Integrated management of Canada thistle (Cirsium arvense) in the Great Plains and Intermountain West using a biocontrol agent (Puccinia suaveolens)

Integrated management of Canada thistle (Cirsium arvense) in the Great Plains and Intermountain West using a biocontrol agent (Puccinia suaveolens) | bioRxiv /* */ /* */ <!-- <!-- /*! * yepnope1.5.4 * (c) WTFPL, GPLv2 */ (function(a,b,c){function d(a){return"[object Function]"==o.call(a)}function e(a){return"string"==typeof a}function f(){}function g(a){return!a||"loaded"==a||"complete"==a||"uninitialized"==a}function h(){var a=p.shift();q=1,a?a.t?m(function(){("c"==a.t?B.injectCss:B.injectJs)(a.s,0,a.a,a.x,a.e,1)},0):(a(),h()):q=0}function i(a,c,d,e,f,i,j){function k(b){if(!o&&g(l.readyState)&&(u.r=o=1,!q&&h(),l.onload=l.onreadystatechange=null,b)){"img"!=a&&m(function(){t.removeChild(l)},50);for(var d in y[c])y[c].hasOwnProperty(d)&&y[c][d].onload()}}var j=j||B.errorTimeout,l=b.createElement(a),o=0,r=0,u={t:d,s:c,e:f,a:i,x:j};1===y[c]&&(r=1,y[c]=[]),"object"==a?l.data=c:(l.src=c,l.type=a),l.width=l.height="0",l.onerror=l.onload=l.onreadystatechange=function(){k.call(this,r)},p.splice(e,0,u),"img"!=a&&(r||2===y[c]?(t.insertBefore(l,s?null:n),m(k,j)):y[c].push(l))}function j(a,b,c,d,f){return q=0,b=b||"j",e(a)?i("c"==b?v:u,a,b,this.i++,c,d,f):(p.splice(this.i++,0,a),1==p.length&&h()),this}function k(){var a=B;return a.loader={load:j,i:0},a}var l=b.documentElement,m=a.setTimeout,n=b.getElementsByTagName("script")[0],o={}.toString,p=[],q=0,r="MozAppearance"in l.style,s=r&&!!b.createRange().compareNode,t=s?l:n.parentNode,l=a.opera&&"[object Opera]"==o.call(a.opera),l=!!b.attachEvent&&!l,u=r?"object":l?"script":"img",v=l?"script":u,w=Array.isArray||function(a){return"[object Array]"==o.call(a)},x=[],y={},z={timeout:function(a,b){return b.length&&(a.timeout=b[0]),a}},A,B;B=function(a){function b(a){var a=a.split("!"),b=x.length,c=a.pop(),d=a.length,c={url:c,origUrl:c,prefixes:a},e,f,g;for(f=0;f<d;f++)g=a[f].split("="),(e=z[g.shift()])&&(c=e(c,g));for(f=0;f<b;f++)c=x[f](c);return c}function g(a,e,f,g,h){var i=b(a),j=i.autoCallback;i.url.split(".").pop().split("?").shift(),i.bypass||(e&&(e=d(e)?e:e[a]||e[g]||e[a.split("/").pop().split("?")[0]]),i.instead?i.instead(a,e,f,g,h):(y[i.url]?i.noexec=!0:y[i.url]=1,f.load(i.url,i.forceCSS||!i.forceJS&&"css"==i.url.split(".").pop().split("?").shift()?"c":c,i.noexec,i.attrs,i.timeout),(d(e)||d(j))&&f.load(function(){k(),e&&e(i.origUrl,h,g),j&&j(i.origUrl,h,g),y[i.url]=2})))}function h(a,b){function c(a,c){if(a){if(e(a))c||(j=function(){var a=[].slice.call(arguments);k.apply(this,a),l()}),g(a,j,b,0,h);else if(Object(a)===a)for(n in m=function(){var b=0,c;for(c in a)a.hasOwnProperty(c)&&b++;return b}(),a)a.hasOwnProperty(n)&&(!c&&!--m&&(d(j)?j=function(){var a=[].slice.call(arguments);k.apply(this,a),l()}:j[n]=function(a){return function(){var b=[].slice.call(arguments);a&&a.apply(this,b),l()}}(k[n])),g(a[n],j,b,n,h))}else!c&&l()}var h=!!a.test,i=a.load||a.both,j=a.callback||f,k=j,l=a.complete||f,m,n;c(h?a.yep:a.nope,!!i),i&&c(i)}var i,j,l=this.yepnope.loader;if(e(a))g(a,0,l,0);else if(w(a))for(i=0;i (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0];var j=d.createElement(s);var dl=l!='dataLayer'?'&l='+l:'';j.src='//www.googletagmanager.com/gtm.js?id='+i+dl;j.type='text/javascript';j.async=true;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-M677548'); Skip to main content Home About Submit ALERTS / RSS Search for this keyword Advanced Search New Results Integrated management of Canada thistle ( Cirsium arvense ) in the Great Plains and Intermountain West using a biocontrol agent ( Puccinia suaveolens) View ORCID Profile Caitlin Henderson , View ORCID Profile Kristi Gladem , View ORCID Profile Stephen L. Young , View ORCID Profile Dan W. Bean , View ORCID Profile Robert N. Schaeffer doi: https://doi.org/10.1101/2025.03.19.644225 Caitlin Henderson 1 Department of Biology, Utah State University , Logan, UT, USA Roles: Graduate Research Assistant Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Caitlin Henderson For correspondence: caitlin.henderson{at}usu.edu kristi.gladem{at}state.co.us Kristi Gladem 2 Palisade Insectary, Colorado Department of Agriculture , Palisade, CO, USA Roles: Biological Control Specialist Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Kristi Gladem For correspondence: caitlin.henderson{at}usu.edu kristi.gladem{at}state.co.us Stephen L. Young 3 United States Department of Agriculture, Agricultural Research Service , Beltsville, MD, USA Roles: National Program Leader Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Stephen L. Young Dan W. Bean 4 Palisade Insectary, Colorado Department of Agriculture , Palisade, CO, USA Roles: Director Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Dan W. Bean Robert N. Schaeffer 5 Department of Biology, Utah State University , Logan, UT, USA Roles: Assistant Professor Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Robert N. Schaeffer Abstract Full Text Info/History Metrics Preview PDF Abstract Canada thistle [ Cirsium arvense (L.) Scop.] is an invasive perennial plant that threatens agricultural landscapes and natural ecosystems worldwide. The extensive regenerative root system of Canada thistle complicates control efforts, with current strategies having limited success. Puccinia suaveolens (Pers.) Rostr (syn. Puccinia punctiformis (F. Strauss) Rohl), an obligate biotrophic rust fungus, has shown potential as a biological control agent by systemically infecting the root system, reducing root mass and shoot growth, and limiting vegetative