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Host resistance is an important strategy for managing Globodera tabacum subsp. solanacearum and G. tabacum subsp. tabacum, important nematode pests of flue-cured tobacco (Nicotiana tabacum) in Virginia, and cigar wrapper tobacco (N. tabacum) in Connecticut and Massachusetts, respectively. Field research from 1992 to 2005 evaluated reproduction of G. tabacum subsp. solanacearum on genotypes with and without a chromosome segment from N. plumbaginifolia containing a gene (Ph p ) that conferred resistance to race 0 of Phytophthora nicotianae (causal agent of tobacco black shank). Ratios of G. tabacum subsp. solanacearum eggs/500 cm 3 soil at the end versus the beginning of experiments (P f /P i ) were significantly lower in cultivars and breeding lines possessing the Ph p -containing chromosome segment from N. plumbaginifolia compared with genotypes without the segment. Numbers of vermiform G. tabacum subsp. solanacearum juveniles in roots were similar among genotypes but numbers of swollen and pyriform nematodes were significantly lower for the known G. tabacum subsp. solanacearum resistant cv. NC 567 and in genotypes possessing the Ph p gene compared with genotypes and cultivars without the gene. In a 2003 greenhouse test, the percentage of plants with visible G. tabacum subsp. tabacum cysts was also significantly lower for parental and progeny genotypes homozygous and heterozygous, respectively, for Ph p compared with similar lines without the gene. These results indicate a close linkage or association between a likely single, dominant gene (Ph p ) for resistance to P. nicotianae and suppressed reproduction by G. tabacum subsp. solanacearum and G. tabacum subsp. tabacum. Further research to accurately elucidate the relationships among these genes could lead to significant improvements in tobacco disease control.

Bulked segregant (BSA) and random amplified polymorphic DNA (RAPD) analyses were used to identify markers linked to the dominant black shank resistance gene, Ph, from flue-cured tobacco (Nicotiana tabacum) cv. Coker 371-Gold. Sixty RAPD markers, 54 in coupling and 6 in repulsion phase linkage to Ph, were identified in a K 326-derived BC 1 F 1 (K 326-BC 1 F 1 ) doubled haploid (DH) population. Thirty RAPD markers, 26 in coupling and 4 in repulsion phase linkage to Ph, were used to screen 149 K 326-BC 2 F 1 haploid plants. Complete linkage between the 26 coupling phase markers and Ph was confirmed by screening 149 K 326-BC 2 F 1 DH lines produced from the haploid plants in black shank nurseries. RAPD markers OPZ-5 770 in coupling and OPZ-7 370 in repulsion phase linkage were used to select plants homozygous for the Ph gene for further backcrossing to the widely grown flue-cured cultivar K 326. Black shank disease nursery evaluation of 11 K 326-BC 4 S 1 lines and their testcross hybrids to a susceptible tester confirmed linkage between Ph and OPZ-5 770 . The results demonstrated the efficiency of marker-assisted selection for Ph using a RAPD marker linked in coupling and repulsion. Complete linkage between 26 RAPD markers and the Ph gene was confirmed in the K 326-BC 5 generation, and RAPD phenotypes were stable across generations and ploidy levels. These RAPD markers are useful in marker-assisted selection for Ph, an important black shank resistance gene in tobacco.

Flue-cured tobacco (Nicotiana tabacum) cultivar Coker 371-Gold (C 371-G) possesses a dominant gene, Ph, that confers high resistance to black shank disease, caused by race 0 of the soil-borne pathogen Phytophthora parasitica var. nicotianae. The origin of this gene is unknown. Breeding lines homozygous for the Ph gene were hybridized with NC 1071 and L8, flue-cured and burley genotypes known to possess qualitative resistance genes from Nicotiana plumbaginifolia and N. longiflora, respectively. The F 1 hybrids were out-crossed to susceptible testers and the progenies evaluated in field black shank nurseries and in greenhouse disease tests with P. parasitica var. nicotianae race 0. Results showed that Ph was allelic to Php from N. plumbaginifolia in NC 1071. Testcross populations of hybrids between burley lines homozygous for Ph and L8, possessing Phl from N. longiflora, showed that Ph and Phl integrated into the same tobacco chromosome during interspecific transfer. Nevertheless, the two loci were estimated to be 3 cM apart. Random amplified polymorphic DNA (RAPD) analyses of the testcross progenies confirmed that recombination between the two loci was occurring. Forty-eight RAPD markers linked to Ph in doubled haploid lines were used in cluster analyses with multiple accessions of N. longiflora and N. plumbaginifolia, breeding lines L8, NC 1071, and DH92-2770-40, and cultivars K 326, Hicks, and C 371-G. A cladogram or region tree confirmed the data obtained from field and greenhouse trials, that Ph, transferred from C 371-G to DH92-2770-40, and Php in NC 1071 were allelic and originated from N. plumbaginifolia.

Random amplified polymorphic DNA (RAPD) analysis was conducted to map the Rk gene in tobacco which conditions resistance to races 1 and 3 of the root-knot nematode, Meloidogyne incognita. Resistant burley tobacco genotype NC 528, containing the Rk gene, and the susceptible cultivar Ky 14 were screened with 1,500 random decamers. A low rate of genetic polymor-phism (<10%) was detected among these lines. Two populations (F 1 and F 3 ) of maternally de-rived doubled haploid (MDH) lines of burley tobacco, developed from the cross NC 528 × Ky 14, were used to map the Rk gene. NC 528, Ky 14, three Rk-resistant (Rk-R) DNA bulks, andthree Rk-susceptible (Rk-S) bulks generated from F 1 -derived MDH individuals were screenedwith the primers that amplified bands polymorphic between Rk-R and Rk-S lines. A total of 67 F1MDH lines and 59 F3MDH lines were screened with the primers that amplified bands polymorphic between Rk-R bulks and Rk-S bulks to confirm linkage between candidate markers and the Rk gene. Sixteen RAPD markers were positioned at six loci in a map 24.1 centimorgans long. Six RAPD markers, including one identified in the F3MDH population, were mapped at the Rk locus.

Black shank, caused by Phytophthora parasitica var. nicotianae, is a widespread and severe disease of tobacco throughout the southeastern United States. Partial resistance derived from the cigar tobacco cultivar Florida 301 has been the primary means of reducing losses to the disease for many years. The recently released tobacco cultivar, Coker 371-Gold (C 371-G), was found to provide an additional source of resistance to P. parasitica var. nicotianae. Although the resistance in C 371-G is being used widely by breeders, the origin and inheritance of this resistance mechanism was unknown. Two populations of doubled haploid lines derived from C 371-G were used to determine that C 371-G possesses a single, dominant gene designated Ph, which confers a very high level of resistance to race 0 of P. parasitica var. nicotianae. A greenhouse inoculation procedure was developed that provided an efficient means of screening for the presence of this resistance gene prior to selection in the field, and confirmed that Ph provides complete resistance to race 0 but no resistance to race 1 of P. parasitica var. nicotianae. Because Florida 301 resistance is effective against both races of the pathogen that occur in the major tobacco growing areas of the United States, combination of these two sources of resistance should provide enhanced protection of new tobacco cultivars to P. parasitica var. nicotianae.