Effect of a Chromosome Segment Marked by the Ph p Gene for Resistance to Phytophthora nicotianae on Reproduction of Tobacco Cyst Nematodes.

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.