Cheatgrass facilitates spillover of a seed bank pathogen onto native grass species

Abstract

Summary 1.Pathogen spillover occurs when disease levels for a given population are driven by transmission from a reservoir species that carries a high pathogen load. Pathogen spillover is widely documented in crop systems, but has been little studied in natural plant communities. 2. The abundant seed production of weedy species may create a scenario where spillover of a generalist seed pathogen onto less abundant seeds of native hosts is possible. The invasive annual weed cheatgrass (Bromus tectorum) is a potential reservoir species for Pyrenophora semeniperda, a multiple-host fungal seed pathogen that naturally occurs in the semi-arid western United States. 3. To investigate potential community-level consequences of spillover by this pathogen in plant communities invaded by cheatgrass, we first used artificial inoculation studies to determine the relative susceptibility of seeds of cheatgrass and five co-occurring native grasses to P. semeniperda. Secondly, we quantified the pathogen reservoir (density of pathogen-killed seeds) in the spring seed bank for cheatgrass monoculture, cheatgrass-invaded native grass, and uninvaded native grass patches. Thirdly, potential pathogen spillover onto co-occurring native grasses was quantified by planting native grass seeds into field-collected seed-zone samples from each vegetation patch type and scoring subsequent seed mortality. 4. All species tested were susceptible to infection by P. semeniperda, but their vulnerability to seed death varied as a function of germination time and degree of susceptibility. 5. Seed bank samples from cheatgrass-dominated patches contained seed densities over four times higher than samples from uninvaded native grass patches, and P. semeniperda-killed seeds were also present at much higher densities, indicating that cheatgrass can function as a reservoir for P. semeniperda. Native seeds planted into seed-zone samples from cheatgrass-dominated patches were more likely to be killed by P. semeniperda than those planted into samples from uninvaded native patches. Seed mortality also varied across years, sites and host species. 6. Synthesis. Pathogen spillover onto native seeds is likely to operate within seed banks of semi-arid communities invaded by cheatgrass, and perhaps other weeds, and may have broad consequences for community structure. Our findings also demonstrate the ecological significance of multiple-host pathosystems that operate at the seed stage.

Ecological consequences of pathogen accumulation on an invasive grass

“…Similar dynamics were found when the invasive plant Chromolaena odorata hosted a soil-borne Fusarium species that had significantly greater inhibitory effects on neighboring native species than on Chromolaena itself (Mangla et al 2008). Thus, disease may decrease the growth of all plants in a community or alter the competitive balance among species (Eppinga et al 2006, Mangla et al 2008, Beckstead et al 2010. Second, rapid range expansion of invaders may allow for a period of pathogen-free growth and reproduction if pathogens experience a lag phase before colonizing invasive populations (Hawkes 2007).…”

Changes in soil microbial community and enzyme activity along an exotic plant Eupatorium adenophorum invasion in a Chinese secondary forest

“…Furthermore, these changes in soil communities usually promote the growth of invasive plants, yet inhibit native plant performance, mainly driven by the lack of soil pathogens to invasive plants in the new habitats [14,15], the preferential enrichment of microbial pathogens to native plants [16,17], or the inhibition of microbial mutualism with native plants [18][19][20].…”

Effects of sand burial and overstory tree age on seedling establishment in coastal Pinus thunbergii forests in the northern Shandong Peninsula, China

“…3b), which was similar to Cyperus pitcher (Chen and Maun 1999) and C. capitatus Vand (RedondoGómez et al 2011). The delayed germination will increase the chances of infection by seed borne and soil borne pathogens (Beckstead et al 2010), related to low germination rates at approximately 5 cm depth. Tree age significantly affected seed germination and has been reported in other studies.…”