Port-Orford-cedar (Chamaecyparis lawsoniana) is a forest tree native to a small area of Oregon and California. A root disease caused by Phytophthora lateralis causes widespread mortality of Port-Orford-cedar. This dissertation examines three important elements of the Port-Orford-cedar P. lateralis pathosystem related to breeding for disease resistance: use of resistant rootstocks to maintain genotypes of Port-Orford-cedar for breeding; the heritability and genetic basis of disease resistance; and variability in virulence and DNA fingerprint among a sample of P. lateralis isolates. Port-Orford-cedar was reciprocally grafted to western redcedar (Thuja plicata), incense cedar (Calocedrus decurrens), and Alaska yellow-cedar (Chamaecyparis nootkatensis). Port-Orford-cedar scion graft success was moderate with western redcedar and incense cedar, but extreme overgrowth of the rootstock by the scion indicated incompatibility. Xylem union was good, but phloem union was incomplete or lacking. Nearly all Port-Orford-cedar rootstocks and seedlings exposed to P. lateralis died of root disease. Four percent of the Alaska yellow-cedar exposed also died, confirming this tree as a host for P. lateralis. Resistance of Port-Orford-cedar to P. lateralis is rare. A small number of trees have been identified exhibiting resistance. A number of families were tested to determine the genetic basis for resistance. Estimates of narrow-sense and family mean heritability of resistance, as exhibited by restriction of lesion length after inoculation, were determined. Both narrow-sense and family mean heritabilities were between 0.61 and 0.98 in most tests. Between 21% and 32% of the variance was due to differences among families. Thirteen isolates of P. lateralis were collected from three hosts throughout the geographic range of the fungus. Variation in growth rate on artificial media at three temperatures, virulence when used to inoculate Port-Orford-cedar, and DNA fingerprint were compared. There were significant differences in growth rate among isolates at 24C, but fewer differences at lower temperatures and on a rich medium. One isolate produced significantly shorter lesions in three different inoculation tests. Isolates differed at only two of 189 bands produced by Inter Simple Sequence Repeat (ISSR) DNA primers, indicating very little genetic variation among isolates. / Graduation date: 2001