regeneration; however, its efficacy when integrated with other control methods remains unclear. We conducted experiments from 2020 to 2022 at two sites in Colorado and Utah to evaluate P. suaveolens efficacy when applied alone and in combination with mowing, tillage, and herbicide. Treatments were applied in Fall (2020 and 2021), with monitoring of thistle stem density, vegetative cover, as well as P. suaveolens incidence before and after treatments through 2022. While P. suaveolens alone contributed to a decrease in thistle density, it was less effective compared to herbicide treatments, and its impact when integrated with mowing or tillage was inconsistent. Herbicide application (alone and when combined with P. suaveolens ) generated the greatest immediate reduction in thistle stem density and vegetative cover, although it resulted in the greatest amount of bare ground exposure. Grass coverage present within treated plots varied significantly between treatments, ranging from 0-75%, with the highest percentage observed in herbicide treatments compared to control and tillage in both years. Forb cover remained below 10% across treatments and years. Although P. suaveolens continues to show promise as a biological control agent, further research is needed to improve efficacy, and optimize integration with other control strategies. Introduction Cirsium arvense (Asteraceae), commonly referred to as Canada thistle, is a pervasive perennial weed found throughout temperate regions globally. In the Western United States (U.S.), Canada thistle ranks as one of the highest among the most commonly occurring noxious weeds, posing significant threats to both managed and natural landscapes ( Bodo Slotta et al. 2010 ; Moore et al. 1975; Nuzzo 1997 ). In agricultural systems, Canada thistle competes for light, nutrients, and water, leading to reduced crop yield and quality, generating significant economic losses for producers (Jacobs et al. 2006; Moyer et al. 1991 ; O’Sullivan 1982). In natural systems, it similarly competes with and displaces native plant species ( Jacobs 2006 ). Canada thistle is commonly found in disturbed areas, including roadsides, streambanks, ditches, clear cuts, forest openings, and wet or wet-mesic grasslands and rangelands, as part of the initial post-disturbance community ( Morishita 1999 ; Nuzzi 1997). It is prevalent in nearly every upland herbaceous community within its range, particularly prairie communities and riparian habitats ( Nuzzo 1997 ). Canada thistle survives and spreads through two reproductive strategies: sexual reproduction via seeds and clonal vegetative growth. Seeds are small and light, with highly variable germination success (Hodgson 1964; Moore 1975 ; Nuzzo 1997 ); however seeds are important for range expansion as shown by the genetic diversity of North American populations ( Bodo Slotta et al 2010 ). Once established, the plant develops a creeping root system, up to 2-3m belowground, with adventitious root buds resulting in clonal vegetative growth that enables rapid propagation and spread (Donald 1994; Lalonde and Roitberg 1994 ). The adventitious buds develop into new rosettes and lateral roots that continue to grow throughout spring, summer, and into fall. As temperatures decrease in fall, the aboveground vegetation dies off, and the roots overwinter; in the spring, root growth resumes and new shoots emerge ( Lalonde and Roitberg 1994 ; Tiley 2010 ). Fragments of roots as small as 1cm are capable of regenerating and forming new colonies ( Nadeau and Vanden Born 1989 ; Thomsen et al. 2015 ). The complex and extensive root system of Canada thistle allows for propagation, spread, and recovery making it particularly problematic and challenging for management ( Nadeau and Vanden Born 1989 ; Tiley 2010 ). A number of management tactics are regularly utilized in efforts to control Canada thistle. Chemical control is common and effective, generally providing rapid results. Mowing can also be an effective control method by reducing photosynthetic capacity aboveground and depleting root reserves used for regrowth, leading to a reduction of new shoots the following season ( Bourdôt et al. 2011 ; Graglia et al. 2006 ). Similarly, cultivation and tillage fragment the root system and force the plant to use root reserves for recovery, however this may also promote new shoot development and further spread of Canada thistle ( Graglia et al. 2006 , Thomsen et al. 2015 ). Both tillage and mowing are most effective when integrated into a weed management program to control established populations. While most current management of Canada thistle relies on either herbicides, mowing, or cultivation, these tactics can be costly, labor-intensive, and often not ideal for environmentally sensitive areas (e.g., riparian zones) ( Bourdôt et al. 2011 ; Graglia et al. 2006 ; Peterson et al. 2020 ; Thomsen et al. 2015 ). In contrast, biological control tactics are often more suitable for managing weeds in natural areas, as they can be self-perpetuating, and more economical in landscape-wide suppression of target species (Cripps et al. 2011; Guske et al. 2004 ; Peterson et al. 2020 ; Sciegienka et al. 2011 ). Effective biological control has long been sought for Canada thistle. Puccinia suaveolens (Pers.) Rostr (syn. Puccinia punctiformis (F. Strauss) Rohl), an obligate biotrophic rust fungus, was first proposed as a control method for Canada thistle in 1893 in North America ( Wilson 1969 ). Puccinia suaveolens can be naturally found on Canada thistle plants throughout its growing region, commonly