and Connecticut (Miller and Gray, 1972). Globodera tabacum virginiae occurs in Virginia (Miller and Gray, 1972) and North Carolina (Johnson, 1998) and primarily parasitizes common horsenettle (Solanum carolinense L.). Globodera tabacum solanacearum reproduces on most commercially grown flue-cured tobacco cultivars, certain cultivars of other tobacco classes, horsenettle, and some cultivars of tomato (Lycopersicon spp. L.), eggplant (Solanum melongena L.), and sweet peppers (Capsicum spp. L.) (Harrison and Miller, 1969; Adams et al., 1982).

Approximately one-third of all flue-cured tobacco acreage in Virginia is infested with G. t. solanacearum, costing farmers an estimated $5 000 000 annually in crop losses and pesticide expenses (C.S. Johnson, unpublished data, 2000). Fields infested with G. t. solanacearum average 15% yield reduction annually (Komm et al., 1983). The primary G. t. solanacearum management tactic is the application of nematicides at an average cost of $250/ha (C.S. Johnson, unpublished data, 2000). These nematicides are highly toxic and limited in their availability. Other control measures for G. t. solanacearum include crop rotation, sanitation, and planting resistant cultivars (Reed et al., 2001).

Resistant cultivars can provide an effective means of controlling nematode populations in infested fields. There has been limited success in finding or developing tobacco cultivars that are both resistant to G. t. solanacearum and have acceptable agronomic quality. The mode of inheritance of resistance to G. t. solanacearum is poorly understood, which makes breeding for resistance difficult. Resistance to G. t. solanacearum in dark fire-cured (DVA 606) and burley (BVA 523) tobacco breeding lines has been reported to be multigenic (Spasoff et al., 1971; Miller et al., 1972). No information on the pedigree of these breeding lines is available. A diallel analysis of eight tobacco accessions demonstrated that general combining ability (GCA) effects accounted for the majority of variation observed among crosses (Hayes et al., 1995) and suggested that additive gene action plays a significant role in the inheritance of resistance to G. t. solanacearum. ‘Burley 64’, ‘Kutsaga 110’, and ‘Bright Cospaia MI 22528’ (TI 1597) significantly contributed to increased resistance to G. t. solanacearum on the basis of GCA effects. Specific combining ability (SCA) effects, which correspond to nonadditive gene action, were not significant in this study (Hayes et al., 1995). LaMondia (1988) demonstrated that the flue-cured tobacco breeding line VA 81 and the flue-cured cultivar PD 4 were resistant to G. t. tabacum. Subsequent studies determined that resistance to G. t. tabacum in VA 81, PD 4, ‘Burley 21’, and ‘Burley 49’ is conferred by a single dominant gene (LaMondia, 1991; LaMondia, 2002). This finding differs from research conducted with G. t. solanacearum (Spasoff et al., 1971; Miller et al., 1972) that suggests inheritance of resistance is quantitative and not qualitative.

Nematode reproductive parameters have been used to evaluate accessions for G. t. solanacearum resistance. Early sources of G. t. solanacearum resistance were reported in N. longiflora Cavanilles, N. glutinosa L., and N. plumbaginifolia Viviani (Baalawy and Fox, 1971). Herrero et al. (1996) observed low levels of G. t. solanacearum reproduction in the accessions Burley 21, ‘Speight G-80’, ‘NC 567’, ‘Kutsaga Mammoth 10’, Cyst 913, 9025-1, PD 4, Kutsaga 110, and VA 81. Hayes et al. (1997) identified 21 accessions resistant to G. t. solanacearum including previously unreported resistance in N. miersii Remy, N. cordifolia Philippi, ‘TN 90’, Burley 49, ‘Burley 64’, ‘MD 40’, ‘Pennbell 69’, ‘Pennlan’, and Bright Cospaia MI 22528.

