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ARS Home » Northeast Area » Frederick, Maryland » Foreign Disease-Weed Science Research » Research » Research Project #429926

Research Project: Utilizing Plant Pathogens as Biological Control Agents of Invasive Weeds in the United States

Location: Foreign Disease-Weed Science Research

2017 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.

Progress Report
Under objective 1, several endemic fungi have been isolated this year from infected oriental bittersweet (Celastrus orbiculatus) plants from Pennsylvania. Seeds have been obtained of C. orbiculatus and C. scandens (American bittersweet) for inoculations. An endemic fungal pathogen isolated from infected Japanese hops (Humulus japonicas) was identified recently as Colletotrichum firoinae using morphological characteristics and nucleic multi-locus sequence analysis. Tests are underway with various spray adjuvants to increase pathogenicity on Japanese hops. Two endemic fungal pathogens isolated from Japanese honeysuckle (Lonicera japonica), were inoculated on Japanese honeysuckle seedlings but no infection was observed. These pathogens are most likely saprophytes and will not be pursued further as potential biological control agents of honeysuckle. An endemic fungal pathogen was isolated from infected mile-a-minute (Persicaria perfoliata), and it was identified as a putative Phoma sp. based up morphology and nucleic sequence analysis. The pathogen was re-isolated following inoculation of mile-a-minute plants and will be explored further as potential biological control agent. Host range testing on native, non-target plants was concluded with Septoria lepidii, a candidate for control of hoary cress by measuring shoot and root weights of inoculated plants. Another Septoria species from Brazil that previously produced disease symptoms on Brazilian peppertree, an invasive species in Florida, was evaluated further by inoculating additional accessions of Brazilian peppertree. However, very little disease was observed on the four additional accessions of Brazilian peppertree. As a result, the project was terminated due to low levels of infection and minimal potential for biological control. Additional risk evaluation studies were conducted for Uromyces salsolae, a candidate for biological control of Russian thistle (Salsola tragus), by comparison with a native rust disease collected in Morro Bay, California, on the endangered native plant Suaeda californica. Inoculation results indicated U. salsolae is specific to S. tragus, while the rust isolated from S. californica is specific to S. californica. Additional inoculations of U. salsolae on the native, sympatric plant, Sarcocornia pacifica, revelaed that S. pacifica was not susceptible to U. salsolae. These experiments addressed reviewer concerns of non-target effects on a proposal submitted for release of U. salsolae. In response to regulator comments on a petition to release Colletotrichum salsolae, a potential biological control agent for Russian thistle (Salsola tragus), target (S. tragus) and non-target (Salicornia rubra) plants were inoculated with serial dilutions of C. salsolae. Likewise, serial dilutions of Boeremia exigua were inoculated onto the target weed Russian knapweed (Acroptilon repens) and the non-target, Safflower (Carthamus tinctorious) to address regulators concerns of no-target effects.


Review Publications
Bruckart, W.L., Eskandari, F., Michael, J.L., Smallwood, E.L. 2017. Differential aggressiveness of Bipolaris microstegii and B. drechsleri on Japanese Stiltgrass. Invasive Plant Science and Management. 10:44-52.