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ARS Home » Northeast Area » Frederick, Maryland » Foreign Disease-Weed Science Research » Research » Research Project #420470


Location: Foreign Disease-Weed Science Research

2013 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:
New diseases of invasive weeds in the U.S. were discovered in 2013. Nineteen pathogens, from diseases of Convolvulus arvensis and Calystegia sepium collected in Turkey, were isolated and proofs of pathogenicity completed. Two pathogens, Phoma macrostoma and Stagonospora convolvuli, show particular promise as biological control agents of these weeds. In Kyrgyzstan, dying Ailanthus altissima trees were found and diseased tissue is being sent to our laboratory for pathogen isolation and testing. The rust fungus Puccinia punctiformis was evaluated in field tests, in Maryland, Greece, Russia, and New Zealand for its potential to control Canada thistle (CT). In all locations, epiphytotics (plant disease outbreaks analogous to epidemics) of systemic disease caused by the rust were successfully initiated, demonstrating that epiphytotics of systemic disease can be routinely established on CT. Data from these locations show that thistle density consistently, and significantly, declined following establishment of systemic disease. A specific cooperative agreement with the Colorado Dept. of Agriculture has been established to deploy this rust in Colorado. A new disease caused by Bipolaris microstegii was identified on Japanese stiltgrass (JSG) from Maryland. Koch’s postulates have been satisfied and results of a study involving 12 isolates of Bipolaris and four accessions of JSG suggest a complex of two Bipolaris species and possibly two JSG types. Tests of Armenian blackberry rust, caused by Phragmidium violaceum, continue to show a differential response among Armenian blackberry accessions from the U.S. supporting the conclusion that Armenian blackberry in the U.S. is really a complex of Rubus species that differ in susceptibility. The rust pathogen, Uromyces salsolae from Russian thistle (RT) has been approved by the Technical Advisory Group of APHIS and final review and consultation with our laboratory is being conducted by APHIS for release in the U.S. A petition for release of Colletotrichum gloeosporioides on RT is being developed following re-naming of the pathogen to C. salsolae. Petitions for release of two facultative pathogenic fungi, Boeremia exigua and Ramularia crupinae, which cause significant damage on Russian knapweed (RK) and common crupina, respectively, are under review by the Technical Advisory Group of APHIS (TAG). As part of the host range determination process, ITS sequences of target (Crupina vulgaris, Rhaponticum repens, and Salsola tragus) and non-target weeds were obtained from GenBank and generated at FDWSRU. Genetic distance matrices and phylograms among target and non-target plants were constructed on the basis of these sequences, and Best Linear Unbiased Predictors (BLUPs) of species were generated after integrating disease reaction data and genetic distance matrices into Mixed Model Equations. The host range determination process has been completed for Boeremia exigua var. rhapontica on RK and Ramularia crupinae on common crupina. Only the target weeds were found susceptible to the pathogens.

4. Accomplishments
1. Successful initiation of epiphytotics (plant disease outbreaks analogous to epidemics) of Canada thistle rust. Canada thistle rust (Puccinia punctiformis) is a potential biological control for Canada thistle, considered to be one of the world’s worst weeds. Despite being studied for over 120 years the disease cycle had not been completely understood, and this had prevented the rust from being used for biological control. ARS researchers at Ft. Detrick Maryland have found that inoculation of Canada thistle rosettes with fungal spores or spore-bearing leaves in September (in the northern hemisphere) and February (in the southern hemisphere) consistently leads to significant disease in subsequent seasons. Collaboration between Foreign Disease-Weed Science Research Unit and scientists in Greece, New Zealand, and Russia demonstrated that disease caused by the fungus can be routinely established in these countries and the USA following this procedure. Canada thistle density consistently, and significantly, declined in these locations following establishment of systemic disease. As a result of this discovery, widespread implementation of successful biological control of Canada thistle is now possible in the USA and worldwide. A specific cooperative agreement has been established with the Colorado Department of Agriculture to deploy this rust in Colorado, where Canada thistle is a major weed problem.

Review Publications
Berner, D.K., Cavin, C.A., Erper, I., Tunali, B. 2012. First report of anthracnose of mile-a-minute (Persicaria perfoliata) caused by Colletotrichum gloeosporioides in Turkey. Plant Disease. 96:1578.