Genetic linkage between wildfire [caused by Pseudomonas syringae pv. tabaci (Wolf & Foster) Young et al.] resistance derived from N. longiflora and G. t. solanacearum resistance has been reported (Spasoff et al., 1971; Komm and Terrill, 1982). Nicotiana longiflora supports only limited reproduction of G. t. solanacearum (Baalawy and Fox, 1971; Hayes et al., 1997). Hayes et al. (1997) observed a high correlation between wildfire and G. t. solanacearum resistance. A linkage between resistances to these two pathogens was not evident in some accessions. ‘Itztepeque’ (TI 551) and ‘KY 190’ were resistant to wildfire and susceptible to G. t. solanacearum, whereas N. miersii was resistant to G. t. solanacearum and susceptible to wildfire. This situation suggests the genes responsible for resistance to these two pathogens may not be closely linked. Spasoff et al. (1971) and Gwynn et al. (1996) also observed a linkage break between G. t. solanacearum and wildfire resistance among F3 lines.

An association between resistance to G. t. solanacearum and the Ph gene for black shank [Phytophthora parasitica Dastur var. nicotianae (Breda de Haan) Tucker syn. P. nicotianae Breda de Haan var. nicotianae G.M. Waterhouse] resistance has also been detected (Johnson, 2001). Flue-cured tobacco cultivar Coker 371 Gold has a high level of resistance to race 0 of P. parasitica var. nicotianae which originated from N. plumbaginifolia (Johnson et al., 2002). Nicotiana plumbaginifolia has also been reported as a source of resistance to G. t. solanacearum (Baalawy and Fox, 1971; Hayes et al., 1997). Several other flue-cured tobacco cultivars with Coker 371 Gold in their pedigree have been released in recent years. Planting Coker 371 Gold, ‘NC 71’, ‘NC 72’, ‘NC 297’, ‘RG H51’, ‘Speight 168’, and ‘Speight 179’ are recommended to growers because these cultivars reduce G. t. solanacearum population densities (Reed et al., 2001). A nematicide is also advised when these cultivars are planted in highly infested fields to ensure acceptable yield and quality.

PD 4 has been used as the conventional source of G. t. solanacearum resistance in flue-cured tobacco breeding programs for many years. PD 4 has inferior agronomic quality compared with commercially available susceptible cultivars. New sources of resistance to G. t. solanacearum have recently been identified (Herrero et al., 1996; Hayes et al., 1997; Johnson, 2001) and the genetics of resistance in these new sources will influence their use in future breeding programs. The objective of this study was to determine the mode of inheritance of resistance to G. t. solanacearum in the flue-cured tobacco cultivars Coker 371 Gold and Kutsaga 110.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Greenhouse experiments were conducted at the Southern Piedmont Agricultural Research and Extension Center (SPAREC) in Blackstone, VA, in 1999 and 2000. Crosses were made between the G. t. solanacearum susceptible commercial flue-cured tobacco cultivar K 326 and two resistant cultivars, Coker 371 Gold and Kutsaga 110, to produce F1, F2, BC1Ps (susceptible parent x F1), and BC1Pr (resistant parent x F1) seed. Coker 371 Gold (PI 552524) was developed in the USA and released in 1988. It has a high level of resistance to race 0 of P. parasitica var. nicotianae and also suppresses G. t. solanacearum reproduction (Johnson, 2001). Resistance to G. t. solanacearum has also been observed in Kutsaga 110 (Herrero et al., 1996; Hayes et al., 1997), which is resistant to P. syringae pv. tabaci. Kutsaga 110 was developed in Zimbabwe and is not grown commercially in the USA. It tends to be lower yielding and to sucker more than the typical cultivars grown in the USA.

Each test was conducted twice as a randomized complete block design with 15 replications. One replication consisted of one Ps (susceptible parent), one Pr (resistant parent), one F1, seven F2, seven BC1Ps, and seven BC1Pr plants for a total of 15 plants for nonsegregating generations and 105 plants for segregating generations evaluated in each test. Four-week-old seedlings were transplanted into 110-mm clay pots containing 300000 mm3 of a 2:1 sterilized fine quartz sand:sandy loam soil (840 g kg-1 sand, 100 g kg-1 silt, 60 g kg-1 clay) mix. A piece of No. 2 Whatman filter paper was placed in the bottom of each pot to retain the soil in the pot. Transplants were grown for 2 wk before inoculation. Globodera tabacum solanacearum cysts were extracted from infested field soil collected from the B.J. Coffee farm, Kenbridge, VA, and SPAREC, Blackstone, VA, with a modified Fenwick can (Caswell et al., 1985). Cysts were dried on filter paper and stored in glass vials at room temperature before inoculation. Globodera tabacum solanacearum eggs were obtained by crushing cysts in a blender at high speed for 60 s. Each test pot was inoculated with 6000 tobacco cyst nematode eggs and juveniles by pipetting the egg suspension into a trench around the root zone of a single tobacco plant. An additional 100 000 mm3 of soil was added to each pot after inoculation to cover exposed eggs and encourage additional root growth (Hayes et al., 1997).

