Project Overview
Canada thistle (Cirsium arvense) is among the most difficult weeds to manage in temperate agricultural production systems worldwide. A highly-aggressive perennial weed, Canada thistle reproduces both sexually (by seed) and vegetatively, through horizontal growth of its root system. Traditional methods of managing Canada thistle infestations include tillage, mowing, and repeated herbicide applications; however, additional NOP-compliant methods of controlling Canada thistle populations are much sought-after.
Puccinia punctiformis is a rust fungus that targets Canada thistle. Puccinia can systemically infect individual Canada thistle plants, resulting in permanent infection of the root system. Some of the newly emerged Canada thistle shoots from systemically diseased roots may appear tall, spindly, deformed, and with leaves bearing orange-colored fungal spermatogonia that can be carried by wind, rain, or pollinators (i.e. insects) to other Canada thistle plants. Canada thistle plants are most susceptible to infection by Puccinia during the fall, when the young plants are still rosettes.
Due to Puccinia‘s natural parasite-host relationship with Canada thistle, the fungus has received attention as a potential biological control agent. This study assessed the efficacy of Puccinia inoculation methods (targeted vs. broadcast), inoculum quantity, and inoculation timing to successfully induce rust disease outbreaks in Canada thistle populations.
Farmer Takeaways
- The rust fungus Puccinia punctiformis can act as an effective, natural biological control agent against Canada thistle in temperate agroecosystems and can establish systemic rust disease outbreaks when inoculated via targeted and broadcast methods.
- Puccinia punctiformis may be most effective as a biological control agent against Canada thistle when inoculated frequently at high doses into dense thistle stands.
Project Objectives and Approach
To assess the efficacy of Puccinia inoculation methods, inoculum quantity, and inoculation timing to successfully induce systemic rust disease outbreaks (epiphytotics) in Canada thistle populations
- Over the course of eight years (2013-2021), a total of 181 Canada thistle-infested sites in Colorado were identified for inclusion in the study.
- Telia*-containing leaves from Puccinia-infected Canada thistle patches were collected, dried, and ground into a coarse powder to be used for inoculation at all study sites.
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- Telia: brown spores that form on the undersides of infected Canada thistle leaves in the fall during Puccinia‘s dormant, overwintering life stage. Telia are an important for spreading rust fungal infection to new plants.
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- Inoculation timing was coordinated to coincide with optimal temperature and dew point for systemic Puccinia infection, when leaf-surface moisture was likely.
- At each study site, a 12×4 m transect was marked for Canada thistle x Puccinia monitoring, with transect placement targeting healthy, dense stands of Canada thistle. Transects were inoculated, with inoculation method treatments including: (1) Targeted; ~0.5g inoculum was placed in the crown of each Canada thistle rosette observed; used for sites with ~40-50 rosettes present in transect; (2) Broadcast; inoculum was hand-distributed throughout transect; used for sites with no rosette availability; and (3) Targeted + Broadcast; combined inoculation approach; used for sites with an intermediate rosette availability.
- Sites were monitored annually for evidence of systemic disease (i.e., spore-bearing diseased shoots).
To monitor the impacts of Puccinia infection on Canada thistle population densities
- Sites were monitored annually for Canada thistle shoot density. Data was collected on healthy and infected Canada thistle stems within each transect, as well as weed stage and disease progression stage.
Key Findings
Canada thistle stem counts declined at most study sites from initial measurement to final observation over the eight-year study period, due to climatic/geographic factors and Puccinia rust infection
- Overall Canada thistle stem counts declined from the time of initial measurement to final observation (~8 years) at 77% of study sites.
- Annual decreases in Canada thistle stem counts were correlated to the previous year’s thistle population density, suggesting that Puccinia rust may spread more effectively between plants that are spaced closely together and may be most effective as a biological control agent in dense Canada thistle stands.
- Climatic/geographic factors* leading up to the inoculation and data collection events at each study site accounted for between 22 and 53% of the variance in annual Canada thistle stem counts between treatments, indicating that Canada thistle and Puccinia performance may vary significantly based on external conditions.
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- *The climatic/geographic factors evaluated in the study included: maximum temperature, minimum temperature, average temperature, total precipitation, water vapor pressure, relative humidity, dew point, shortwave radiation flux, and elevation.
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Larger doses of Puccinia inoculum, as well as more frequent inoculation may be more effective at reducing annual Canada thistle stem counts
- Annual Canada thistle stem counts were positively correlated with time since last inoculation and negatively correlated with quantity of inoculum at the last treatment, suggesting that frequent, high-dose Puccinia inoculation may be the most effective at curbing the spread of Canada thistle.
Aboveground evidence of systemic Puccinia rust disease outbreaks were only observed in a small portion of the study sites, but additional tools for detecting belowground (root system) infection may be necessary to understand asymptomatic Puccinia infection mechanisms
- Systemic Puccinia rust disease outbreaks, which occur when the rust fungus spreads throughout the entire plant’s vascular system (not just to localized spots on the leaves), were only observed in 17% of treated study sites. Previous studies have indicated that infected Canada thistle can be asymptomatic aboveground for a time while exhibiting signs of belowground (root system) infection, so the researchers postulate that the percentage of sites with systemic disease outbreaks may have been higher than the observed 17%; however, belowground infection detection methods and diagnostic tools are needed.
Resources
Bean, D. W., Gladem, K., Rosen, K., Blake, A., Clark, R. E., Henderson, C., Kaltenbach, J., Price, J., Smallwood, E. L., Berner, D. K., Young, S. L., & Schaeffer, R. N. (2024). Scaling use of the rust fungus Puccinia punctiformis for biological control of Canada thistle (Cirsium arvense (L.) Scop.): First report on a U.S. statewide effort. Biological Control, 192, 105481.
Read MoreMontana State University Article - "Thistle Rust: A Potential Biocontrol Agent to Help in the Management of Canada Thistle"
Read MoreLocation
ColoradoCollaborators
Kristi Gladem, Colorado Department of Agriculture
Karen Rosen, Colorado Department of Agriculture
Alexander Blake, EcoData Technology
Robert Clark, EcoData Technology
Caitlin Henderson, Utah State University
John Kaltenbach, Colorado Department of Agriculture
Joel Price, Oregon Department of Agriculture
Emily Smallwood, USDA
Dana Berner, USDA
Stephen Young, USDA
Robert Schaeffer, Utah State University
Region
Plains
Topic
Weed Management
Category
Grain and Field Crops
Year Published
2024
