Title: Factors affecting host range in a generalist seed pathogen of semi-arid shrublands Authors
|Beckstead, Julie -|
|Meyer, Susan -|
|Bergen, Kellene -|
|Holden, Sandra -|
|Boekweg, Heather -|
Submitted to: Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 30, 2013
Publication Date: March 11, 2014
Citation: Beckstead, J., Meyer, S.E., Reinhart, K.O., Bergen, K.M., Holden, S.R., Boekweg, H.F. 2014. Factors affecting host range in a generalist seed pathogen of semi-arid shrublands. Plant Ecology. 215:427-440. Interpretive Summary: Tens of millions of acres of rangelands in the Great Basin and Intermountain West regions have been invaded by cheatgrass. Controlling this pest over such a large area though is likely to be cost prohibitive. Identification of natural enemies that can function as a biocontrol of this weed is likely to be a cost effective means of managing this weed over such a large area. However, biocontrol agents can have adverse effects on desirable plants. So accurate assessment of relevant risks is necessary to justify development of any biocontrol agent. This research addresses the effect of a seed pathogen on cheatgrass and nontarget (desirable) species. Pathogenicity trials in the laboratory and field helped identify that laboratory trials, which are typical when assessing biocontrol specificity, yielded overly conservative estimates of the pathogen’s fidelity to the focal weed. This information is essential to helping further justify development of the focal pathogen as a biocontrol of cheatgrass.
Technical Abstract: 1. In weed biocontrol, the potential impact of the biocontrol organism on non-target species is a major concern traditionally addressed with laboratory experiments that measure potential or maximum host range. Several factors may operate to reduce realized host range relative to potential host range, particularly under field conditions. This reduction in host range may have major implications for risks associated with biocontrol. 2. We explored factors influencing the host range of the generalist ascomycete grass seed pathogen Pyrenophora semeniperda. We measured potential host range in laboratory experiments at high inoculum loads with 26 grass species, including the target weed Bromus tectorum, and developed models to predict host susceptibility. We also examined pathogen and host density effects on infection and mortality in the laboratory and field. 3. All species tested were at least somewhat susceptible to the pathogen at high inoculum loads, but infection and mortality varied widely. Grass species more closely related to the original host (B. tectorum) were more susceptible to infection, whereas species with slower germination were also much more likely to suffer mortality. Infection and mortality were both sharply reduced when inoculum load was reduced but the shape of the response was species-specific. For example, intermediate inoculum loads had major negative impacts on dormant B. tectorum seeds but had minimal effects on native species with slow germination rates. 4. Pathogen density in field seed banks of native grasses was low or nil, probably because of low host seed density, while both seed and pathogen density in B. tectorum seed banks were orders of magnitude higher. When samples from plots effectively inoculated for B. tectorum biocontrol were planted with two native grasses, impacts were minimal, even though both species were highly susceptible in laboratory experiments defining potential host range. 5. Synthesis and applications. Laboratory host range experiments at high inoculum loads present a worst-case scenario for non-target host impacts. This may be appropriate for classical biocontrol of exotics, but it appears overly conservative for naturally-occurring pathogens used as mycoherbicides. A realistic assessment of mycoherbicide risks to non-target species requires examination of additional factors that influence host range expression under field conditions.