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Research Project: DISCOVERY AND CHARACTERIZATION OF PLANT PATHOGENS FOR BIOLOGICAL CONTROL OF INVASIVE WEEDS FROM THEIR NATIVE RANGE

Location: Foreign Disease-Weed Science Research

2015 Annual Report


1a. Objectives (from AD-416):
1. Discover, identify and evaluate the efficacy of exotic pathogens as classical biological control agents of invasive weeds such as Canada thistle, Russian thistle, Russian knapweed, yellow starthistle, and medusahead. 1A - Discovery 1B - Isolation and identification 1C - Evaluation of pathogen efficacy 2. Conduct risk analyses to determine the agricultural and ecological safety for the release of pathogens as classical biological control agents, such as Colletotrichum gloeosporioides f. sp. salsolae and Phoma exigua. 2A - Development of test plant lists 2B - Evaluation of disease reaction among non-target and target species 3. Develop an improved process of risk assessment using plant pathogens as a model system. 3A - Integrate DNA sequences of species on the basic test plant list into host- range evaluation with Mixed Model Equations 3B - Generate BLUPs of species on the basic test plant list 3C - Determine true host range of each pathogen by including DNA sequences and disease reaction data of other closely related species. 4. Release and, with collaborators, monitor and evaluate impact of pathogens on weed populations and non-target effects in the field. 4A - Develop and submit a proposal for release that describes the importance of the target weed as a pest and the efficacy and safety of the candidate pathogen. 4B - Participate in the regulatory decision process as needed. 4C - Participate with cooperators in release (with permit from state and federal regulators) and post-release monitoring of the pathogen).


1b. Approach (from AD-416):
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, yellow starthistle, and medusahead. Other targets include, but are not limited to: Carduus thistles, milk thistle, knapweeds, common crupina, whitetop, broadleaved pepperweed, Himalaya 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, an improved method of risk assessment will be developed and used. This improved method 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. 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.


3. Progress Report:
Developing beneficial plant pathogens for biological control of weeds included attention to 15 diseases from 10 weeds that are invasive in the United States (U.S.). Each candidate pathogen is at a different stage of a multi-step evaluation process that ranges from discovery and identification (Objective 1), to risk assessment (Obj. 2), evaluation (Obj. 3), through to proposal, release, and monitoring (Obj. 4). First inoculations were made with exotic fungi from broadleaved pepperweed (no disease), tree of heaven (a generalist, soilborne fungus; not considered a good candidate), and a leaf killing disease of mile-a-minute, now under consideration for additional tests. Early evaluation of two leaf-spot diseases of field bindweed has identified a fungal candidate as worthy of further tests. One of two leaf-spotting fungal candidates from Brazilian peppertree has also been established at this lab, and plans have been made with cooperators in the US (Florida) and Brazil for tests to validate these results. Of five species of invasive blackberry (Rubus fruticosus Aggregate) along the Pacific Coast, the most common, Armenian blackberry (R. armeniacus), is not susceptible to an exotic rust fungus strain that attacks another morphologically similar invasive species in the US, thus identifying need for additional biological control agents of R. armeniacus from Europe or for other control measures. Also, pathogen collections at the Foreign Disease-Weed Science Research Unit and the European Biological Control Lab were reviewed for untested foreign candidates. Since the mid-1970s, more than 5,000 pathogen accessions have been stored, and although many have been completely processed, several remain that may be good candidate pathogens of very important weeds, e.g., thistles and knapweeds, field bindweed, swallow-worts, teasel, hoary cress, Russian thistle, and some grasses. Thus far, more than 200 accessions are being considered for initial screening. Pending results of initial evaluations and identification, decision about further assessments, e.g., about risk, can be made. (Obj. 1). Risk assessment of a leaf-spotting disease was initiated for biological control of hoary cress. Significant reduction in root biomass was recorded on the target, but clear symptoms of disease were also noted on four related native species. Studies are in process to evaluate and clarify the meaning of non-target responses. (Obj. 2, 3) Petition for release of a fungus to control common crupina (petition # 13-03) remains in review by the Technical Advisory Group (TAG) following response to reviewer questions. Field efficacy of an indigenous rust fungus on Canada thistle is being carried out in Maryland and Colorado (>100 sites). Petitions for release of two other fungi for biological control of Russian knapweed (petition # 13-02) and Russian thistle (petition # 14-01) did not receive recommendation by the TAG and thus were not sent up for final APHIS review. Additional host range studies are in progress for each of these proposals, and resubmissions are being considered pending results of additional tests and evaluation of reviewer questions. (Obj. 2, 4).


4. Accomplishments
1. Rubus armeniacus is not susceptible to rust disease by Phragmidium violaceum in the United States. Invasive blackberry causes significant economic loss and significant changes to stream and terrestrial ecosystems. In 2005, an exotic rust disease that is potentially important for biological control was discovered on invasive blackberry in the United States (U.S.). Five species of invasive Rubus are established in the U.S., but surveys of blackberry thickets in Oregon and artificial inoculations of accessions in greenhouse tests by ARS researchers at Ft. Detrick, MD, revealed that the most common of the invasive blackberry, R. armeniacus(Armenian blackberry), is not susceptible to the strain of P. violaceum along the Pacific Coast. Thus a critical gap has been identified in pursuit of biological control or other approaches to blackberry management, particularly concerning Armenian blackberry.


Review Publications
Berner, D.K., Eskandari, F., Cavin, C.A., Dubin, H. 2015. Fulfillment of Koch’s postulates and partial host range of Septoria lepidii Desm., a fungal pathogen for potential biological control of hoary cress (Lepidium spp.). Biocontrol Science and Technology. 25:732-737.