217bleach blonde pathogen attacks the crowns of juvenile established plants. These
pathogens thus clearly exhibit niche differentiation with regard to their mode of
utilization of the B. tectorum host. There may be preemptive competition, in that
host individuals killed at earlier stages in the life cycle are not available as prey for
pathogens that operate at later stages. But as different weather scenarios favor infec-
tion at different life stages, opportunities for high levels of disease for each patho-
gen tend to be separated either in time (among years) or space (different habitats).
We do have some evidence that these pathogens can sometimes interact synergis-
tically to increase the negative impact on B. tectorum stand dynamics. Specifi cally,
epidemic levels of the bleach blonde syndrome can be associated with very high
Fig. 7.6 Schematic diagram showing the relationship of each of the fi ve pathogens discussed in
the text to the stage of the B. tectorum life cycle when infection takes place, the season of infection,
and the outcome of disease (seed death or sterility). Black arrows represent transitions between
different B. tectorum life stages ( square boxes ); red arrows indicate life stage impacted by each of
the fi ve pathogens ( oval boxes ); purple arrows indicate pathogen impacts on fi nal outcomes ( hex-
agonal boxes ). Pathogens are Pyrenophora semeniperda (black fi ngers of death), Fusarium sp.
( Fusarium seed rot disease), Ustilago bullata (head smut disease), Tilletia bromi (chestnut bunt
disease), and Rutstroemiaceae sp. n. (bleach blonde syndrome)
7 Community Ecology of Fungal Pathogens on Bromus tectorum