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Abstract Detail



Mycological Section

Meyer, Susan E. [1], Clement, Suzette [2].

General resistance in the Bromus tectorum - Pyrenophora semeniperda pathosystem.

Pyrenophora semeniperda (black fingers of death) is a generalist fungal seed pathogen that can cause high mortality in Bromus tectorum (cheatgrass) seed banks. We have documented considerable variation in virulence (aggressiveness) among strains of this pathogen as well as variation in resistance among host species. Here we examine whether genotypes of cheatgrass that are characteristically found in specific habitats have differential general resistance to attack by this pathogen, and whether this variation is related to the probability of pathogen attack. We hypothesized that selection for resistance should be low in mesic habitats, where very few seeds enter the pathogen-vulnerable secondarily dormant state and where inoculum levels are low. Selection for resistance should be high in xeric habitats where a large seed fraction commonly enters secondary dormancy and inoculum levels are high. Selection should be especially intense in areas prone to cheatgrass stand failure, where the need for recovery from the seed bank should apply strong selection for genotypes able to survive pathogen attack. Greenhouse-produced dormant seeds of 33 cheatgrass lineages belonging to genotypes characteristic of a range of habitats were inoculated with pathogen conidia belonging to two strains of intermediate virulence at a level comparable to inoculum levels in the field. Each treatment combination included two replicates, and the entire experiment was repeated twice in time. Inoculated seeds were incubated at 20C for two weeks and scored for pathogen-caused mortality (appearance of fungal stromata on ungerminated seeds). Mean mortality varied among host lineages from 15 to 60%, indicating a fourfold difference in general resistance. Genotypes from mesic upland environments were significantly less resistant to the pathogen (mean mortality 37%) than genotypes from sagebrush steppe (mean mortality 32%) and salt desert environments (mean mortality 24%), supporting our hypothesis. The least resistant genotypes were from warm desert habitats, however, an apparent contradiction (mean mortality 46%). In spite of this, when we regressed general resistance (survival percentage) of host lineages on pathogen-killed seed density in the seed bank at the location of origin, we found significantly higher resistance in lineages from habitats with more disease (r=0.575, n=17, P=0.016). The reason for low levels of disease in warm desert environments is not known. Our results help explain why P. semeniperda inoculum augmentation is not always effective at completely eliminating the cheatgrass carryover seed bank, especially in xeric habitats prone to stand failure. 


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1 - UNITED STATES DEPARTMENT OF AGRICULTURE, USFS SHRUB SCIENCES LABORATORY, 735 NORTH 500 EAST, Provo, UT, 84606, USA
2 - USDA Forest Service, Rocky Mountain Research Station, Shrub Sciences Laboratory, 735 North 500 East, Provo, UT, 84606, USA

Keywords:
fungal plant pathogen
seed bank
natural selection
black fingers of death
cheatgrass
resistance variation.

Presentation Type: Oral Paper:Papers for Sections
Session: 36
Location: Cottonwoods North/Boise Centre
Date: Wednesday, July 30th, 2014
Time: 8:45 AM
Number: 36002
Abstract ID:143
Candidate for Awards:None


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