Location: Foreign Disease-Weed Science Research2020 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.
Utilizing plant pathogens of invasive weeds as biological control agents is an effective and sustainable weed management approach that helps mitigate the spread of invasive weeds to our natural and agricultural systems. Evaluating pathogens of invasive weeds is a multistep process that includes discovery, evaluation, petitioning, release, and monitoring disease development in the field following a release. Over the course of our five-year project plan, under Objective 1, our research has pursued both foreign and endemic plant pathogens of at least 23 invasive weed species that are prevalent throughout the Eastern, Western, Southeastern, Pacific, and/or Mid-Atlantic regions of the United States, including: mile-a-minute (Persicaria perfoliata), Japanese hop (Humulus japonicus), garlic mustard (Alliaria petiolata), Japanese honeysuckle (Lonicera japonica), oriental bittersweet (Celastrus orbiculatus), Japanese stiltgrass (Microstegium vimineum), tree of heaven (Ailanthus altissima), teasel (Dipsacus fullonum), field bindweed (Convolvulus arvensis), hoary cress (Lepidium draba), Brazilian peppertree (Schinus terebinthifolia), European blackberry (Rubus fruticosus), common crupina (Crupina vulgaris), Russian Knapweed (Rhaponticum repens), Russian thistle (Salsola tragus), Canada thistle (Cirsium arvense), shiny Xylosma (Xylosma congestum), Pennsylvania smartweed (Polygonum pensylvanicum), black swallow-wort (Vincetoxicum nigrum), poison hemlock (Conium maculatum), English ivy (Hedera helix), yellow nutsedge (Cyperus esculentus), and periwinkle (Vinca minor). Moreover, over 200 foreign acquisitions in storage at the Foreign Disease-Weed Science Research Unit (FDWSRU) are under consideration for screening. In addition to our inventoried foreign isolates, dozens of endemic pathogens are stored and awaiting evaluation to determine their potential use as biological control agents. The discovery, identification, and evaluation of potential biological control candidates has resulted in the submission of three petitions to the Technical Advisory Group (TAG) for pathogen release. Of the three petitions, two proposals that petitioned for the release of fungi for the biological control of Russian knapweed (petition # 13-02) and Russian thistle (petition # 14-01) did not receive TAG recommendation. However, the petition for controlling common crupina with a fungal pathogen (petition # 13-03) was recommended by TAG and is nearly through the Animal and Plant Health Inspection Service review and permitting process. In anticipation of petition approval, protocols for inoculum preparation for field release are being developed. Furthermore, collaborations were initiated with ARS scientists in Sidney, Montana, to better characterize the genetic diversity of North American common crupina populations and determine present locations of weed infestations. Field releases and post-release efficacy studies continue to be carried out by cooperators in Colorado on a naturalized Canada thistle rust fungus. More recent research is progressing on a novel, endemic bacterial pathogen of garlic mustard that was recently discovered in Maryland. Host range studies to ensure host plant specificity are currently in progress. Under Objective 2, additional studies evaluating the efficacy and host specificity of several promising fungal candidates of invasive black swallow-wort are also being pursued.
Tancos, M.A., Frederick, R.D. 2020. First Report of Xanthomonas campestris infecting Invasive Garlic Mustard in the United States. Plant Disease. 104:1251. https://doi.org/10.1094/PDIS-09-19-1963-PDN.
Bruckart, W.L., Thomas, J.L., Abbasi, M., Aime, C.M., Frederick, R.D., Tancos, M.A. 2020. Uromyces rebeccae sp. nov., a newly described rust on the federally endangered plant, California sea-blite (Suaeda californica). Mycologia. 112(3):543-551. https://doi.org/10.1080/00275514.2020.1739602.