Frequency of infection, main effects and interactions among four geographic sources of white pine blister rust (WPBR) (Cronartium ribicola J.C. Fisch.) and 10 Ribes-sites (Ribes L. specie × site combinations) was investigated using leaf disk assay. Two clones, one of R. hudsonianum (Dougl.) Jancz. and one of R. viscosissimum Prush., were not infected by any sources of WPBR. One clone of R. viscosissimum that was not infected by two sources of WPBR was susceptible to the other sources. Highly significant WPBR sourc × Ribes-site analysis of variance interaction for incubation period and infection efficiency precluded testing main effects. Profile plots of incubation period interaction means showed orderly interaction by all WPBR sources and plots of infection efficiency showed that aggressively virulent WPBR from Oregon (Champion Mine) ranked near or at the bottom for infection efficiency for all Ribes-sites. Meanwhile, aggressively virulent WPBR from Idaho (Merry Creek) ranked near the bottom for infection efficiency when inoculated onto Ribes obtained from the Cascade Mountains but switched to the highest ranking when inoculated onto Ribes obtained from Idaho. Geographic interaction of white pine blister rust and Ribes for incubation period and infection efficiency may help to explain geographic patterns recently observed in WPBR molecular markers.
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