Stewart, Thomas , Meyer, Susan E. , Clement, Suzette .
Virulence Variation in the Generalist Seed Pathogen Pyrenophora semeniperda.
The generalist pathogen Pyrenophora semeniperda is a weak pathogen when it infects the leaves or stems of grasses, but it has been shown to be a major pathogen of grass seeds. It is particularly abundant in seed banks of the weedy annual grass Bromus tectorum. The pathogen is most active in spring, when B. tectorum seeds are in a state of secondary dormancy. It appears from field studies to have limited ability to kill nondormant seeds in the autumn seed bank. In this investigation, we measured the virulence of 91 P. semeniperda isolates from 19 B. tectorum populations. We defined virulence as the ability to kill nondormant B. tectorum seeds. For each of two replicate treatments, we dusted 50 nondormant seeds with 0.003g of conidial inoculum from a single isolate and incubated the seeds at 20C for fourteen days, then scored the seeds as germinated or killed (as evidenced by the presence of fungal stromata on ungerminated seeds). We determined that pathogen-caused mortality on nondormant seeds ranged from 0-44%. Most of the variation in virulence was distributed among isolates within populations (P<0.0001), with no significant among-population effect. In addition, there was no clear relationship between molecular marker (ITS) genotype and virulence phenotype, suggesting that perhaps virulence can evolve quite rapidly in this organism. Further studies are planned to evaluate fitness in pathogen isolates that exhibit wide differences in virulence, to test whether there may be a fitness tradeoff that prevents the pathogen from evolving ever higher levels of virulence. The considerable variation in virulence that we encountered is also promising in terms of the use of this organism as a biocontrol agent against B. tectorum.
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1 - Brigham Young University, Plant and Wildlife Sciences, 287 Widtsoe Building, Provo, Utah, 84602 , United States
2 - USDA Forest Service, Rocky Mountain Research Station, Shrub Sciences Laboratory, 735 N 500 E, Provo, Utah, 84606, USA
Presentation Type: Oral Paper:Papers for BSA Sections
Location: Cottonwood C/Snowbird Center
Date: Wednesday, July 29th, 2009
Time: 4:45 PM