Black shank (caused by Phytophthora nicotianae) is among the most significant causes of tobacco yield losses in the United States. Host plant resistance is the most effective and cost-efficient means of control. Some sources of resistance have been of the race-specific type that can breakdown over time. Marker-assisted identification of resistance quantitative trait loci (QTLs) associated with partial resistance would help breeders move more robust sources of resistance into elite backgrounds.

Previous work using a Hicks × Beinhart 1000 doubled haploid mapping population resulted in identification of Beinhart 1000 QTLs conferring moderate resistance to P. nicotianae. Through marker assisted backcrossing into the BC₄ and BC₆ generations, we developed nearly isogenic lines (NILs) in the genetic backgrounds of Hicks and K326 background for the two largest effect QTLs. These NILs have been evaluated for two years at two locations in black shank nurseries. The data suggest the QTLs provide moderate levels of resistance, with a synergistic effect between the two loci.

The previously characterized gene NtCPS2 involved in the synthesis of cis-abienol co-segregates with the resistance QTL on linkage group 15. A closely linked but currently unknown gene is also involved with accumulation of sucrose esters on the leaf surface. Evaluation of genetic materials suggests that the two genes act in an additive manner to affect accumulation of these leaf surface chemistries.

Further efforts are in progress to fine map these QTL using additional DNA markers, as well as to characterize their correlated effects on yield and leaf quality.