co-introduced in the invasive range and causing periodic outbreaks of disease ( Berner et al. 2013 ; French and Lightfield 1990 ). Highly host specific to Canada thistle, P. suaveolens has only been reported on two other thistles native to Eurasia; Silybum marianum (L.) Gaertner in 2002 under greenhouse conditions and C. setosum (Willd.) M. Bieb. in 2023 under field conditions in China ( Berner et al. 2002 ; Liang et al. 2024 ). The potential for P. suaveolens to be utilized more broadly as a biocontrol for Canada thistle will increase with continued research ( Bean et al. 2024 ; Berner et al. 2015a ; Berner et al. 2015b ; Cripps et al. 2014 ; Thomas et al. 1994 ). The lifecycle of P. suaveolens can be divided in two stages: the vegetative mycelium within the root system and the spore-producing aboveground systemic and local infections. It is thought that P. suaveolens remains latent within the root system until abiotic or biotic conditions are adequate to produce spore-bearing systemically infected stems ( Mendgen and Hahn, 2002 ). Puccinia suaveolens infection reduces Canada thistle belowground biomass as root resources are parasitized by mycelia and also allocated to plant defense compounds instead of growth ( Chichinsky et al., 2023 ; Clark et al. 2020 ). This reduces the aboveground shoots and competitive ability of Canada thistle ( Chichinsky et al., 2023 ). Systemically infected stems typically do not flower, can die off early in the season and may help to further reduce root resources of infected Canada thistle colonies ( Van den Ende et al. 1987 ; Chichinsky et al. 2023 ). The rate and distance of spread of P. suaveolens caused by underground mycelia or aboveground spores remains unknown. Puccinia suaveolens produces five spore types that develop consecutively beginning with emergence of systemically infected shoots in early spring that are deformed, chlorotic, strongly floral scented, and covered in yellow-orange pycnia pustules ( Buller 1950 ; Menzies 1953 ; Petersen 1974 ). After outcrossing, pycniospores give rise to chain-like formations of the dark orange-brown aeciospores giving the systemically infected stems a characteristic rusty appearance ( Berner et al. 2013 ; Connick and French et al. 1991). Production of urediniospores is indicated by the darkening of the spores ( Buller 1950 ; Peterson 1974). Urediniospores and aeciospores are morphologically indistinguishable as single celled spores but only urediniospores produce localized infections on neighboring plants throughout summer (Kirk et al. 2001, Menzies 1953 , Peterson 1974). Localized infections produce pustules on the leaves but the shoots do not have the same growth abnormalities associated with systemic infection, and will still appear relatively normal (Baka and Lösel 1992; Thomas et al. 1994 ). Localized infections can produce two-celled teliospores on leaves that senesce heading into fall, then the spores will either blow off and overwinter in the soil or germinate on new rosettes to initiate new vegetative infection in the roots or systemic infection aboveground. ( Alexopoulos et al. 1996 ; Berner et al. 2015b ; Menzies 1953 ). Optimal teliospore germination occurs when temperatures are between 16-20°C ( Berner et al. 2013 ; French and Lightfield 1990 ) with optimal dew periods of 2-3 hours ( Morin et al. 1992a , 1992b ). Integrated weed management (IWM) is a holistic approach implementing one or more biological, physical, cultural or chemical control tactics (Harker and Donovan 2013). IWM aims to reduce weed adaptation and resistance to any single control tactic by using several possible tactics that take into account threshold populations, critical periods, and environmental outcomes. Utilizing IWM may lead to reduced environmental impacts of any given control method, decreased control costs by reducing pests to economically and ecologically insignificant levels, increased sustainability and reduced herbicide resistance (Harker and Donovon 2013). In a meta-analysis, Davis et al. (2018) found that combined treatments had better long-term outcomes for control of Canada thistle then reliance on herbicide treatment alone. Mowing and tillage have been shown to affect Canada thistle populations, but results have varied between significant reductions in population size to virtually no impact at all ( Beck and Sebastian 2000 ; Bourdôt et al. 2011 ). A stem-mining weevil, Hadroplontus litura, and a bacterial plant pathogen, Pseudomonas syringae pv. tagetis, have shown to have an additive effect in suppressing Canada thistle treated with herbicide (Sciegienka 2011). Puccinia suaveolens has also previously been used in conjunction with other control methods. Mowing combined with P. suaveolens strongly reduced the proportion of fertile flower heads of Canada thistle compared to infection alone ( Kluth et al. 2003 ). Demers et al. (2006) found that systemically infected P. suaveolens shoots increased, while healthy shoots decreased when combined with mowing. In a greenhouse experiment with a crop sequence of winter wheat, spring pea and summer safflower, crop competition reduced Canada thistle but when inoculated with P. suaveolens , the effect was increased ( Chichinsky et al 2023 ). The potential of using P. suaveolens in an IWM approach for Canada thistle is supported, but more research is needed to develop and refine best management practices. Canada thistle is a problematic weed that is difficult to control and current methods have varying degrees of success in both managed and natural ecosystems. As a biological