Drip irrigation was used in the first test for each cross while subirrigation was used in the second test. Plants were watered daily with an automatic watering system in the drip-irrigated test. Fertilizer was applied once a week with an injector at a rate of 125 mg/kg N of 20-10-20. The injector was set to deliver 1 L of nutrient solution per 64 L of water. The automatic watering system was set up to deliver water/nutrient for 1 min/d and was increased to 2 min/d as the plants matured. Plants in the subirrigated tests were fertilized as described above. Drip irrigation is recommended for future cyst nematode studies because of greater nematode reproduction as reflected by the mean nematode counts (Table 1). Greenhouse temperatures were maintained at approximately 24°C during the day and 18°C during the night.


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  		Table 1. Means from two tests for tobacco cyst nematode resistance based on the sum of swollen, pyriform, and adult nematodes per 1-g root sample, number of cysts per 400000 mm3 of soil, and number of eggs per 400000 mm3 of soil for tobacco crosses K 326 x Coker 371 Gold and K 326 x Kutsaga 110.

 
Plants were assessed for G. t. solanacearum resistance approximately 8 wk after inoculation. The shoot of each plant was removed and discarded. The root ball was removed from each pot and rinsed over a 38-µm-pore sieve with a stream of tap water. The contents of the sieve were rinsed into an Erlenmeyer flask. The root ball of each plant was weighed and a 1-g sample of fibrous root was randomly collected. The feeder root subsample and root wash materials were combined and stained with acid fuchsin (Byrd et al., 1983). The number of vermiform, swollen, pyriform, and adult nematodes were counted by means of a dissecting microscope. Cysts were extracted from the remaining soil in pots with a modified Fenwick can (Caswell et al., 1985). The number of cysts per pot were counted by means of a dissecting microscope and crushed in a blender for 60 s to release the eggs. Eggs were suspended in tap water and stained with acid fuchsin. The numbers of G. t. solanacearum eggs in two 10-mL aliquots were counted for each sample and averaged to estimate the total number of eggs per pot. The numbers of cysts per pot and eggs per pot were standardized and expressed as number of cysts per 400 000 mm3 of soil and number of eggs per 400 000 mm3 of soil for statistical analysis.

The sum of swollen, pyriform, and adult nematodes was calculated. Vermiform nematodes were not included in the new parameter because they invade both resistant and susceptible roots. Host resistance was evaluated by the ability of the nematode to establish a feeding site and thereby develop into an adult cyst nematode (Baalawy and Fox, 1971). The sum of swollen, pyriform, and adult nematodes was used as a measure of resistance because summing these life stages eliminates variability associated with nematode development and provides a more useful indicator of resistance. Standard deviations were proportional to the means, so the data were log transformed [log10(x+1)] before statistical analysis (SAS Institute, Inc., 1998). The numbers of vermiform, swollen, pyriform, and adult nematodes observed in the roots, number of cysts per 400 000 mm3 of soil, and number of eggs per 400 000 mm3 of soil were analyzed by an analysis of variance for each test. The GLM procedure of SAS was used to perform each analysis of variance and treatment means were compared by Duncan's multiple range test (Gomez and Gomez, 1984, p. 207–215).

