Community Ecology of Fungal Pathogens on Bromus tectorum

Abstract

Bromus tectorum L. (cheatgrass or downy brome) presents a rich resource for soil microorganisms because of its abundant production of biomass, seeds, and surface litter. Many of these organisms are opportunistic saprophytes, but several fungal species regularly found in B. tectorum stands function as facultative or obli-gate pathogens. These organisms interact dynamically with abiotic factors such as interannual variation in weather, with other soil microorganisms, with their hosts, and with each other to create spatially and temporally varying patterns of endemic or epidemic disease. Five principal soilborne pathogens, Ustilago bullata Berk. (head smut pathogen), Tilletia bromi (Brockm.) Nannf. (chestnut bunt pathogen), Pyrenophora semeniperda (Brittlebank & Adams) Shoemaker (black fi ngers of death pathogen), Fusarium Link sp. n. (Fusarium seed rot pathogen), and a new species in the Rutstroemiaceae (bleach blonde syndrome pathogen), are known to have sometimes major impacts on B. tectorum seed bank dynamics, seedling emer-gence, and seed production. These pathogens exhibit niche specialization, so that they are rarely in direct competition. They sometimes interact to increase the total impact on B. tectorum stand structure, which can result in stand failure or " die-off. " Die-offs represent areas where B. tectorum has been controlled by natural processes, suggesting that these areas might be suitable targets for restoration. Naturally occur-ring fungal pathogens that can have a strong negative impact on B. tectorum success have also been considered as candidate organisms for B. tectorum biocontrol using an augmentative mycoherbicidal strategy. Monocultures of exotic annuals in the Bromus genus represent a valuable resource for many pathogens, which in turn can have strong impacts on stand dynamics, although stands are remarkably well buffered from these impacts in most years. Most of the information we have on stand dynamics and almost all the information on the effects of fungal pathogens come from work with B. tectorum , and this infor-mation forms the basis for most of the discussion that follows. What we know about other annual bromes largely confi rms that they have similar life histories and seed bank dynamics. For Bromus tectorum L. (downy brome or cheatgrass), very high seed production, effi cient seed dispersal, and the ability to form short-persistent (1–2 years) seed banks contribute to the buffering of disease impacts (Meyer et al. 2007a ; Smith et al. 2008). Productivity varies dramatically from year to year due to variation in precipitation, but years without seed production are rare. The seeds of B. tectorum would seem to present a major resource for granivores , but native rodents do not prefer them, and even harvester ants and birds apparently consume only a small fraction of the crop (Connolly et al. 2014). There is no indication of secondary metabolites produced by the plant or its endophytes that would deter granivores, although this has not been specifi cally examined. Fall seed bank densi-ties are generally similar to seed production estimates, suggesting that few seeds are removed. Densities range from 5000 to as high as 50,000 seeds m −2 , which is approximately equivalent to a grain yield of up to 1500 kg ha −1 (Meyer et al. 2007a ; Smith et al. 2008). However, this surfeit of seeds does not go to waste. A diverse community of soilborne pathogens is ready to take advantage of this resource as soon as the fi rst rains arrive following dispersal. Bromus tectorum seeds are dormant in early summer at dispersal and lose dormancy through dry afterripening under dry summer conditions (Christensen et al. 1996). They are poised to germinate rapidly with the fi rst substantial rains of autumn, but if rainfall is intermittent or insuffi cient to trigger complete germination, or if precipitation arrives late in the fall, ungermi-nated seeds may enter secondary dormancy (Allen et al. 2010). These dormant seeds comprise the persistent seed bank. Seedling emergence can take place any time from late summer through early spring depending on precipitation patterns. Regardless of emergence time, the plants bolt and set seed in spring, that is, they exhibit a facultative winter annual life cycle. The seeds, more accurately referred to as fl orets containing single caryopses, are quite large, weighing an average of 3 mg. As with most grasses, the primary storage compound is starch. Plant ecologists and range managers working in B. tectorum -dominated systems have long been aware of the presence of fungal pathogens in B. tectorum stands and S.E. Meyer et al.