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USFS Scientist First to Define Blister Rust Resistant Allele in Limber Pine

Limber pine (Pinus flexilis) is being threatened by the lethal disease white pine blister rust (WPBR), expanding bark beetle pressure, and climate change in mountain environments. The upper subalpine and treeline ecosystems dominated by limber pine are ecologically valuable to landscape dynamics, watershed protection and biodiversity; maintaining their health is important for ecological sustainability and to society. Consequently, limber pine is approved for endangered species status under The Wildlife Act in Alberta (Canada)(Government of Alberta, 2012) and active conservation efforts in the Southern Rocky Mountains are underway (Burns et al 2008, Schoettle et al. 2011). Impacts of WPBR are evident in the northern populations; the pathogen is still spreading and many southern populations are threatened but not yet invaded (Kearns et al. 2013). The types and frequencies of genetic resistance to the rust will likely determine the potential success of proactive or restoration measures (Schoettle and Sniezko 2007).

A susceptible diseased limber pine seedling (left) and a healthy seedling with the Cr4 resistance allele (right) ten months post-inoculation
AW Schoettle
A susceptible diseased limber pine seedling (left) and a healthy seedling with the Cr4 resistance allele (right) ten months post-inoculation. Premature senescence of infected needles on seedlings (red needles, right) is associated with the Cr4 allele

In a recent publication in the journal Phytopathology, Forest Service researchers report on the first of a series of studies using individual families to examine complete resistance to WPBR and its inheritance in limber pine. Dr. Anna Schoettle and colleagues report that 14% of the 105 tested limber pine families show segregation of a disease-free trait consistent with inheritance by a single dominant gene, which they name Cr4, and the Cr4 resistance allele appears to be distinct from the Cr2 resistance allele in western white pine (Schoettle et al. 2014). This confirms limber pine as the fourth North American white pine species to show a major gene for resistance to WPBR. The frequency of the Cr4 allele across healthy and recently invaded populations in the Southern Rocky Mountains was unexpectedly high (5.0%; ranging from 0 to 13.9%) compared to resistance allele frequencies in western white pine or sugar pine. The research suggests that Cr4 is not a product of a recent mutation and may have other adaptive significance within the species possibly related to other abiotic or biotic stress factors. Field verification of the expression of the Cr4 allele under natural conditions are underway and the research team is also conducting studies to identify and quantify other resistance mechanisms to WPBR in limber pine and bristlecone pines. The identification of Cr4 and other resistance types in native populations of limber pine early in the invasion progress in this region enables the Forest Service to optimize cost effective treatments by timing interventions to maximize forest health benefits, consistent with the recommendations of the National Strategic Framework for Invasive Species Management (Dix et al. 2013).

References

Burns, KS, AW Schoettle, WR Jacobi, MF Mahalovich. 2008. Options for the management of white pine blister rust in the Rocky Mountain Region. Gen. Tech. Rep. RMRS-GTR-206. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 26 p.

Dix, ME and others. 2013. Forest Service National Strategic Framework for Invasive Species Management, FS-1017, Aug 2013.

Government of Alberta Species assessed by Alberta’s Endangered Species Conservation Committee: short list [accessed 3 March 2014].

Kearns, HSJ, WR Jacobi, RM Reich, RL Flynn, KS Burns and BW Geils. 2013. Risk of white pine blister rust to limber pine in Colorado and Wyoming, USA. For. Path. 44 (2014) 21–38

Schoettle, AW, BA Goodrich, JG Klutsch, KS Burns, S Costello, RA Sniezko, J Connor. 2011. The Proactive Strategy for Sustaining Five-Needle Pine Populations: An Example of its Implementation in the Southern Rocky Mountains. In: Proceedings of the High Five Symposium. 28-30 June 2010 Missoula, MT. Proceedings RMRS-P-63. Fort Collins, CO: U.S. Pp 323-334.

Schoettle, AW, RA Sniezko. 2007. Proactive intervention to sustain high-elevation pine ecosystems threatened by white pine blister rust. Journal of Forest Research 12:327-336.

Schoettle, AW, RA Sniezko, A Kegley, and KS Burns. 2014. White pine blister rust resistance in limber pine: evidence for a major gene. Phytopathology 104:163-173.

Infection spot types on limber pine needles following artificial inoculation with C. ribicola
Schoettle et al 2014
Infection spot types on limber pine needles following artificial inoculation with C. ribicola. Susceptible-type needle spots on seedlings that develop WPBR (A, secondary needle; B, primary needle); discrete necrotic-type spots on a cotyledon (C), primary needle (D) and secondary needle (E and F) on seedlings with the Cr4 allele that remain disease-free. Spots such as shown in image C were rare.
The research publication made the cover of the February 2014 issue of Phytopathology journal
The research publication made the cover of the February 2014 issue of Phytopathology journal
Natural white pine blister rust infection on a susceptible limber pine tree
AW Schoettle
Natural white pine blister rust infection on a susceptible limber pine tree