Generation means analysis was performed by the SAS procedure IML (Mather and Jinks, 1982, p. 72–76; SAS Institute, Inc., 1998) to assess the inheritance of resistance to G. t. solanacearum. A three-parameter model (m, a, and d) was fitted and tested for goodness of fit by a chi-square test with three degrees of freedom. The three genetic parameters were defined as follows: m = the midparent value, a = the amount of variation among the means resulting from the additive effect of the genes, and d = the amount of variation among the means resulting from the dominance effect of the genes. A six-parameter model (m, a, d, aa, ad, and dd) was fitted if a significant chi-square value (poor fit) was obtained for the three-parameter model. The six genetic parameters were defined as follows: m = the mean of the inbred population, a and d as defined for the three parameter model, aa = the amount of variation among the means attributed to additive x additive epistasis, ad = the amount of variation among the means resulting from additive x dominance epistasis, and dd = the amount of variation among the means resulting from dominance x dominance epistasis. Standard errors of genetic estimates were compared with the estimated genetic values to determine significance. If the absolute value of an estimate was greater than twice its standard error, the estimate was considered significantly different from zero.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Error variances were homogeneous between the Coker 371 Gold tests for all parameters measured, except for the number of cysts per 400000 mm3 of soil. Therefore, a combined analysis was performed. Significant differences were detected between tests for the K 326 x Coker 371 Gold cross on the basis of egg counts only. Significant test x generation mean interactions were detected for cyst and egg counts, but were due to changes in magnitude, not to changes in genotype rank. In contrast, error variances between the Kutsaga 110 tests were heterogeneous for most parameters. Genotype ranks were similar between the Kutsaga 110 tests, allowing for a combined analysis (Herrero et al., 1996). When a combined analysis was performed for the K 326 x Kutsaga 110 cross, significant differences were detected between tests for all measured parameters. Significant test x generation mean interactions were detected for sum of swollen, pyriform, and adult counts. Interactions were the result of changes in magnitude between the generation means and not changes in rank; therefore, a combined analysis could be justified (Herrero et al., 1996).

The sum of swollen, pyriform, and adult nematodes for the resistant parent was significantly lower than the susceptible parent for the K 326 x Coker 371 Gold cross (Table 1). Although the F1 generation mean was intermediate between the two parents, it was not significantly different from the resistant parent in either test. The F2 generation mean was not significantly different from the F1 generation mean in either test. The BC1Ps generation mean was intermediate between the two parents and was significantly different from both parent generation means in both tests. The BC1Pr generation mean was also intermediate between the two parents and was not significantly different from the resistant parent generation mean in either test. The BC1Ps generation mean was more susceptible than the BC1Pr generation mean.

Similar trends were observed for cysts per 400000 mm3 of soil generation means for both tests for the K 326 x Coker 371 Gold cross (Table 1). The F2 generation mean in the drip irrigation test was significantly higher than the F1 generation mean. Egg count means per 400000 mm3 of soil also showed similar generation mean ranks in the drip irrigation test. The F1 generation mean was not significantly different from the susceptible parent mean in the subirrigation test and the F2 generation mean was not significantly different from any other generation mean in the subirrigation test.

Generation means for each parent for the sum of swollen, pyriform, and adult nematodes in the K 326 x Kutsaga 110 cross were significantly different from each other in both tests (Table 1). The F1 generation mean was intermediate between the parental means and was not significantly different from the resistant parent generation mean in either test. The F2 generation mean was not significantly different from the F1 generation mean in either test. The BC1Ps generation mean was significantly different from both parent generation means in both tests, but was not significantly different from the F2 generation mean in the drip irrigation test. The BC1Pr generation mean was not significantly different from the resistant parent generation mean in either test.

Parent generation means were significantly different from each other on the basis of cyst counts per 400000 mm3 of soil for both tests for the K 326 x Kutsaga 110 cross (Table 1). The F1 generation mean was significantly higher than the resistant parent in the drip irrigation test, but not in the subirrigation test. The F2 generation mean was significantly higher than the F1 generation mean in subirrigation test, but not in the drip irrigation test. The BC1Ps generation mean was significantly different from both parents in drip irrigation test, but not significantly different from the susceptible parent generation mean in subirrigation test. The BC1Pr generation mean was significantly different from both parents in the drip irrigation test, but not significantly different from the resistant parent generation mean in the subirrigation test. The BC1Pr generation mean was not significantly different from the F2 generation mean in either test. Egg counts per 400000 mm3 of soil showed similar generation mean ranks between tests.

