Barberry as Alternate Host Is Important for Puccinia graminis f. sp. tritici But Not for Puccinia striiformis f. sp. tritici in the U.S. Pacific Northwest.

Common barberry (Berberis vulgaris) is the alternate host of the wheat stem rust pathogen, Puccinia graminis f. sp. tritici, under natural conditions in the U.S. Pacific Northwest. Barberry was recently shown to be infected by basidiospores of the wheat stripe rust pathogen, Puccinia striiformis f. sp. tritici, under controlled conditions, but it is unclear if barberry plays any role in stripe rust epidemics under natural conditions. Aecial samples of Puccinia spp. collected from barberry plants in the Pacific Northwest from 2010 to 2013 were characterized to species by inoculation on wheat plants under controlled conditions and by molecular markers and sequences of the internal transcribed spacer (ITS) region of nuclear ribosomal DNA. Inoculation of wheat plants with bulked aecia-bearing barberry samples resulted in most P. graminis f. sp. tritici uredia and some P. striiformis f. sp. tritici uredinia. Virulence tests demonstrated that the P. graminis f. sp. tritici isolates were sexually produced, whereas the P. striiformis f. sp. tritici isolates were clonal based on both virulence and simple sequence repeat marker tests, indicating urediniospores from wheat fields landing on barberry leaves as the possible source of P. striiformis f. sp. tritici inoculum. A method for simultaneously testing individual aecia for identifying of P. graminis f. sp. tritici and P. striiformis f. sp. tritici by pathogenicity and ITS markers. Using the method together with ITS sequencing, tested individual aecia were mostly P. graminis f. sp. tritici and occasionally some other formae speciales of P. graminis, but not P. striiformis. The results imply that barberry is essential for stem rust epidemics, but not for stripe rust under the natural conditions in the U.S. Pacific Northwest.