Genetic models of host-pathogen gene interaction based on inoculation of loblolly pine seedlings with the fusiform rust fungus

Loblolly pine (Pinus taeda) seedlings from 12 full-sib families obtained from a six-parent half diallel mating design were challenged in a greenhouse using two basidiospore inocula of the fusiform rust fungus (Cronartium quercuum f.sp. fusiforme) at extremely high spore density. Each basidiospore inoculum originated from a mixed gall collection of aeciospores obtained from field-infected trees. Assessments at 4.5 months after inoculation showed that rust disease levels were high for every full-sib family and were typically above 90% for most full-sib families for both inocula. However, disease (% galled) for family E by A progeny, even at 9 months post inoculation, was lower, around 75%. A genetic model for interaction of two pairs of genes was proposed to explain the observed infection levels (% galled) in this diallel based on a gene-for-gene hypothesis. The putative genotypes of host parents and virulence compositions of mixed inocula were postulated. A bulk-segregant analysis approach based on phenotype (gall vs. no gall) was used to search for dominant molecular markers associated with the potential resistance genes in the host parents. A few candidate marker polymorphisms were observed between the gall vs. no gall bulks; however, none of the candidates appropriately co-segregated with phenotype when tested across the progeny set. An alternative model involving recessive resistance controlled by a single locus was also considered, but as with the two gene model, no markers to support the appropriateness of the recessive resistance model were observed.