Resistance and virulence interactions between two white pine species and blister rust in a 30-year field trial

We monitored infection and mortality of full-sib families of sugar pine (SP) and western white pine (WWP) selected for different mechanisms of resistance to white pine blister rust for more than 30 years in a field test in northern California. Natural infection was enhanced by interplanting alternate host Ribes spp. among test seedlings. Parents of the families were from three geographic provinces, representing the northern Rocky Mountains of Idaho (WWP), the Cascade Range of Oregon and Washington (WWP), and the Siskiyou Mountains of California and Oregon (SP). Several families of each pine species had major gene resistance (MGR), or genes for partial resistance (PR), or both types. Under the chronic epidemic conditions that prevailed, susceptible controls of both species became infected and died within a few years. Families with MGR segregated in expected Mendelian ratios (susceptibility/resistance), but each R gene—Cr1 in sugar pine and Cr2 in western white pine—was eventually defeated by biotypes of the rust with specific virulence to it (vcr1 and vcr2, respectively). WWP families from Idaho, which lacked Cr2, performed the best overall, ranging from 44 to 64% rust-free; a few WWP families from the Cascades, both with and without Cr2, performed comparably. Families with PR were not specifically vulnerable to vcr1 or vcr2. PR was less frequent in SP than WWP, and SP families had steeper infection rate curves than WWP. Combining PR in pedigrees with MGR may prolong protection of MGR by inhibiting selection and epidemic increase of vcr genes in the rust.