Location: Foreign Disease-Weed Science Research2016 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.
Evaluating beneficial plant pathogens for biological control of weeds is a multi-step process that ranges from discovery and identification, to risk assessment and evaluation, through to proposal, release, and post-release monitoring. Under objective 1A, field trips were initiated in the vicinity of Frederick, Maryland, to survey for pathogens where invasive weeds occur. Fungal isolates have been obtained from Japanese hops, mile-a-minute (Polygonum perfoliatum), tree of heaven, and vine honeysuckle (Lonicera japonica). Under objective 1B, review continued of the >5,000 foreign acquisitions in storage at the Foreign Disease-Weed Science Research Unit (FDWSRU), more than 200 of which are under consideration for initial screening. Tests were initiated on pathogens of teasel and field bindweed from this resource. Plan for these unknowns, in addition to identification of the fungus, is to satisfy Koch’s Postulates and thus facilitate decision about potential for biological control. Additional isolates of Bipolaris microstegii and populations of Japanese stiltgrass were also acquired to include in ongoing comparative studies and host range determinations. Presently, evaluations involve 15 diseases from 9 weeds, in addition to the unidentified candidates. Under objective 1C, there were two main focal points in ongoing research. First is to complete evaluation of Septoria lepidii, a candidate for control of hoary cress. Several experiments were conducted to measure damage by S. lepidii both to hoary cress and to selected related, native species that had developed symptoms. The result is that measurable damage occurred to two non-target species by S. lepidii. In tests with another Septoria species from Brazil, very good symptoms developed on Brazilian peppertree, an invasive species in Florida. Plans are to initiate studies on efficacy (damage) and risk (host range determination) with the encouragement and cooperation of scientists from the U.S. (Florida, ARS) and Brazil. Evaluation of a rust disease on European blackberry (Rubus fruticosus aggregate), invasive along the Pacific Coast, is being completed and has involved scientists from the Oregon Department of Agriculture and the Czech Republic. Outcome of the research indicates the most common species, Armenian blackberry (R. armeniacus), is not susceptible to the exotic rust fungus, whereas another morphologically similar invasive species in the U.S. (R. praecox) is susceptible. The conclusion is that additional biological control agents or other control measures will be needed for management of R. armeniacus. Under objective 1D, petition for release of a fungus to control common crupina (petition # 13-03) was approved by the Technical Advisory Group (TAG) last year and sent forward to the USDA, Animal and Plant Health Inspection Service (APHIS) for review and permitting. 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. Review of data for each of these candidates suggested some level of non-target effect, justifying additional research before considering the resubmission of either of these proposals to the TAG. Additional host range studies are in progress for each of these candidates. Evaluation of Puccinia crupinae, vs. common crupina, was completed and consideration of a proposal for release is being made. Additional risk evaluation studies are also being conducted for Uromyces salsolae, a candidate for control of Russian thistle, and it includes comparison with a native rust disease collected in Morro Bay, California, on the endangered Suaeda californica. Under objective 1E, field efficacy of an indigenous rust fungus on Canada thistle continues to be carried out by cooperators in Maryland and Colorado (>100 sites).
1. Rust fungus infects select species of invasive blackberry. Invasive blackberry causes significant economic loss and significant changes to stream and terrestrial ecosystems. In 2005, an exotic rust disease caused by Phragmidium violaceum was discovered on invasive blackberry in the U.S. that may have utility for biological control. 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 in Ft. Detrick, Maryland, revealed that the most common of the invasive blackberry, Rubus armeniacus (Armenian blackberry), is not susceptible to the strain of P. violaceum along the Pacific Coast. While this rust does not infect R. armeniacus, it may be useful as a biological control agent against other invasive blackberry species.
Bruckart, W.L., Michael, J.L., Coombs, E.M., Pirosko, C.B. 2015. Puccinia jaceae is established on Yellow Starthistle (Centaurea solstitialis) in Oregon. Plant Disease. doi: 10.1094/PDIS-09-15-1042-PDN.
Bruckart, W.L., Eskandari, F., Berner, D.K. 2016. Risk assessment and implications of common crupina rust disease for biological control. Journal of Invasive Plant Science Management. 9:33-40.