Bromus tectorum (cheatgrass) is an invasive winter annual grass that has spread across the Intermountain West, reducing rangeland productivity and crop yields. Bromus tectorum seeds can remain dormant in the soil up to five years and seedbank densities can range from 10,000 to 30,000 seeds per m2. Multiple herbicide applications are commonly used for B. tectorum control; however, this fails to target the seedbank. Pyrenophora semeniperda, a potential biological control agent, is a soil-borne fungal pathogen that attacks B. tectorum seeds, thereby reducing propagule pressure. However, P. semeniperda use is currently limited because little is known about how environmental and biological variables impact its efficacy. The goals of this study were to a) assess the impact of temperature on B. tectorum seed mortality caused by P. semeniperda and b) determine if B. tectorum susceptibility to P. semeniperda varies among B. tectorum ecotypes. This experiment was conducted on a temperature gradient table, arranged as a split split-plot design. Pyrenophora semeniperda inoculation (inoculated, non-inoculated) was the whole plot factor. The subplot was temperature (T1: 13.03°C, T2: 17.19°C, T3: 20.86°C, T4: 24.60°C, T5: 32.03°C) and B. tectorum ecotype (farming, ranch, subalpine creek) was randomized within temperature. This experiment was repeated two times.
Results/Conclusions
Results indicated that fungal-caused seed mortality varied among temperature, ecotype and inoculation status (p = 0.03). While there was no difference among B. tectorum susceptibility at 32.03°C, 17.19°C, and 13.03°C for all ecotypes, we found that B. tectorum susceptibility varied among ecotypes at 24.60°C and 20.86°C in inoculated whole plots. In particular, at 24.60°C, the ranch ecotype was less susceptible to infection when compared to the subalpine creek ecotype (p = 0.013). The susceptibility of B. tectorum from the ranch ecotype was 60% ± 11.5%, compared with 93.3% ± 6.7% from the subalpine creek ecotype. At 20.86°C, the susceptibility of the ranch and farming ecotypes was lower than that from the subalpine creek ecotype (p = 0.013 and <0.001, respectively). The susceptibility of the farm and ranch ecotypes were 33.3% ± 6.7 and 60% ± 11.5% respectively compared with 93.3% ± 6.7% from the subalpine creek ecotype. This study will enhance the understanding of the interactions between environmental and biological variables in determining the impact of P. semeniperda caused mortality on B. tectorum. It will also improve our understanding of how rising soil temperatures could impact the potential use of P. semeniperda for a B. tectorum biological control agent.