INOCULUM DENSITY AND EXPRESSION OF MAJOR GENE RESISTANCE TO FUSIFORM RUST DISEASE IN LOBLOLLY-PINE

Inoculum densities of 25 x 10(3) to 200 x 10(3) per mi of basidiospore s from single aeciospore isolates avirulent or virulent to the Fr1 (fu siform resistance-1) gene were used to inoculate a control-pollinated loblolly pine family heterozygous for this gene. With two avirulent is olates, the regression curve of gall frequency 9 months after inoculat ion went from 26 to 50% as inoculum density increased to 100 x 10(3) s pores. The regression curve flattened at higher inoculum densities. Wi th two virulent isolates, gall frequency increased from 47% to a plate au at 97% as spore density increased. A double-blind dement of the stu dy correlated the occurrence of the genetic marker (RAPD marker J(7-)4 85A) for Fr1 resistance in haploid megagametophyte tissue and the pres ence or absence of galls on seedlings after artificial inoculations. W ith avirulent isolates at the two higher densities of 100 x 10(3) and 200 x 10(3), marker presence-absence and phenotypic assessments of gal l presence-absence agreed for 95% of the seedlings. At the 50 x 10(3) 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 s pore densities. The double-blind study indicates that resistant indivi duals can be identified from the megagametophyte tissue of germinating seedlings. With virulent isolates, marker and disease phenotype did n ot correlate, even at the lowest inoculum density. The virulent isolat es appear to be homozygous for virulence because infection of marker-p ositive resistant seedlings equaled or exceeded that of marker-negativ e susceptible seedlings at the lowest inoculum density.