DISTRIBUTION AND FREQUENCY OF A GENE FOR RESISTANCE TO WHITE-PINE BLISTER RUST IN NATURAL-POPULATIONS OF SUGAR PINE

The gametic frequency of a dominant allele (R) for resistance to white pine blister rust, a disease caused by an introduced pathogen (Cronartium ribicola), in natural populations of sugar pine was estimated by the kind of leaf symptom expressed after artificial inoculation of wind-pollinated seedlings from susceptible seed-parent genotypes (rr). Gene frequency increased clinally from near 0 in the southern Cascade Range to 0.08 in the southern Sierra Nevada, but it was not correlated with any major climatic gradient. Because R expresses a typical hypersensitivity response to pathogenesis, it has probably always functioned in disease resistance. A candidate pathogen for this function is C occidentale, cause of pinyon blister rust, and a close relative of C. ribicola. Increase in allele frequency was positively associated with the proximity of sugar pine to single-leaf pinyon pine populations. Although sugar pine is not presently a natural host of pinyon rust, R may be a relict gene that protected sugar pine from this endemic pathogen in recent geologic epochs, when both pines are known to have been more intimately associated.