Thomas Stewart and Dr. Phil Allen, Plant and Wildlife Science
Bromus tectorum, an invasive annual grass weed, is one of the main plants responsible for an increase in fire frequency and intensity in western wildlands. Wildfires cost the U.S. Forest Service over a billion dollars a year to fight. These fires destroy native vegetation communities and reduce biodiversity. Pyrenophora semeniperda, a native pathogen of grass seeds, has the capability of reducing the field seed bank of B. tectorum. Using P. semeniperda as a biocontrol on B. tectorum could help reduce the risk of fire to the western wildlands.
Pyrenophora semeniperda is a generalist pathogen that is known to attack grass seeds. It has been found to attack B. tectorum seeds after dispersal. This is evident from the development of fungal stromata on the seed . We have found that P. semeniperda appears to be able to infect slow germinating seeds, and that quick germinating seeds usually escape infection. This is due to a race for the resources stored in the seed endosperm. If P. semeniperda uses the resources first, then the seed is killed. If the seed germinates quickly enough, P. semeniperda does not have time to use up the resources and the seed escapes death . This means that the primary target of the pathogen is secondarily dormant seeds in the spring seed bank.
Through existing research by Dr. Susan Meyer of the USDA Forest Service we know that Pyrenophora semeniperda eliminates 95% of the Bromus tectorum seed bank at her White Rocks Utah research plots. However, this is only dormant seeds that are getting eliminated by the fungus. This means that during a fall precipitation event the non-dormant B. tectorum seeds are able to germinate and escape death from P. semeniperda.
Through this project we tested for a variation in the virulence of P. semeniperda from 4 different populations. They were tested on non-dormant B. tectorum seeds, because any strain of P. semeniperda can kill dormant seeds. Our 4 populations are from 10-Mile Creek, Dog Valley, House Mountains, and White Rocks. We then isolated 5 isolates from each population.
With these 20 isolates we grew them out a V8 auger to promote stomata production. After 4 weeks of growth the stomata were transferred to a dish containing modified alphacel media. This media promotes conidia production instead of stomata production. For each isolate we grew out 100 dishes on the modified alphacel media. After 6 weeks of growth the plates were dried and the conidia were harvested.
Due to contamination issues and limited time to grow more conidia only 9 of the 20 isolates produced enough conidia to run the virulence test on. These isolates were 10-Mile Creek 1, 3, 4, 7, 10; House Mountains 2; Dog Valley 3, 4; and White Rocks 0. Table 1 shows the amount of conidia production per 100 plates for each isolate.
After the collection of the conidia 18 reps of 50 non-dormant Bromus tectorum seeds were taken and placed into glass jars. Into these jars we placed .01g of conidia inoculum. 2 reps of each isolate were used. These jars were hooked up to a jitterbug sander and shook for 3 minuets to evenly distribute the conidia onto the B. tectorum seeds. The seeds were placed into Petri dishes on blotters to germinate. After 7 days each plate was read and the seeds were scored as germinated or killed. These numbers were then taken and a percentage of killed seeds was obtained.
White Rocks was the most virulent with a seed kill of 21% while House Mountains was the least virulent with a seed kill of 2%. The other isolates fell in-between these two figures. See figure 1 for a break down of the percentage of seeds killed for each isolate. These figure show that there is a variation in the virulence of the seed pathogen Pyrenophora semeniperda.
Due to the small size of this study further research with more populations and more isolates needs to be done. This is an important step in the fight against B. tectorum. The ability to find a more virulent strain of P. semeniperda could be a major help in the control efforts against B. tectorum.
There are further plans to explore this topic further. It is believed that a strain more virulent than any in this study does exist. This strain could be located in areas known as dead spots, areas that no vegetation is growing due to an unknown reason, which can be found scattered throughout the west. Pyrenophora semeniperda is going to be gathered at these locations and tested for virulence.
References
- Meyer SE, D Quinney, DL Nelson, J Weaver, 2007. Impact of the pathogen Pyrenophora semeniperda on Bromus tectorum seedbank dynamics in North American cold deserts. Weed Research. 46:1-9.
- Beckstead J, Meyer SE, Smith C, Molder C, in review. A race for survival: can Bromus tectorum seeds escape Pyrenophera semeniperda-caused mortality by germinating quickly? Weed Research.