control for Canada thistle, P. suaveolens has significant potential, as it can self-perpetuate, spread to surrounding areas, and contribute to population suppression at large scales when applied alone ( Bean et al. 2024 ). Our objectives were to determine the efficacy and compatibility of different control methods (mowing, tillage, herbicide, and P. suaveolens ) when applied alone and in combination to suppress Canada thistle. We highlight the benefits and limitations of using P. suaveolens in an IWM program, along with considerations for improved application efficacy. Materials and Methods Study Sites Two experimental sites were established in 2020: one in the Tamarack Ranch State Wildlife Area of Colorado (CO, 40.8320°N, 102.80437°W) and the other in Park City, Utah (UT 40.674330°N, 111.491324°W). The CO site is within the High Plains Ecoregion, while the UT site is within the Wasatch and Uinta Range Ecoregion (Omernik and Griffith 2014). The CO site is a 12 by 600 m plot of land, characterized as a shortgrass prairie with seasonal water inundation. Historically, the site had been maintained as a food crop plot for wildlife. At the end of each growing season, glyphosate had been applied and the plot was tilled for several years, resulting in the formation of a near monoculture of Canada thistle (Levi Kokes, personal communication May 2020). The CO site typically has precipitation occurring throughout the year (Supplementary Table S:1). The UT site is a small preserve nestled within a suburban development. Historically, spraying, particularly for musk thistle ( Carduus nutans L.), and goat/sheep grazing had been occasionally employed at the UT site but had not occurred for many years; the area is largely left untouched (Sara Jo Dickens, personal communication, 2020). The UT site generally has the majority of precipitation occurring in the winter months (S:1) ( PRISM Climate Group, 2023 ). Puccinia suaveolens Inoculum Dried inoculum was prepared following Berner et al (2013) and Bean et al (2024) . Briefly, Canada thistle leaves bearing telia (small pustules on yellowing leaves), were collected in late summer from a site near Colorado Springs, CO. Leaves were harvested and stored in paper bags to allow foliage to dry at room temperature. Dried leaves were ground to a coarse powder in a kitchen blender and used as inoculum within season, or stored at −80 ◦C for future use. Samples of ground leaf preparations were viewed under a microscope to ensure the majority of spores were two-celled teliospores, which are necessary for initiating systemic infection ( French and Lightfield, 1990 ; Berner et al 2013 ; Van Den Ende et al. 1987 ). Experimental Design In both UT and CO, an experiment site was established using a randomized complete block design, consisting of 10 treatment combinations applied across replicates. Treatments included an untreated control, P. suaveolens inoculation alone, tillage, tillage plus P. suaveolens inoculation, mowing, mowing plus P. suaveolens inoculation, herbicide (aminopyralid and chlorsulfuron tank mix), herbicide plus P. suaveolens inoculation, herbicide, mowing, and tillage (HMT), and HMT plus P. suaveolens inoculation ( Table 1 ). Each treatment was applied once in 2020 and 2021 to field plots (CO: 2 by 6 m; UT: 2 by 5 m). Plots were spaced (CO:4m; UT:2m) apart to avoid edge effects with 8 replicates in CO and 4 in UT. Differences in experimental set up between sites were due to the size and accessibility of Canada thistle populations. Herbicide and mowing treatments were applied in the fall during the first week of September. An herbicide tank mix was applied (aminopyralid 7 fl. oz/acre; chlorsulfuron at 1 fl. oz/acre) using a backpack sprayer calibrated in the field. Aminopyralid was chosen specifically as it is more effective at lower rates compared to other herbicides (e.g., picloram and clopyralid), and may also be used in areas where other chemicals are not appropriate or recommended (Enloe et al. 2017). In herbicide and mowing combination treatments, mowing was applied first to provide an opportunity for more even herbicide application and uptake given the physical damage to Canada thistle ( Carpinelli 2004 ). Fourteen days after initial treatments with mowing and herbicide, tillage (30 cm depth) and P. suaveolens inoculum (40 g CO, 33.3 g UT) were applied to select plots. When applying P. suaveolens inoculum, the entire plot was first sprayed with water using a backpack sprayer to create a mist on Canada thistle leaves, then P. suaveolens was broadcast by hand no higher than 1m above the ground to avoid excessive dispersal by wind. The 14-day period allowed the herbicide to translocate through the roots and other tissues before tillage following recommended manufacturer (Corteva) guidelines. Puccinia suaveolens inoculum was applied last either alone or in combination ( Table 1 ). The later timing for inoculum application in IWM treatments allowed for new growth and rosettes of Canada thistle in response to mowing and possibly tillage, which may improve the chance for infection ( Demers et al. 2006 ). Applications of P. suaveolens inoculum before mowing, tillage, or herbicide spray, would have been detrimental to germinating teliospores, which may have begun developing mycelia in the live tissue and subsequently been destroyed ( Berner et al. 2013 ; Petersen 1974 ). The initial monitoring of plots at both sites occurred in fall prior to first treatments. Monitoring occurred two weeks prior to the optimal timing for P. suaveolens teliospore inoculum application at each