The F2 progeny data were skewed toward the resistant parent with a large portion of the progeny being resistant and a smaller portion exhibiting a range of susceptibility. Progeny were classified as resistant when the number of nematodes for the measured parameter was below a threshold of 10% of the susceptible parent mean. Chi-square values below 3.84 (df = 1) were obtained for the sum of swollen, pyriform, and adult nematodes, the number of cysts per 400000 mm3 of soil, and the number of eggs per 400000 mm3 of soil, indicating acceptable fits to a 3:1 (resistant:susceptible) segregation ratio in the drip irrigation test for the K 326 x Coker 371 Gold cross (Table 2). The sum of swollen, pyriform, and adult nematodes in the subirrigation test also fit a 3:1 ratio, but the number of cysts and eggs did not. Poor fit to the 3:1 ratio for cysts and eggs is probably related to poor nematode development in the subirrigation test. BC1Ps data fit a 1:1 segregation ratio when F2 data fit the 3:1 ratio, which supports the conclusion that a single dominant gene confers G. t. solanacearum resistance in Coker 371 Gold.


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  		Table 2. Phenotypic ratios of resistant to susceptible (R:S) plants for tobacco crosses K 326 x Coker 371 Gold and K 326 x Kutsaga 110 based on the sum of swollen, pyriform, and adult nematodes, number of cysts per 400000 mm3 of soil, and number of eggs per 400000 mm3 of soil.

 
Fit to a 3:1 ratio, based on 10% of the susceptible parent mean threshold values, was rejected for the K 326 x Kutsaga 110 cross. Graphical representation of the F2 data appeared to be more continuous than F2 data for the K 326 x Coker 371 Gold cross, although data were also skewed toward the resistant parent. To obtain a good fit to the 3:1 ratio, threshold values would have to be larger and would include genotypes not considered to be resistant.

The three-parameter model was sufficient for explaining the inheritance of resistance to G. t. solanacearum for the K 326 x Coker 371 Gold cross. Mean, additive, and dominance effect estimates were significant in both tests and the combined analyses for sum of swollen, pyriform, and adult nematodes (Table 3) and the number of cysts per 400000 mm3 of soil (Table 4). Additive effects were positive and similar in magnitude to dominance effect estimates, which were all negative. Mean effect estimates were similar to additive effect estimates. Although mean, additive, and dominance effect estimates were significant in the combined analysis for number of eggs per 400000 mm3 of soil, analyses of the individual tests resulted in significant chi-square values indicating a poor fit to the three-parameter model (Table 5).


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  		Table 3. Estimates of genetic effects for tobacco cyst nematode resistance based on the nontransformed sum of swollen, pyriform, and adult counts from two flue-cured tobacco crosses fitted to a three-parameter model.

 

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  		Table 4. Estimates of genetic effects for tobacco cyst nematode resistance based on nontransformed cyst counts per 400000 mm3 of soil from two flue-cured tobacco crosses fitted to a three-parameter model.

 

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  		Table 5. Estimates of genetic effects for tobacco cyst nematode resistance based on nontransformed egg counts per 400000 mm3 of soil from two flue-cured tobacco crosses fitted to a three-parameter model.

 
The three-parameter model was sufficient for explaining the inheritance of resistance to G. t. solanacearum for the K 326 x Kutsaga 110 cross on the basis of all parameters measured except for the sum of swollen, pyriform, and adult nematodes in the subirrigation test (Tables 3, 4, and 5). The poor fit to the three-parameter model in the subirrigation test for the cross can be attributed to the poor reproduction in the test. Mean effect estimates were similar to or larger than additive effect estimates for all parameters. Additive effect estimates were all positive and significant. Dominance effect estimates were all negative and were significant in both tests for all parameters except in the second test for egg counts per 400000 mm3 of soil. Additive effect estimates were larger in magnitude than dominance effect estimates for all parameters. Additive genetic effects account for a greater portion of variation in the K 326 x Kutsaga 110 cross as compared with the K 326 x Coker 371 Gold cross.

A six-parameter model was tested when the three-parameter model was insufficient in explaining the inheritance of resistance to G. t. solanacearum. Therefore, resistance based on egg counts per 400000 mm3 of soil for the K 326 x Coker 371 Gold cross and resistance based on the sum of swollen, pyriform, and adult nematodes for the K 326 x Kutsaga 110 cross were tested for fit to a six-parameter model. Chi-square values indicate fits to the six-parameter model for both tests and the combined analyses for both crosses (Table 6).


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  		Table 6. Estimates of genetic effects for tobacco cyst nematode resistance based on nontransformed egg counts per 400000 mm3 of soil for the flue-cured tobacco cross K 326 x Coker 371 Gold and the sum of swollen, pyriform, and adult nematodes for the flue-cured tobacco cross K 326 x Kutsaga 110 fitted to a six-parameter model.

