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United States Department of Agriculture

Agricultural Research Service

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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

2011 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
Under objective 1, several new pathogens were discovered in 2011 from Russia and Turkey. One of these pathogens, Colletotrichum lineola from swallow-wort in Russia is a potentially important biological control agent of two species of this weed in the Northeastern U.S. Under objective 2, two previously discovered fungal pathogens, Colleototrichum gloeosporioides and Uromyces salsolae from Russian thistle have been thoroughly evaluated for host range and safety. The latter pathogen has been approved by the Technical Advisory Group (TAG) of APHIS and final review of the proposed action is under consideration by APHIS for release in the U.S. A petition for release of C. gloeosporioides will be prepared in the coming year (This research relates to objective 4 of the project). As part of the host range determination process, a new approach integrating DNA sequences and disease reaction data was developed (This research relates to objective 3 of the project). This approach, which generates Best Linear Unbiased Predictors (BLUPs) of disease reaction among non target plant species, was presented at the Biological Control for Nature conference in Northhampton, MA. The author of the presentation was subsequently invited to develop a manuscript on this for the journal BioControl (This research relates to objective 3 of the project). The fungal pathogens, Puccinia crupinae and Ramularia crupinae on common crupina are still in the host range determination process. The facultative pathogenic fungus Boeremia exigua (synonym=Phoma exigua) causes significant damage on Russian knapweed and causes no significant non-target effects that we have so far determined (This research relates to objective 2 of the project). A petition to release this fungus will be prepared in the near future (This research relates to objective 4 of the project). The naturalized rust fungus Puccinia punctiformis is being evaluated in field tests for its potential to control Canada thistle. In 2011 research was continued under an NRI grant on biological control of Canada thistle and was extended to the European Biological Control Laboratory substation and Aristotle University both in Thessaloniki, Greece. In both Maryland and Greece, epidemics of the rust were successfully initiated. Previous inoculations at FDWSRU showed a 47 times increase in systemically diseased shoots. It is now possible to routinely establish systemic disease on Canada thistle which should lead to implementation of successful biological control with the rust (This research relates to objective 1 of the project). A survey in Oregon for Himalaya blackberry rust resulted in acquisitions of 32 plants and 10 isolates of Phragmidium violaceum. Reciprocal inoculations were initiated in order to learn if there is variability in susceptibility and aggressiveness of P. violaceum within Oregon (This research relates to objective 1 of the project).


4.Accomplishments
1. Improved methodology for host range determination. Determining host range of classical biological control agents has been a cumbersome process, often involving tests of 50 or more non-target species under conditions that make it difficult to predict results in the field. Recently ARS researchers at Ft. Detrick, Maryland, developed a procedure to generate best linear unbiased predictors (BLUPs), based on disease reaction and plant species DNA sequences, to determine the probable host-range, in the field, of plant pathogens for classical biological control of weeds. Results of host-range tests using this approach have been published in several articles, and the approach was presented at the Biological Control for Nature conference in Northhampton, MA. Comparison of BLUPs with actual damage on non-target plants indicated that BLUPs successfully predict damage by pathogens to non-target plants while having a lower probability of erroneously predicting a species not susceptible.

2. Successful initiation of in-field epidemics of Canada thistle rust. Canada thistle rust (Puccinia punctiformis) is either native or naturalized in all areas of the world where the weed occurs. Despite being studied for over 100 years the disease cycle had not been completely understood and this had prevented epidemics of the rust from being successfully initiated and used for biological control. Our research results indicate that inoculation of Canada thistle rosettes with teliospores or teliospore-bearing leaves in September (in the northern hemisphere) consistently leads to establishment of systemically diseased shoots the following spring and epidemics of the rust in subsequent seasons. These successes indicate that epidemics of the rust fungus can be initiated and have the potential to control Canada thistle. This has now been accomplished in Maryland and in Greece, and, through collaboration epidemics are being initiated in New Zealand, Russia, and Turkey.


Review Publications
Berner, D.K., Cavin, C.A. 2011. Finalizing host range determination of a weed biological control pathogen with BLUPs and damage assessment. Biocontrol. DOI:10.1007/S10526-011-9399-X.

Last Modified: 10/1/2014
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