respective site. At both sites, a quadrat (m 2 ) was placed in the center of each plot and the number of thistle stems was counted and percent groundcover of Canada thistle, grass, forbs, litter, and bare ground was estimated. Across the entire plot, a two-minute timed count of Canada thistle stems systemically infected with P. suaveolens was also performed. Statistical Analyses All analyses were performed with R (R Core Team 2023), using packages tidyverse, ggplot2, glmmTMB, DHARMa, emmeans and car ( Brooks et al. 2017 ; Fox and Weisberg 2019 ; Hartig 2022; Lenth 2024 ; Wickham et al. 2019 ). Data from the two sites were analyzed separately because of the imbalance in design due to the difference in site accessibility and size of Canada thistle population. Stem density change as a function of P. suaveolens inoculum application (Yes or No); management approach [Control, Herbicide (H), Mowing (M), Tillage (T), and HMT]; or year (Fall 2020, Fall 2021, and Fall 2022); and the interaction between combinations of these parameters were analyzed with a generalized linear model (GLM). Stem density was modeled (with negative binomial) also using GLM. Significance was tested using ANOVA type II Wald chi-square tests, followed by post-hoc pairwise Tukey test. The percent change in stem density was calculated with standard deviation. Finally, cover data were analyzed using a GLM (with beta distribution) testing significance with ANOVA type II Wald chi-square tests, followed by post-hoc pairwise Tukey test. Results and Discussion In this study, conducted in two regions of the western U.S., the Intermountain West and the Great Plains, we evaluated the efficacy of P. suaveolens and its compatibility with other control methods for managing Canada thistle by measuring stem density and vegetative cover. While stem density reflects the direct effects on the target weed, vegetative cover can represent biodiversity, forage availability, resiliency of the landscape, nutrient cycling, and be used to predict production costs for livestock producers or fire risk. Consideration of both stem density and resulting vegetative cover will help land managers to make informed decisions about which treatments work in their IWM plan and how P. suaveolens can be incorporated. Stem Density Stem density of Canada thistle differed significantly between treatments (UT: P<0.0001, CO: P<0.0001) ( Figure 1 , Table 3 ). Herbicide treatments resulted in the largest decrease in stem density, while tillage treatments had the largest increase in stem density. There was also a significant interaction between management and year (UT: P<0.0001, CO: P<0.0001), and a marginal interaction between management, season, and P. suaveolens in CO (P=0.057) ( Table 3 ). At the UT site, initial average stem density was 24 ± 3 shoots per m 2 ( Figure 1 ), while the CO site initial average stem density was 80 ± 36 shoots per m 2 ( Figure 1 ). These differences may be attributed to variation in climate conditions and prior management practices employed at each site. During post-treatment monitoring, stem density in the control plots at the UT site increased by 85% (stem count m 2 (sc), 2020: 22±16; 2021: 40±9; 2022: 43±19) the first year, with an additional 7% increase the following year ( Table 2 and Figure 1 ). At the CO site, stem density in the control plots decreased by 3% (sc, 2020: 76±31; 2021: 74±28; 2022: 63±55) in the first year and then decreased another 15% the following year ( Table 2 and Figure 1 ). The UT site has higher average annual precipitation compared to the CO site, although during the first year of treatments they were about equal (Supplementary Table S1). At the UT site, most precipitation occurs as winter snowfall, resulting in extended dry periods that can stress Canada thistle, reducing root and stem growth, and subsequently, stem density and coverage ( Tiley 2010 ). In contrast, Tamarack Ranch, CO, receives more evenly distributed precipitation throughout the year ( PRISM Climate Group 2023 , Supplementary Table S1). At the UT site, temperature ranged from −11.1°C during the coldest months to 29°C in the hottest months of the experiment period (2020-2022). At the CO site, the temperature ranged from −13.5°C during the coldest months to 33°C (2020-2022) during the hottest months (Supplementary Table S1) ( PRISM Climate Group 2023 ). Herbicide and Herbicide+Mowing+Tillage (HMT) Herbicide treatments, whether alone or in combination, were most effective in decreasing Canada thistle stem density, and were significantly different compared to the other treatments (UT: P<0.001; CO: P<0.001; Table 3 ). At both sites, there was an immediate decline in stem density that continued even after the second application with sparse regrowth ( Figure 1 ). In CO, herbicide only treatments decreased stem density 93% in year one and another 77% in year two (sc, 2020: 78±21; 2021: 6±6; 2022: 1±2). When P. suaveolens was applied along with the herbicide, stem density declined 90% in year one and 100% the following year (sc, 2020: 84±40; 2021: 8±9; 2022: 0±0). The UT site had similar decreases in stem density from herbicide, year one 97% and year two 100% (sc, 2020: 29±13; 2021: 1±2; 2022: 0±0) and herbicide with P. suaveolens , year one 98% and year two 100% (sc, 2020: 22±14; 2021: 1±1; 2022: 0±0) ( Table 2 ). Using Tukey’s fence method for determining outliers, the Colorado HMT and HMT with P. suaveolens plots in block 8 in 2022 were determined to be outliers and were removed from analysis. Deep rooted perennial grasses that may provide higher competition