Range-Wide Geographic Variation of Major Gene Resistance ( Cr 3) to White Pine Blister Rust in Pinus strobiformis (Southwestern White Pine)

Southwestern white pine (Pinus strobiformis) is native to the U.S. and Mexico and is susceptible to the pathogen Cronartium ribicola, which causes the disease white pine blister rust. A major gene, Cr3, confers resistance to the disease. While Cr3 has been observed at low frequency in the periphery of the species range, its presence in the core of the range, in Mexico, is unknown, as are the frequency and geographic pattern of this resistance. We conducted one of the first range-wide assessments of resistance to white pine blister rust in a five-needle pine, evaluating 20,533 seedlings from 475 families over ten years of monitoring. Patterns of Mendelian segregation within open pollinated families were used to infer major gene resistance (MGR) genotypes of maternal parent trees and to estimate its geographic frequency. We identified up to 34 of the 475 parent trees as heterozygous for MGR. Four of the heterozygous parents were located within the core of the species range in Mexico. Range-wide, the estimated Cr3 frequency was 3% in zygotes and 2 to 4% in gametes, depending on trial type. The probability of being stem symptom free was frequent in the northern and eastern portions of the range. These findings inform genetic conservation efforts and identify seed sources for further field testing and reforestation and restoration. Because much of the range lies in Mexico and has yet to be impacted by C. ribicola these results provide a unique opportunity for proactive management in the face of a spreading disease.