A major gene for resistance to white pine blister rust in Western white pine from the Western cascade range.

ABSTRACT A dominant gene for resistance to white pine blister was indicated by Mendelian segregation in full-sib families of western white pine parent trees selected for phenotypic resistance in six heavily infected stands in the Western Cascades of Oregon and Washington. Seedlings were artificially inoculated three times between 1959 and 1964 and observed for development of stem infection. Segregation at this locus (Cr2) occurred in only two of the six parent populations sampled: one a natural stand, Champion Mine (CM), and the other a plantation of unknown seed origin. At CM, reduced penetrance of this gene was expressed by altered Mendelian ratios (mostly less-than-expected resistant phenotypes) in families of specific combinations of certain parents, indicating the presence of modifier genes with effects that ranged from mild to almost complete suppression of Cr2. Between 1968 and 1994, an apparent shift in virulence at CM caused all of the resistant selections to become infected and die. Recent inoculations of many of the same or related families from these parents, made from grafted ramets in a seed orchard, showed that Cr2 conditions a classical hypersensitive reaction (HR) in needle tissues, the primary infection courts. In the latter tests, seedlings were challenged with wild-type and four other sources of inoculum at and near CM that were also suspected of having wider virulence than wild type. No seedlings segregating for HR that were inoculated with wild type subsequently developed stem symptoms, but the other inocula induced both susceptible and HR needle spots on Cr2- genotypes, and many of these seedlings did develop stem infections. This implied that spore genotypes with specific virulence to Cr2 are carried in these inocula.

Evidence of Cytoplasmic Inheritance of Virulence in Cronartium ribicola to Major Gene Resistance in Sugar Pine.

ABSTRACT Tests for Mendelian segregation of virulence and avirulence in Cronartium ribicola, causal agent of white pine blister rust, to a major gene (R) for resistance in sugar pine were made using haploid basidiospore progenies from single diploid telia as inoculum on resistant genotypes. The telia were sampled from a small deme in the Siskyou Mountains of northern California, where a few mature sugar pines known to be Rr genotypes had become infected after withstanding the chronic blister rust epidemic for several decades and where intermediate frequencies of virulence in the ambient basidiospore population were subsequently measured. Infection type on inoculated seedlings with R was qualitative: all progenies of 81 single telia tested over 3 different years were either virulent (compatible) or avirulent (inducing hypersensitive necrosis), never a mixture of both reactions. The complete absence of heterozygotes in the telia population is strong evidence that virulence is not controlled by a nuclear gene. The data are consistent with earlier tests showing that basidiospore inoculum derived from aeciospores isolated from infected Rr trees produced mostly (>90%) virulent reactions on R- seedlings. The evidence indicates that transmission of virulence is uniparental via the cytoplasm of aeciospores. Exchange of spermatia between haploid thalli does not appear to be involved.

Random amplified polymorphic DNA markers tightly linked to a gene for resistance to white pine blister rust in sugar pine.

We have genetically mapped a gene for resistance to white pine blister rust (Cronartium ribicola Fisch.) in sugar pine (Pinus lambertiana Dougl.) by using an approach which relies on three factors: (i) the ability to assay for genetic markers in the haploid stage of the host's life cycle, using megagametophyte seed tissue; (ii) a simple and clearly defined pathosystem; and (iii) the use of random amplified polymorphic DNA (RAPD) markers that can be quickly and efficiently evaluated. Resistance to white pine blister rust in sugar pine is known to be controlled by a single dominant gene (R). Maternal segregation of R and dominant RAPD markers were scored simultaneously following collection of megagametophytes for DNA assays and seedling inoculation with C. ribicola. Bulked samples of haploid megagametophyte DNA from resistant and susceptible offspring of segregating full-sib and half-sib families were used to evaluate 800 random decanucleotide primers. Ten loci linked with the gene for resistance to white pine blister rust were identified and segregation data were obtained from five families. Six of the linked markers were within 5 centimorgans of the gene, and one marker was 0.9 centimorgan from R. These and other markers derived by this approach may provide starting points for map-based cloning of this important gene.

White pine blister rust: simply inherited resistance in sugar pine.

Segregation ratios of offspring from disease-free sugar pines suggest that resistance to the white pine blister rust fungus is under major gene control and simply inherited.