Does Fusarium-caused seed mortality contribute to Bromus tectorum stand failure in the Great Basin?

Summary: Bromus tectorum (cheatgrass, downy brome) is an important invader in western North America, dominating millions of hectares of former semi-arid shrubland. Stand failure or 'die-off' is relatively common in monocultures of this annual grass. The study reported here investigated whether soil-borne pathogens could be causal agents in die-offs. Soils from two die-off areas and adjacent B. tectorum stands were used in a glasshouse experiment with sterilised and non-sterilised treatments. Soil sterilisation did not increase emergence, which averaged 80% in both die-off and non-die-off soils. Seedling biomass was higher in die-off soils, probably due to increased nitrogen availability. Fusarium was isolated from 80% of killed seeds in non-sterilised soil treatments. In pathogenicity tests with 16 Fusarium isolates, host seeds incubated under water stress (-1.5MPa for 1 week prior to transfer to free water) suffered over twice the mortality of seeds incubated directly in free water (25-83% with water stress vs. 5-43% without water stress). These results suggest that soil-borne Fusarium could play a role in B. tectorum stand failure in the field, but that low water stress conditions in the glasshouse experiment were not conducive to high levels of disease. Pathogenic Fusarium isolates were obtained from seeds planted in both die-off and non-die-off soils, suggesting that microenvironmental factors that affect levels of water stress might be as important as relative abundance of soil-borne pathogens in mediating spatial patterns of disease incidence in the field.