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

Abstract

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 J7-485A) 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.

Identification of Nine Pathotype-Specific Genes Conferring Resistance to Fusiform Rust in Loblolly Pine (Pinus taeda L.)

“…The Family 10-5 × Isolate 3327-13-3 cell was tentatively classified as compatible and Fr1-linked marker analysis failed to detect a significant association with phenotype confirming compatibility. The interaction of Family 10-5 progeny challenged with isolate NC2-36 was marker verified as compatible [19,40] and isolate NC 2-40 was also marker confirmed as virulent against Fr1 resistance [22]. Following the discovery of Fr1, eight additional Fr genes (Fr2-Fr9), as previously noted and described here in Section 3.2, have been found in loblolly pine.…”

“…Hence, when we speak of challenging pine progeny with a given single-genotype isolate, such as 0-5-32, this is a shorthand way of saying that the progeny were challenged with basidiospores derived from that given isolate. Greenhouse inoculations of pine seedlings (eight weeks post seed-sowing or approximately six weeks seedling age) were performed using the concentrated basidiospore inoculation system (CBS) of Matthews and Rowan [8], typically at a concentration of 100,000 basidiospores per mL, as recommended by Kuhlman et al [22]. Inoculations were conducted at both the USDA Forest Service Southern Research Station in Athens, Georgia and at the USDA Forest Service Resistance Screening Center in Asheville, North Carolina.…”

“…Powers [16] and Kuhlman and Matthews [17] reported methods for the development of single-genotype Cqf isolates with the Kuhlman and Matthews work showing virulence variation among single-genotype isolates derived from the same single gall. Contemporaneous with and subsequent to Kuhlman and Matthews [17], the use of basidiospore inocula derived from single-genotype isolates has greatly advanced the understanding of the genetics of fusiform rust disease, and the hypothesis that this pathosystem largely conforms to a classic gene-for-gene model has yet to be rejected [18][19][20][21][22][23][24][25]. Recently, Nelson et al [26] concluded that gene-for-gene interactions largely determine fusiform rust gall formation in loblolly and slash pine, with that conclusion being firmly supported for loblolly pine by the demonstration that host resistance gene Fr1 and pathogen avirulence gene Avr1 interact in a gene-for-gene fashion [27].…”

Rust resistance and mating system in Eucalyptus grandis Hill ex Maiden progenies

“…The variance analysis (ANOVA) in this study was based on polygenic inheritance of rust resistance, even if some studies have found that it is associated to other major genes (KUHLMAN et al, 1997). ANOVA was carried out on individual tree data for each progeny test using the GLM procedure (SAS, 1999).…”

Performance of the loblolly pine fusiform rust disease resistance gene (Fr1) in a slashXloblolly pine hybrid family

“…Loblolly pine seeds used as a control/check were obtained from a full-sib cross of loblolly tree 10-005, as the female, with pollen from a highly rust-susceptible loblolly individual, USDA Forest Service tree 4666-4 (Kuhlman 1992;Wilcox et al 1996;Kuhlman et al 1997) that lacks Fr1 resistance (Wilcox et al 1996;Kuhlman et al 1997). …”