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 western North America. The systemic, seedling-infecting head smut pathogen Ustilago bullata Berk. commonly infects cheatgrass stands, often at epidemic levels. We examined factors controlling U. bullata infection levels in greenhouse studies with parental lines of four B. tectorum populations from contrasting habitats and U. bullata bulk teliospore collections from within the four populations. The U. bullata infection process appeared to have broad environmental tolerances, so that it was relatively simple to develop a protocol for obtaining high infection percentages in susceptible lines. Bromus tectorum populations generally showed highest infection levels when inoculated with locally collected U. bullata teliospores. This effect was most marked for the warm desert population, which was completely resistant to U. bullata collected from other areas, but 100% susceptible to locally collected inoculum. Two of the four populations showed major differences in susceptibility among parental lines, with the differences most pronounced when nonlocal inoculum was used. In preliminary trials with paired monosporidial isolates, two paired isolates infected all nine inbred lines to levels near 100%, while a third paired isolate was pathogenic on only five of the nine lines. These results demonstrate resistance polymorphism both among and within B. tectorum populations. This polymorphism may be important in developing strategies for the use of U. bullata as a biocontrol agent for B. tectorum.