were disturbed by tillage and replaced with annual grasses. Herbicide effects were perhaps lessened by seasonal water inundation. However the explanation of these outliers is unknown and further research would be needed to confirm the potential cause of increased Canada thistle stem density in these plots. In CO, the combined treatment (HMT) without P. suaveolens had a 97% decrease in stem density in year one and a 93% decrease in year two (sc, 2020: 67±44; 2021: 2±4; 2022: 0±0). When P. suaveolens was applied along with the HMT treatment, stem density decreased 86% in year one and 94% in year two (sc, 2020: 84±47; 2021: 12±15; 2022: 1±1) ( Table 2 ). At the UT site, stem density in the HMT plots without P. suaveolens decreased 69% in year one and another 95% in year two (sc, 2020: 25±10; 2021: 8±5; 2022: 1±3). When P. suaveolens was applied along with the HMT treatment, stem density decreased 84% in year one and 100% in year two (sc, 2020: 27±7; 2021: 4±5; 2022: 0±0) ( Table 2 ). Aminopyralid, which is selective against broadleaf weeds in rangelands and pastures, provided near 100% control of Canada thistle in herbicide-treated plots with additive effects from P. suaveolens inoculum application. Limited thistle regeneration was observed, likely emanating from neighboring plants, seeds, or remaining root fragments. Puccinia suaveolens Puccinia suaveolens was present at both sites in plots after treatments, however, there were only a few symptomatic stems. In CO, no symptomatic plants were found during the fall monitoring. In UT, the symptomatic shoots were found in the tillage plus P. suaveolens treatment, first appearing in year one (1 shoot) and also in year two (4 shoots). There was an overall lack of statistical significance ( Table 3 ) of P. suaveolens application at both sites, however there was a general declining trend in stem density indicating that the P. suaveolens had a slight suppressing effect on Canada thistle ( Figure 1 , Table 2 ). In UT, P. suaveolens treatments alone appeared to slow the increase of stem density (48%) by year two (sc, 2020: 26±8; 2022: 38±13) when compared to the untreated control, which had an increase of 98% (sc, 2020: 22±16; 2022: 43±19) ( Table 2 ). In CO, P. suaveolens treatment (sc, 2020: 93±28; 2022: 54±35), reduced stem density 24% more than the untreated control (sc, 2020: 76±31; 2022: 63±55) ( Table 2 ). The decrease in Canada thistle stem density following P. suaveolens is similar to the findings of Bean et al. (2024) , who recorded stem density decreases at 77% of monitored sites in Colorado over 3-8 years that went from 87.9 ± 6.5 stems to 44.7 ± 4.2 stems. Sites with more frequent and higher quantities of P. suaveolens inoculum applied had a lower stem density over time. We suspect that Canada thistle stem densities within P. suaveolens treated plots will continue to decrease with or without additional inoculations. While stem decline was observed, the lack of symptomatic thistle stems could potentially be attributed to genotypic differences and associated resistance within Canada thistle, or the compatibility of the host-pathogen interaction. Alternatively the abiotic or biotic factors that induce production of systemically infected stems may not have been met though vegetative mycelium within the root system could still be present. Puccinia suaveolens may continue to have an impact on Canada thistle or additional inoculation treatments might be required. This could be the case at both sites, but particularly at the CO site, where stem density decline was more obvious in Fall 2022 after a second inoculation ( Figure 1 ). Mowing A slight interaction occurred between season and P. suaveolens in CO (P=0.058), the significance occurring in the second year (P=0.0116) with mowing plus P. suaveolens having a greater decrease in stem density (65%) (sc, 2020: 62±41; 2022: 22±25) compared to mowing alone (49%) (sc, 2020: 83±37; 2022: 43±35). In UT, mowing (sc, 2020: 22±9; 2022: 21±11) and mowing plus P. suaveolens (sc, 2020: 25±17; 2022: 22±8) resulted in a small decrease in stem density of 1% and 13% respectively with no significance. The reduction in stem density between mowing with P. suaveolens inoculation and mowing alone was not statistically significant in UT and only slightly in CO (P=0.099, Table 3 ). In CO, mowing with P. suaveolens inoculation initially had a smaller impact compared to mowing alone. However, in year two mowing with inoculations showed a slight decrease in stem density compared to mowing alone, which had a slight increase in stem density ( Figure 1 ) ( Table 3 ). In UT, Canada thistle stem density resulting from mowing (averaged over P. suaveolens ) was significantly lower compared to control (averaged over P. suaveolens ), (UT: P=0.003, CO P=0.005) ( Table 3 ). In UT, mowing had significantly lower stem density (P=0.006) compared to tillage, with no significance in CO ( Table 2 ). Mowing has been used to enhance the occurrence of systematically infected stems ( Bourdôt et al. 2011 ), and increases localized infection by spreading spores ( Demers et al. 2006 ). Very few systemically infected stems were found during the 2-year study, which may have resulted in less additive effects from mowing with P. suaveolens compared to mowing alone. However, mowing should still be utilized with P. suaveolens in an IWM program, as the two treatments can be compatible and mutually beneficial. Tillage In CO, there was a significant difference (P=0.043, Table 3 ) between tillage and control. Further analysis showed that tillage treatments had