Ustilago sp. are smut fungi that systemically infect grasses, including exotic annual Bromus grass species of the subgenus Genea that are causing vegetation type conversions in western U.S landscapes. Ustilago bullata renders these plants sterile and may lead to complete stand failure under high plant density. Depending on patterns of host-specificity, U. bullata may be a promising brome-specific biological control agent. However, little is known about niche specialization of Ustilago pathotypes and specific environmental conditions (e.g., timing and amount of precipitation or temperature) that determine the degree of plant infection and Ustilago teliospore germination.
We sought to understand the necessary environmental conditions for host-plant infection. We expected increased incidence of infection with earlier and more frequent rain events, as the fungal teliospores must germinate prior to Bromus seed germination to increase incidence of infection. We collected both infected and uninfected samples from sites in southern California, prepared axenic cultures, georeferenced samples, and imported collection data into ArcGIS. Precipitation and temperature data were extracted from PRISM datasets and compiled for analysis from July and through the following June of each year. Data were analyzed daily to investigate whether timing of rain events had an effect on infection.
Our survey included smut-infected B. diandrus, B. rubens and B. tectorum across a range of desert, Mediterranean, and montane ecosystems in California. Our data suggest that each Ustilago pathotype may have a preferred niche, similar to the distinct niche requirements of each host-plant species. These Ustilago pathotypes may also exhibit niche specialization, such that temperature or precipitation requirements may differ among pathotypes, and each pathotype may require different environmental conditions to promote infection.
In order to infect, the teliospores must germinate concurrently with Bromus seeds and produce sporidia, which conjugate into dikaryotic infection hyphae, and subsequently penetrate the coleoptile during an infection window. However, infection hyphae require increased access to nutrients and slightly acidic conditions, which may influence restoration techniques aimed at employing Ustilago taxa as a biocontrol agents.
Characteristics of our isolated cultures indicate that different pathotypes are infecting each host plant species. These host-specificity patterns suggest that targeted biocontrol strategies with Ustilago pathotypes may be effective. Our sampling efforts continue to investigate the environmental requirements necessary to promote infection. Research into host-specificity and niche requirements of these plant pathogens may elucidate whether Ustilago would be a viable biological control agent for brome grasses in southern California.