Evidence for resistance polymorphism in the Bromus tectorum Ustilago bullata pathosystem: implications for biocontrol

Bromus tectorum L. (cheatgrass or downy brome) is an important exotic weed in natural ecosystems as well as in winter cereal cropland in semiarid west ern North America. The systemic, seedling-infecting head smut pathogen Usti lago bullata Berk, commonly infects cheatgrass stands, often at epidemic le vels. We examined factors controlling U. bullata infection levels in greenh ouse studies with parental lines of four B. tectorum populations from contr asting habitats and U. bullata bulk teliospore collections from within the four populations. The U. bullata infection process appeared to have broad e nvironmental tolerances, so that it was relatively simple to develop a prot ocol for obtaining high infection percentages in susceptible lines. Bromus tectorum populations generally showed highest infection levels when inocula ted with locally collected U. bullata teliospores. This effect was most mar ked for the warm desert population, which was completely resistant to U. bu llata collected from other areas, but 100% susceptible to locally collected inoculum. Two of the four populations showed major differences in suscepti bility among parental lines, with the differences most pronounced when nonl ocal inoculum was used. Ln preliminary trials with paired monosporidial iso lates, two paired isolates infected all nine inbred lines to levels near 10 0%, while a third paired isolate was pathogenic on only five of the nine li nes. These results demonstrate resistance polymorphism both among and withi n B. tectorum populations. This polymorphism may be important in developing strategies for the use of U. bullata as a biocontrol agent for B. tectorum .