Inoculum Density and Expression of Major Gene Resistance to Fusiform Rust Disease in Loblolly Pine

Inoculum densities of 25 × 10³ to 200 × 10³ per ml of basidiospores from single aeciospore isolates avirulent or virulent to the Fr1 (fusiform resistance-1) gene were used to inoculate a control-pollinated loblolly pine family heterozygous for this gene. With two avirulent isolates, the regression curve of gall frequency 9 months after inoculation went from 26 to 50% as inoculum density increased to 100 × 10³ spores. The regression curve flattened at higher inoculum densities. With two virulent isolates, gall frequency increased from 47% to a plateau at 97% as spore density increased. A double-blind element of the study correlated the occurrence of the genetic marker (RAPD marker J_7-485A) for Fr1 resistance in haploid megagametophyte tissuend the presence or absence of galls on seedlings after artificial inoculations. With avirulent isolates at the two higher densities of 100 × 10³ and 200 × 10³, marker presence-absence and phenotypic assessments of gall presence-absence agreed for 95% of the seedlings. At the 50 ×10³ level, marker-phenotype agreed for 86% of the seedlings. The increased marker-phenotype association resulted from a reduction or elimination of disease escapes as Fr1 resistance remained stable even at higher spore densities. The double-blind study indicates that resistant individuals can be identified from the megagametophyte tissue of germinating seedlings. With virulent isolates, marker and disease phenotype did not correlate, even at the lowest inoculum density. The virulent isolates appear to be homozygous for virulence because infection of marker-positive resistant seedlings equaled or exceeded that of marker-negative susceptible seedlings at the lowest inoculum density.