USDA-ARS researchers at Oregon and Washington State Universities have recently publicized* the results of a long-term field trials investigating the utility of native soil bacteria for selective control for the invasive annual grasses cheatgrass (Bromus tectorum, also known as “downy brome”), jointed goatgrass (Aegilops cylindrica), and medusahead (Taeniatherum caput-medusae).
Research into this new management tool started in the 1980s in eastern Washington when Pseudomonas (a common genus of soil bacteria native to much of the western United States and elsewhere) was found on the roots of winter wheat that had demonstrated poor early spring growth (but without negatively affecting yields). Researchers wondered if the suppressive effect of the bacteria would be similar or worse for species that are closely related to wheat, such as Eurasian winter annual grasses, which negatively affect biodiversity and productivity of rangelands and croplands on millions of acres in the Western U.S. To investigate this possibility, researchers have tested over 20,000 potential bacterial candidates through greenhouse experiments and field trials over the last 20 years located around the inland Pacific Northwest.
Recently, they made a breakthrough with Pseudomonas fluorescens strain ACK55; a Gram-negative rod. Treating ACK55 similarly to a preemergence herbicide, researchers found that at a use rate of one pint of active culture (1 x 108 colony forming units mL-1) per acre, sprayed in the fall prior to emergence of the target species, P. fluorescens ACK55 is extremely effective at controlling cheatgrass, jointed goatgrass, and medusahead by inhibiting root cell elongation and tiller initiation. In fact, a single application appears to be adequate for almost eliminating these annual weeds from the seed bank in the long-term (4-5 years). Also similarly to preemergence herbicides, researchers have stressed that several other factors, such as soil properties, temperatures, and precipitation, can mediate treatment effectiveness, specifically noting that the bacteria may not perform as well under hot and arid conditions experienced in summer months.
Adding to the potential for its utility in an Integrated Pest Management (IPM) program for Eurasian winter annual grasses, tests of over 200 non-target plants demonstrate that the bacteria do not affect crop species or native species, meaning that P. fluorescens is selective as well. In the presence of P. fluorescens, crops and natives are able to competitively dominate the suppressed weeds and increase in cover. Research has also found that P. fluorescens does not inhibit fish, birds, bees, and other insects. Moreover, since the strain has no known anti-fungal or anti-bacterial activity, it is not expected to disturb the native microbial communities.
P. fluorescens ACK55 is currently undergoing the EPA registration process, which can take about 17 months. If acceptable by EPA standards, the inoculate could be available for widespread use. Other than state-specific restrictions on using pathogens for invasive plant control, there are also the hurdles of propagation- what is the cost of growing enough of the bacteria to supply it to farmers, ranchers and conservationists interested in using it to control invasive winter annual grasses in the western U.S.? Because it's likely to be cost-prohibitive to apply the bacteria to all of the millions of acres currently infested by these problem species, the researchers suggest that a more targeted approach be considered. Direct application of the bacteria to leading edges of existing infestations to prevent spread into new areas, and prioritize application to recently burned areas where the existing population of invasive grasses is restricted to the seed bank, where the bacteria can have the largest impact.
Additionally, as previously mentioned, there are likely site specific factors that will limit effectiveness of the bacteria for invasive grass control. As we often hear restoration ecologists and weed managers stress, P. fluorescens likely has great utility that will be most effective when used as part of an IPM program. In fact, as the researchers themselves suggest, the bacteria is unlikely to be successful if simply applied to invasive grass monocultures, as they will simply regenerate given sufficient time. Instead, they suggest that post emergence herbicides be applied to reduce the standing crop of invaders, while also applying the bacteria to attack germinating seedlings and provide seed bank control over the longer term. Finally, seeding with natives or desirable forage species can prevent recolonization.
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*Coverage by the NYT (http://www.nytimes.com/2015/10/06/science/researcher-finds-way-to-fight-cheatgrass-a-western-scourge.html)
Coverage by High Country News (https://www.hcn.org/articles/researchers-find-formidable-foe-for-invasive-cheatgrass)