Association of Mmdl, a major gene for resistance to Melampsora medusae f. sp. deltoidae, with quantitative traits in poplar rust

A single gene, Mmdl, which conditions resistance and a necrotic flecking response to a monouredinial isolate of Melampsora medusae f. sp. deltoidae and is presumed to possess the corresponding avirulence gene, was previously shown to segregate 3:1 (resistant to susceptible) in an interspecific hybrid poplar F-2 progeny. Some inoculated clones of the resistant phenotypic class of this progeny were completely resistant and bore no uredinia, but most bore some sporulating uredinia with accompanying necrotic flecking. The dominant allele at the Mmdl locus in these incompletely resistant clones was significantly associated with reduced uredinial density and diameter and longer latent period in a growth-room assay and with reduced disease incidence and infection efficiency and longer latent period in a leaf-disk assay. However, high clone-mean heritabilities within the susceptible phenotypic class indicated that genes other than Mmdl also contribute to control of quantitative traits. Leaf age had a significant effect on uredinial density and latent period but not on uredinial diameter in the growth-room assay. All quantitative traits were intercorrelated to varying extents. A principal components analysis (PCA) demonstrated that uncorrelated components associated with uredinial diameter and uredinial density explained two-thirds of the total variation. Since uredinial diameter (PCI) and necrotic flecking are the visual components of an infection-type rating scale, genetic analyses of poplar rust should be based on infection type. Mmdl is a major gene for resistance associated with a significant effect on all quantitative traits measured. Gene complexes in domesticated, agricultural rust pathosystems known to provide durable resistance consist of similar "pivotal" major genes and inferred ancillary genes or quantitative trait loci.