 
For the K 326 x Coker 371 Gold cross, additive gene effect estimates were positive and significant for both tests and the combined analysis. Dominance effect estimates were larger in magnitude than additive effect estimates but were not significant. Significant positive additive x dominance and negative dominance x dominance epistatic effect estimates were obtained for the subirrigation test. Both estimates were larger in magnitude than the additive effect estimate. Additive, additive x dominance, and dominance x dominance effect estimates were important in the subirrigation test for the K 326 x Coker 371 Gold cross on the basis of egg counts.

For the K 326 x Kutsaga 110 cross, additive effect estimates were positive and significant for both tests. Dominance effect estimates were negative, smaller in magnitude than additive effect estimates, and nonsignificant. A significant positive additive x additive effect estimate was obtained in the subirrigation test. However, no other significant epistatic effects were obtained.


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Inheritance of resistance to G. t. solanacearum in Coker 371 Gold and Kutsaga 110 appears to differ genetically. Both cultivars possess partial dominance for resistance on the basis of F1 generation means (Table 1). The F1 generation means were intermediate to the parental means, but deviate from the midparent value toward the resistant parent mean. F2 progeny data were skewed toward the resistant parent mean for both cultivars. The F2 progeny distribution for the K 326 x Coker 371 Gold cross fit a 3R:1S segregation ratio when the threshold for resistance was 10% of the susceptible parent (K 326) mean (Table 2). The BC1Ps data support the F2 data by fitting a 1:1 segregation ratio. These findings suggest the presence of a single dominant gene in Coker 371 Gold conditioning resistance to G. t. solanacearum. In contrast, resistance to G. t. solanacearum in Kutsaga 110 appears to be quantitative in nature. The F2 generation data were more continuous, although a large proportion of the plants were similar to the resistant parent. Using 10% of the susceptible parent mean as a threshold value for resistance was not sufficient to achieve a fit to the 3:1 segregation ratio (Table 2). The threshold limit would have to be larger than 10% of the susceptible parent mean to achieve a fit to the 3:1 ratio. Increasing this threshold would include a greater proportion of susceptible phenotypes and would not accurately portray the inheritance of resistance in Kutsaga 110.

Early research on G. t. solanacearum inheritance in BVA 523 and DVA 606 indicated resistance was multigenic because there was a continuous range among F2 progeny for the number of G. t. solanacearum females per plant (Spasoff et al., 1971; Miller et al., 1972). BVA 523, a burley breeding line in which wildfire and G. t. solanacearum resistance is apparently linked, was also evaluated for wildfire resistance (Spasoff et al., 1971). F2 progeny data suggested a single dominant gene controlled wildfire resistance in BVA 523. A single dominant gene for resistance to wildfire was transferred from N. longiflora to a breeding line, TL 106 (Clayton, 1947). Burley 21 was the first burley cultivar to be released with resistance to wildfire (Heggestad, 1966) and breeding line TL 106 is included in the pedigree of Burley 21. All commercial burley cultivars in the USA have resistance to wildfire race 0 which can be traced back to N. longiflora (Legg and Smeeton, 1999).

Resistance to G. t. solanacearum in Kutsaga 110 may be derived from N. longiflora. Kutsaga 110 is an anther-derived dihaploid from the cross TW 438 x Kutsaga 51E (Anne Jack, personal communication, 2000). TW 438 is a breeding line with resistance to wildfire and Tobacco mosaic virus. The wildfire resistance is derived from N. longiflora and incorporated via Burley 21. There are also quantitative genes in Kutsaga 110 for tolerance to wildfire derived from ‘Meadows Giant’ and ‘Kutsaga 51’, which may or may not be associated with G. t. solanacearum resistance. LaMondia (1991) determined a single dominant gene confers G. t. tabacum resistance in VA 81 and PD 4. The source of G. t. tabacum resistance in VA 81 and PD 4 most likely originates from N. longiflora. BVA 523 is in the pedigree of VA 81 and Burley 21 is one of the parents used to develop PD 4 (Currin et al., 1981).