significantly greater decline in stem density compared to control in 2022 (P=0.003). There was no significant difference between tillage alone and tillage with P. suaveolens (UT: P>0.05, CO: P>0.05, Table 3 ). The percent change in stem density is similar for both treatments: tillage in UT had an increase of 139% (sc, 2020: 19±7; 2022: 45±11) and in CO a decrease of 65% (sc, 2020: 93±35;2022: 31±39). In UT, tillage with P. suaveolens had a stem density increase of 147% (sc, 2020: 20±6; 2022: 48±18) and in CO a decrease of 67% (sc, 2020: 85±38; 2022: 28±24) ( Table 2 ). In CO, tillage combined with P. suaveolens resulted in a slightly greater decrease in stem density in the first year ( Figure 1 ) compared to tillage alone. The higher annual precipitation in the first year (Supplementary Table S1) may have contributed to P. suaveolens and tillage having a greater effect than the second year. In our study, application of P. suaveolens inoculum did not have a significant interaction with tillage but could be implemented in an IWM approach. Tillage has been used to manage Canada thistle by reducing stem density through the depletion of root reserves and reduction in shoot biomass (Thomsen 2011; Weigel 2024). Proper timing of tillage can be crucial, as early tillage can allow Canada thistle to recover and rebuild root reserves for overwintering ( Donald 2000 ; Thomsen et al. 2015 ). Applying P. suaveolens inoculum two weeks after tillage may enhance pathogen invasion of the smaller root fragments, and increase systemic infection in the spring. ( Alexopoulos et al. 1996 ; Berner et al. 2015a ). Groundcover Canada Thistle The trends observed in Canada thistle cover align closely with those in stem density. When cover measurements are divided by stem density, an estimate of the biomass of individual shoots can be made which may indicate the health of the population. Treatments with herbicide had the lowest amount of Canada thistle cover, ranging from 0-25% in UT and 0-35% in CO. Of note, in 2022, plots treated with herbicide and P. suaveolens had zero Canada thistle cover while plots treated with herbicide alone still had a low density of Canada thistle stems. Puccinia suaveolens may have an additive effect in herbicide treated areas or help prevent regrowth, suggesting that these two treatments are compatible. Mowing also significantly decreased percent Canada thistle cover compared to control, although no significant difference occurred between mowing alone and mowing with P. suaveolens . The highest Canada thistle cover occurred in the control, which was more easily seen in UT than in CO ( Figure 2 ). Thistle cover was higher in tilled plots as a result of the fragmentation and spread of Canada thistle roots creating many small populations ( Donald 2000 ; Thomsen et al. 2015 ). Vegetation In UT, the herbicide treatment, which reduced broadleaf plants, including Canada thistle, allowed more opportunity for grasses to grow (≤ 80% cover in year 2). Grass cover in UT was significantly higher in herbicide treatments compared to control and tillage (UT: P<0.005), with greater effects observed in the second year. In a three year study of management tactics for a non-native forb, a positive effect on native grass cover was seen as a result of herbicide treatments, however a steady and significant increase in non-native grass cover was also seen ( Skurski et al. 2013 ). Grasses were not identified to the species level and could include undesirable invasive species of concern (e.g., cheatgrass) for management of natural areas. In contrast, grass coverage in CO showed minimal change across treatments (P>0.05), though a significant interaction occurred between management and P. suaveolens (P=0.002). Further analysis showed that HMT and P. suaveolens treatments had significantly lower grass cover compared to control, P. suaveolens , or mowing plus P. suaveolens (Tables 4; Figure 2 ). Tillage to 20cm distributes seeds throughout the entire tillage area reducing accumulation of seeds near the surface, and therefore might result in reduced germination and grass cover as seen in our results ( Feledyn-Szewczyk et al. 2020 ). Differing climatic conditions between the two sites could also affect grass growth and contribute to the difference between treatments. Therefore site characteristics need to be considered in conjunction with management strategy for potential revegetation or secondary invasion by non-native species. Prior to initial treatments, forb cover was low (0-10%) and remained below 10% across the majority of plots with no significant difference between treatments (UT: P>0.05, CO: P>0.05) ( Table 4 , Figure 2 ). Use of broadleaf herbicides against invasive forbs can be expected to also suppress both native and other exotic forbs within the treatment areas ( Skurski et al. 2013 ). As expected, only treatments with broadleaf selective herbicides showed a slightly greater decline in forb cover. In other studies, short-term changes in native forb cover remained insignificant after herbicide application, except for reductions in flowering and seed set for at least 4 years post-treatment ( Crone et al. 2009 ). There may be long term implications in native forb recovery after herbicide is used to control non-native forbs like Canada thistle. Non-Vegetation Bare ground significantly increased as a result of HMT treatments with and without P. suaveolens in UT (P0.05). Combined treatments had more of an impact on bare ground cover in UT than herbicide alone perhaps due to significantly more grass cover in the herbicide alone