Genetic resistance to white pine blister rust, restoration options, and potential use of biotechnology

All nine five-needle white pine species (genus Pinus, subgenus Strobus, subsections Strobus and Balfourianae) native to the U.S. and Canada are highly susceptible to white pine blister rust (WPBR), caused by the non-native fungal pathogen Cronartium ribicola. WPBR is present within the geographic range of eight of the nine species in the U.S. including the four species also present in Canada, but has not yet been documented in Mexico. Genetic resistance to WPBR has been documented in eight of the white pine species present in the U.S., with extensive work on foxtail pine (Pinus balfouriana) just recently started. The development of populations of trees with durable genetic resistance, while also retaining genetic diversity and adaptability, is seen as a fundamental step in restoring white pine species. Major gene resistance (MGR) has been documented in four species, and quantitative resistance (QR) is likely present in all species, but at levels ranging from very low to moderately high. Restoration using seed from WPBR resistant parent trees has been underway for several decades for western white pine (P. monticola), sugar pine (P. lambertiana), and eastern white pine (P. strobus), and has begun more recently for whitebark pine (P. albicaulis) and limber pine (P. flexilis). For many of these white pine species, locating additional resistant parents and acquiring more seed will be needed over the ensuing decades. The previous efforts in developing populations of trees with genetic resistance to WPBR has used conventional tree improvement techniques of tree selection and seedling inoculation trials. However, in the future with the continued development of omics resources and tools in white pines, biotechnology has the potential to aid resistance programs. Candidate genes have been identified for host MGR, QR, and disease susceptibility (S) to WPBR, as well as for C. ribicola effectors. Marker-assisted selection (MAS) tools developed from MGR-linked genes would be useful to combine MGR and QR, which should improve the potential durability of resistance. Integration of breeding programs with omics information and tools can help pave a road towards improvement of WPBR resistance through biotechnological approaches such as MAS, and genomic selection (GS), or potentially helping to incorporate unique resistance not currently found in North American five-needle white pines. In the near future, these tools could potentially aid in the initial search for candidate trees which would increase the efficiency of developing WPBR resistant populations, as well as defining the extent and distribution of adaptive genetic variation in the species, which will aid in planning restoration efforts.