Skip to main content
ARS Home » Northeast Area » Frederick, Maryland » Foreign Disease-Weed Science Research » Research » Research Project #429926

Research Project: Utilizing Plant Pathogens as Biological Control Agents of Invasive Weeds in the United States

Location: Foreign Disease-Weed Science Research

2019 Annual Report

Objective 1: Develop biological control agents for invasive weeds (such as Russian thistle, Russian knapweed, common crupina, and invasive Rubus species) through the identification, isolation, efficacy testing, and host-specificity evaluation of candidate plant pathogens (such as Colletotrichum salsolae, Phoma exigua, and Ramularia crupinae). [NP304, Component 2, Problem Statement 2B3] Sub-objective 1A - Isolation and identification. Sub-objective 1B - Evaluation of pathogen efficacy. Sub-objective 1C - Evaluation of disease reaction among non-target and target species. Sub-objective 1D - Develop and submit a proposal for release that describes the importance of the target weed as a pest and the efficacy and safety of a candidate pathogen. Sub-objective 1E - Participate with cooperators in release and post-release monitoring of the pathogen (under permit from state and federal regulators). Objective 2: Develop methods to rapidly screen and identify plant pathogens that have a high probability of being useful, safe, and effective as biological control agents for high priority invasive weeds (such as Russian thistle, Russian knapweed, common Crupina, and invasive Rubus species), in that they have the desired properties of high virulence and pathogenicity to the target weed, yet are sufficiently host specific that they are not a threat to beneficial and native organisms (such as crop and rare plants). [NP304, Component 2, Problem Statement 2B3] Sub-objective 2A - Refine and improve statistical approaches to risk assessments that combine disease response data with quantitative plant relationship data based upon molecular characteristics. Sub-objective 2B - Refine and apply available protocols in microscopy, physiology, and bioinformatics to improve risk assessments and clarify results of tests in Objective 1, particularly concerning non-target plant species reactions.

Exotic pathogens will be collected from symptomatic target weeds in countries where they are native, evaluated for their potential using standard plant pathology methods, and identified using both classical morphological characters and molecular sequence data. The primary target weeds will be Canada thistle, Russian thistle, Russian knapweed, yellowstarthistle, and medusahead. Other targets include, but are not limited to: Carduus thistles, milk thistle, knapweeds, common crupina, whitetop, broadleaved pepperweed, invasive blackberry, swallow-worts, cheat grass, teasel, and field and hedge bindweed. Pathogens will be evaluated for the risk associated with intended release into ecosystems containing economically and ecologically important North American plant species. Risk will be evaluated, in quarantine, based on disease reaction of species related to the target weed from a test-plant list reviewed and modified according to recommendations of regulators at the USDA Animal and Plant Health Inspection Service. In evaluating disease reaction, protocols for improved risk assessment will be developed and used. These include an advanced statistical analysis that incorporates disease reaction data with genetic relatedness, from DNA sequences, of species on the test-plant list. Output from these analyses will be best linear unbiased predictors of the disease reaction of each species. Other protocols incorporating advancements in histology, microscopy, and genetics will also be used to improve understanding of risk and non-target plant disease responses. Pathogens determined to have an adequately narrow host range will be proposed for release in the U.S.A. Proposals for release of the pathogen will be developed for review by the Technical Advisory Group for Biological Control Agents of Weeds, and subsequent development of an Environmental Assessment, declaration of Finding of No Significant Impact and issuance of federal and state permits for release. Inoculum of the pathogen will be prepared in sufficient quantity for release, and target weeds will be inoculated in the field under conditions that favor disease development and establishment. Establishment and spread of pathogens will be monitored in the field by recording disease symptoms on the target weed and re-isolating the pathogens. Damage to target weed populations and environmental factors important in pathogen establishment, efficacy and spread, will be measured.

Progress Report
Plant pathogens can be an effective management tool to curtail the spread of invasive weed species that colonize both natural and agricultural systems. Under Objective 1A, multiple endemic plant pathogens were isolated and characterized this year from diseased plant material. Within the Mid-Atlantic region, endemic plant pathogens were isolated from the invasive weed species, mile-a-minute (Persicaria perfoliata), Japanese hop (Humulus japonicus), Garlic mustard (Alliaria petiolata), Japanese honeysuckle (Lonicera japonica), and Oriental bittersweet (Celastrus orbiculatus). Approximately ten fungal pathogens were isolated from diseased Japanese hop samples, with one of the unidentified isolates showing great promise due to rapid disease progression and plant death. Isolate characterization and whole plant inoculations are ongoing. Three promising fungal pathogens isolated from diseased mile-a-minute are currently undergoing morphological and molecular characterization for identification. Additional studies are investigating diseased garlic mustard populations that appear to be bacterial and viral in origin, but molecular characterization and inoculation studies are ongoing. A severe dieback event on the invasive plant, black swallow-wort (Vincetoxicum nigrum), was observed in several counties along the east coast of Rhode Island. Collaborators at the University of Rhode Island sent diseased plant tissue for pathogen identification and characterization. Of the twenty fungal samples that were isolated from diseased swallow-wort material, five fungal isolates appear to be promising biological control agents and are undergoing further evaluations. Following pathogen identification and the completion of Koch’s Postulates, all recently collected endemic pathogens will undergo efficacy and host specificity analyses. Under Objective 1B, the previously characterized endemic fungal pathogen (Colletotrichum firoinae) of Japanese hop was inoculated under controlled greenhouse conditions with successful disease development, but due to minimal disease progression, the pathogen will no longer be pursued as a potential biological control agent of Japanese hop. Under Objective 1D & 1E, the petition to release a fungal pathogen (Ramularia crupinae) to control the federally noxious weed common crupina (petition # 13-03) is nearly through the Animal and Plant Health Inspection Service review and permitting process. In anticipation of petition approval, collaborations were initiated with the ARS scientists in Sidney, Montana to better characterize the genetic diversity of North American common crupina populations and determine present locations of common crupina infestations throughout the Western United States.


Review Publications
Bruckart, W.L., Thomas, J.L., Frederick, R.D., Aime, M.C., Abbasi, M. 2019. First report of a rust disease on Suaeda californica in California. Plant Disease.