treated plots in year two (P<0.0001). Combined control tactics have been shown in both cropping and non-cropping systems to have better long-term control of Canada thistle than herbicide alone ( Davis et al. 2018 ). There may have been an additional effect from changes in seedbank availability due to tillage ( Feledyn-Szewczyk et al. 2020 ) or from mowing as mowing alone resulted in more bare ground compared to control (P=0.03). In CO, bare ground cover was significantly greater in both herbicide and HMT treated plots compared to all other treatments (P<0.001) ( Table 4 , Figure 2 ). There was no significant difference found between bare ground cover in HMT and herbicide alone treated plots. Bare ground is an important aspect of land management since it may necessitate reseeding to prevent soil erosion and the creation of niches for other noxious weeds. Revegetation should be included to promote native and desirable plants ( Molvar et al. 2024 ; Rodriguez et al. 2024 ; Weidlich et al. 2020 ). At the UT site, herbicide treatments resulted in significantly more litter compared to tillage (UT: P=0.002). In CO, HMT treatments had significantly lower litter compared to all treatments except for control ( Table 4 ; Figure 2 ). Litter cover can be beneficial in reducing bare ground, retaining soil moisture and increasing nutrient cycling ( Perera et al. 2024 ; Redman 1978) Practical Implications Effective strategies for controlling Canada thistle vary with site conditions and management goals. Chemical treatments are often the most effective at reducing Canada thistle populations, but frequently increase areas of bare ground for invasion from other weeds and even re-invasion by Canada thistle. The use of herbicides is often restricted in natural landscapes or on organically certified farms, leaving stakeholders in search of alternative and complimentary control tactics. Applications of P. suaveolens inoculum in addition to other tactics show promising trends for greater control of Canada thistle than without the inoculum. Continued monitoring will help determine if additional treatment applications are needed, especially where P. suaveolens effects are enhanced on Canada thistle. Although P. suaveolens is slower acting in suppressing Canada thistle, one of the long-term benefits is that recovery of native plant communities is more likely. Puccinia suaveolens is unique among tactics for managing Canada thistle, as there are no direct effects on non-target plants. Applying P. suaveolens alone or with other tactics, provides a safe, sustainable, and effective means for enhancing management of Canada thistle in natural areas. Funding statement This research was supported in part by USDA NIFA (2019-70006-30452; RS, SY, and DB) and APHIS Cooperative grants (AP20PPQFO000C386, AP21PPQFO000C237, and AP22PPQFO000C142). The multistate project was initiated and supported by USFS BCIP agreement #17-CA-1142004-252 (DB) in cooperation with Carol Randall of the USFS. Competing interests The authors declare none. Herbicide, mowing, tillage and rust application methods View this table: View inline View popup Download powerpoint Table 1: Overview of weed management tactics employed for treatment of Canada thistle at experimental sites in Colorado and Utah. Percentage change of Canada thistle stem density Colorado and Utah View this table: View inline View popup Download powerpoint Table 2: Annual average change (percent) of Canada thistle and average stem count with +/- SD in Colorado and Utah from 2020-2022 following treatment with individual and combined weed management tactics. Based on a Tukey’s fence method for determining outliers, plots 74 and 75 from block 8 in CO were excluded from analysis. Canada thistle stem count ANOVA table Colorado and Utah View this table: View inline View popup Download powerpoint Table 3: Statistical results of the impact of rust inoculum application,, management practice, and their combination across seasons on Canada thistle stem count in Colorado and Utah. Ground coverage ANOVA table for Colorado and Utah sites View this table: View inline View popup Download powerpoint Table 4: ANOVA table of the five ground cover types measured in UT and CO sites as a function of rust inoculum application, management strategy, season and the combined effects of these three parameters. Canada thistle Stem count in Colorado and Utah Download figure Open in new tab Figure 1: Canada thistle stem count in Fall 2020-2022 in A) Colorado and B) Utah following treatment with individual and combined weed management approaches. Ground percentage for weed management approaches at Colorado and Utah Download figure Open in new tab Figure 2: Average percent of the 5 measured ground cover types. A) Colorado and B) Utah experimental site, 2020-2022 following treatment with individual and combined weed management approaches. Climate table for Colorado and Utah 2020-2022 View this table: View inline View popup Download powerpoint Supplemental Table 1: Precipitation, maximum and minimum temperatures at the two experiment sites (UT and CO) throughout the experimental years 2020-2022. Acknowledgments The authors wish to thank, Sara Jo Dickens, PhD of Ecology Bridge LLC; Logan Jones of Park City Municipal Corporation, Levi Kokes Colorado Parks and Wildlife, Tamarack Ranch State Wildlife Area, Colorado State University Extension Office, and the Utah Weed Supervisors Association for site access, Park City site information signage, and other logistical and outreach support. 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