White pine blister rust in north america: past and prognosis.

ABSTRACT After a full century in North America, the blister rust epidemic has yet to stabilize, continuing to spread into warmer and drier areas previously considered climatically inhospitable. The disease apparently has no environmental limits wherever white pines and Ribes spp. cohabit and will eventually become pandemic. Although much timber value has been lost, more severe long-term damage is disruption caused to ecosystems by altered patterns of natural succession. During the last half of the century just past, development of genetic resistance superceded other direct control measures-mainly Ribes spp. eradication and antibiotics-which proved ineffective and/or unfeasible in large areas of the white pine range, especially in the West. Several mechanisms of complete (major gene) and partial resistance are common to at least several white pine species. Although North American populations of rust have low genetic variability overall, rust genotypes with specific virulence to major resistance genes exist in some local demes at high frequencies. The challenge will be to package and deploy resistance genes in ways that will dampen sudden increases in rust races of wide virulence. New introductions of blister rust from its gene center in Asia remain the gravest threat to genetic improvement programs.