Using a fungal pathogen to control a non-native invasive plant, despite spillover effects
Natural enemies take on the role of biological control agents for the management of non-native plant species, with varying rates of success. A major concern of biological control is the risk of spillover onto non-target species, which needs to be weighed against biological control efficacy and the ability to mitigate negative effects. Using a randomized complete block glasshouse experiment, we investigated the spillover effects associated with a soil-borne generalist fungal pathogen, Pyrenophora semeniperda, which has potential to control a highly invasive non-native plant in the Intermountain West, Bromus tectorum L. (cheatgrass, downy brome). Bromus tectorum control is notoriously difficult given its winter annual life history that depends on a vast seedbank and continuous germination strategy from fall through spring. Pyrenophora semeniperda is able to kill seeds in the seedbank, providing the missing link to current management strategies that only target B. tectorum seedlings. Bromus tectorum invasions occur in forage, range, and croplands; however, only a handful of non-target species in these environments have been tested for P. semeniperda susceptibility. Thus, in addition to B. tectorum, we tested 15 co-occurring species (5 species in each environment) for P. semeniperda susceptibility. We hypothesized that native rangeland species would demonstrate greater P. semeniperda susceptibility compared to introduced forage species.
An analysis of variance and least significant differences showed that P. semeniperda effectively reduced B. tectorum emergence by 40% (P < 0.05), as well as the emergence of several non-target species. Specifically, P. semeniperda reduced emergence for 4 of the 5 native rangeland species by 20-80% compared to a non-inoculated control. Crop and forage species experienced 10-40% reductions in emergence, suggesting that these species are less susceptible to P. semeniperda spillover effects. These results demonstrate that while P. semeniperda can control B. tectorum, spillover effects are of concern. This is especially true in B. tectorum-invaded rangelands, where revegetation relies on the establishment and recruitment of native species. Thus, utilizing a biological control agent to target an invasive non-native plant such as B. tectorum requires further research on how to mitigate non-target effects. Our glasshouse research thus far has demonstrated that funcigice seed treatments are able to mitigate the spillover of P. semeniperda onto non-target species. We have implemented field experiments that integrate P. semeniperda with traditional management techniques with the hope that a multi-pronged approach will better serve the Intermountain West in its war against B. tectorum and other invasive annuals.