Globodera tabacum solanacearum resistance in Coker 371 Gold is derived from a different source. Coker 371 Gold has a single dominant gene, Ph, that confers a high level of resistance to P. parasitica var. nicotianae race 0 (Carlson et al., 1997). The Ph gene originated from N. plumbaginifolia (Johnson et al., 2002). Nicotiana plumbaginifolia is also resistant to G. t. solanacearum (Baalawy and Fox, 1971; Hayes et al., 1997). Black shank resistance in Coker 371 Gold is also derived from Florida 301 quantitative genetic resistance which may or may not be associated with G. t. solanacearum resistance.

Generation means analyses showed that additive gene action plays a role in the resistance to G. t. solanacearum in Coker 371 Gold and Kutsaga 110 (Tables 3, 4, 5, and 6). Additive gene effects were more important in the inheritance of resistance to G. t. solanacearum in Kutsaga 110. Similar results were obtained by Hayes et al. (1995) who determined that additive gene action plays a significant role in the inheritance of resistance to G. t. solanacearum in Kutsaga 110. In contrast, additive and dominance gene effects were equally important in the inheritance of G. t. solanacearum in Coker 371 Gold. Dominance gene effects were negative in both the K 326 x Coker 371 Gold cross and the K 326 x Kutsaga 110 cross because resistance values are obtained by measuring in the negative direction or corresponding smaller values. Although the additive-dominance model adequately explained the gene effects in both crosses, epistatic interactions were detected in one test for eggs per 400000 mm3 of soil and in one test for the sum of swollen, pyriform, and adult nematodes.

Globodera tabacum tabacum and G. t. solanacearum are part of a species complex and are morphologically very similar to one another, differing slightly in host preference. Reports on mode of inheritance of G. t. solanacearum and G. t. tabacum resistance have been contradictory (Spasoff et al., 1971; Miller et al., 1972; LaMondia, 1991; LaMondia, 2002). Discrepancies can be due to source of resistance, variation in environmental conditions under which plants were evaluated, and criteria used for assessing resistance. Nematode reproduction is highly influenced by environmental factors, complicating the study of resistance. Reproducing results from different experiments can be very difficult because of the variability in nematode reproduction because of environmental influences (Phillips et al., 1989). The rate of development, size of cysts, and number of eggs and larvae in cysts of G. t. solanacearum vary with different soil temperatures (Adams et al., 1982). Egg hatching depends on the concentration of root exudates and temperature (Wang et al., 1997). Root exudates stimulate hatching and hatching seems to increase with increasing temperature (Wang et al., 1997). LaMondia (1995) observed hatching of G. t. tabacum on tobacco, tomato, and black nightshade (Solanum nigrum L.) roots and showed that hatching was stimulated by root exudates. The relationship between dilution of host exudates or plant age and hatch was similar, but the magnitude of hatch was greatly reduced when the experiments were repeated (LaMondia, 1995). These kinds of variability can be reflected in the range of susceptibility observed among plants.

The plants in this study were inoculated with eggs. The sum of swollen, pyriform, and adult nematodes was counted and used as an indicator of resistance to account for differential hatching among pots. LaMondia (1991) inoculated with juveniles, eliminating the variability associated with hatching and rated resistance according to the number of white females observed on the roots. Inoculating with juveniles ensures a more uniform infection of plants and also reduces the amount of time required for the nematode to complete its lifecycle, thus allowing for more rapid screening of plants for resistance.

The choice of breeding strategy for incorporating G. t. solanacearum resistance into agronomically acceptable cultivars depends on the type and magnitude of gene effects. Resistance to G. t. solanacearum appears to be inherited as a single dominant gene in Coker 371 Gold. Additive and dominance gene effects were equally important in the K 326 x Coker 371 Gold cross, indicating that inheritance of resistance may be more complex. Developing hybrids with Coker 371 Gold will allow for quick incorporation of resistance into agronomically superior lines. Additive gene effects were more important in the inheritance of resistance to G. t. solanacearum in Kutsaga 110. Therefore, a breeding program that leads to development of desirable homozygous cultivars is suggested. It may be beneficial to test for allelism between Coker 371 Gold and Kutsaga 110 for G. t. solanacearum resistance. The gene for resistance in Coker 371 Gold may have been part of a quantitative trait loci that was responsible for a major portion of the genetic variation.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Part of a thesis submitted by the senior author in partial fulfillment of the requirements for a Master of Science degree. Research supported by a grant provided by Philip Morris USA.

Received for publication September 12, 2002.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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