Samuel Saunders, Travis Poh and Dr. Bradley Geary, Department of Plant and Wildlife Sciences
INTRODUCTION
Cheatgrass (Bromus tectorum) is a highly flammable invasive annual grass that dominates millions of acres of the Great Basin. A common phenomenon in cheatgrass monocultures is die-off or stand failure (Baughman and Meyer in press). Fungal isolates of the genus Fusarium are frequently cultured from dead cheatgrass seeds in die-off areas and may be an important cause of stand failure (Franke et al. in draft), and making Fusarium fungi a potential biocontrol agent against cheatgrass. In a previous study, the plant pathology lab at BYU found that this pathogen could cause high mortality when seeds were held under water stress at 250C (summer conditions), and that pathogen strains varied in their ability to kill seeds. My purpose in the present study was to investigate whether this pathogen could cause seed mortality under the autumn and winter temperature regimes that usually prevail during precipitation events that could trigger pathogen attack.
METHOD
To test whether these could be die-off causal organisms, we placed 50 cheatgrass seeds with inoculum from twenty Fusarium isolates from die-off areas at 250C in water and at -1.5 MPa (i.e., under water stress). Each Fusarium treatment was blocked four times and the experiment was repeated twice. The five most virulent Fusarium isolates from this study were used to conduct the pathogenicity experiment proposed here. It included testing each isolate at 4 temperatures (250C, 150C, 50C, and -50C) in a combined factorial design with 4 water potentials (0, -1.5, -2, and -2.5 MPa). Spores of each of the five isolates were used to inoculate non-dormant cheatgrass seeds with a load of 125,000 fungal spores/ml. Spores were collected from cultures grown on SNA (Spezieller Nährstoffarmer Agar) for 3 weeks (Leslie and Summerell 2006). The inoculated seeds were then placed in Petri dishes on blotter papers at each water potential. Each dish had 50 seeds and there were 4 blocks per treatment combination. The water potentials were obtained by using blotters soaked in PEG (Poly-Ethylene Glycol) to create water stress (Michel and Kaufman 1972). Each Petri dish was incubated for 1 wk at the designated water potential and temperature. The seeds were then moved to dishes with 2 blotters soaked in ddH20 to remove the water stress and incubated for an additional 4 wks at the same temperature. This enabled evaluation of seed mortality by scoring whether seeds germinated or showed disease signs when germination-inhibiting water stress was removed. The experiment was repeated at least once. We used analysis of variance with appropriate data transformations to statistically evaluate the data set.
RESULTS
These are our results from the analysis only at 25C where there were three stress treatments. Strain, stress, and strain x stress are significant at PStorm>Night Fury=Supernova
Below is our analysis from all temperatures in only PEG and water. All main effects and interactions are significant at P25>20>15>10C
CONCLUSION
Strain by stress interaction is that kill with PEG is inversely related to kill in water. Night Fury and Supernova have the highest kill in water but the lowest kill in PEG. For Avalanche and Fluffy, the effect is vice versa—they have the highest kill in PEG but the lowest kill in water. Kill in water isn’t very high, but the differences among strains in water are significant. Difference between 10 and 15C is small overall and so is difference between 25 and 30C. These differences are not significant in just the water treatment, but they remain significant in just the PEG treatment. Strain x stress—strains that have lower kill in PEG have higher kill in water across all the temperatures, not just at 25C. Strain x temp—Avalanche and Fluffy show less proportional decrease in kill as temp goes down, Night Fury and Supernova show more proportional decrease. This clearly shows that strains differ in their ability to kill at autumn and winter temperature regimes that usually prevail during precipitation events that could trigger pathogen attack.
DISCUSSION
Because of the results obtained future studies are necessary to further the understanding of the pathogenicity of the five fungal strains against Bromus tectorum. Along with recognizing that temperature plays a significant effect on killing efficiency, what will be the effect of varying inoculum loads on Bromus tectorum? As well, since the experiment was performed in a laboratory setting, further tests will need to be run in green houses simulating varying temperatures.