Mapping of quantitative trait loci affecting resistance to Phytophthora nicotianae in tobacco (Nicotiana tabacum L.) line Beinhart-1000

Black shank, caused by Phytophthora nicotianae, is one of the most important diseases affecting tobacco (Nicotiana tabacum L.) production worldwide. While monogenic resistance to Race 0 of this pathogen has been transferred via interspecific introgression, quantitative resistance is currently needed to provide adequate resistance to all races. The objective of this research was to gain increased insight into the inheritance of resistance in the highly-resistant cigar tobacco line, Beinhart-1000, and to identify genomic regions contributing to this resistance. A doubled haploid population was generated from a cross between Beinhart-1000 and the susceptible cultivar, Hicks. The population was evaluated for field resistance in three environments and genotyped with 206 polymorphic microsatellite markers. No doubled haploid line exhibited as high a level of resistance as Beinhart-1000. Heritability was high and multiple interval mapping suggested that at least six quantitative trait loci (QTL) may contribute to the high level of resistance in this line. The two largest QTL explained 25.4 and 20.4% of the observed phenotypic variation for end percent survival. A model including all six significant QTL explained 64.3% of the phenotypic variation and 73.1% of the genetic variation. The two major identified QTL and their associated markers may be of use for employing Beinhart-1000 as a source of black shank resistance in tobacco breeding. The major QTL on linkage group four was found to cosegregate with Abl, a gene involved in accumulation of the trichome exudate cis-abienol. The question of pleiotropy versus linkage needs to be investigated with respect to partial resistance against P. nicotianae.