Exotic Plant Management in California Annual Grasslands

permit for release on yellow starthistle in California. The rustwas originally collected from yellow starthistle in its nativerange of Turkey in 1978 (Woods and Villegas 2004). Pucciniajaceae var. solstitialis is an obligate parasite of yellow starthistleand is associated with the thistle over a wide area, at leastfrom Spain to Turkey (Savile 1970). It completes its life cycleon a single host plant and has all five spore forms. It causesnonsystemic foliar infections that can reduce fresh and dryweights of inoculated yellow starthistle in controlled studies(Bruckart 1989; Shishkoff and Bruckart 1993). Spores, however,may not persist over the winter.Despite considerable research on host specificity of Pucciniajaceae var. solstitialis and nontarget plant safety at the USDA-Agricultural Research Service (ARS) facility in Ft. Detrick,Maryland, little is known about the plant-pathogen interactionunder field conditions, including information on themost effective inoculation timing window to maximize infectionrates and foliar damage. Furthermore, while the rust hasbeen shown to reduce root biomass, it is unknown whetherthis is due to a decrease in lateral root production or to theability of yellow starthistle to produce deep roots. Such informationmay significantly impact our capability to predictthe effect of the pathogen on yellow starthistle’s ability todevelop tolerance to water stress. Moisture levels have beenshown to correlate directly with seed production. If rootdepth is limited by pathogen stress, this may subsequentlyimpact plant growth, seedhead production, and ultimatelyreproductive output. In addition, nothing is known on howthe Puccinia rust will interact with the established biologicalcontrol insects or how it will change the competitive abilityof yellow starthistle compared to other grassland vegetationunder different environmental conditions. Results of thesestudies can greatly improve the capacity to predict the potentialeffectiveness of the Puccinia rust under a number ofabiotic and biotic conditions.Chemical ControlHerbicides are an important method of weed control in grasslandsystems. Herbicides can be applied to grasslands by anumber of methods, including fixed-wing aircraft, helicopters,ground applicators, backpack sprayers, and rope wickapplicators. Herbicides registered for use in grasslands of thewestern United States are listed in Table 22.4, along with pertinentinformation on each compound. Some of these products,including picloram and metsulfuron, are not registeredin California. Of these compounds used in grasslands, theauxinic, or growth regulator, herbicides have played the mostimportant role in broadleaf weed control.For large-scale use, herbicides are typically considered to bethe most economical option. The most widely used grasslandherbicides in California are those that have postemergenceactivity (DiTomaso 2000). These include 2,4-D, triclopyr,dicamba, clopyralid, chlorsulfuron, and glyphosate. Ofthese, clopyralid, 2,4-D, triclopyr, and dicamba are growthregulator herbicides that are selective on broadleaf speciesand have little activity on grasses. Clopyralid also has excellentpreemergence activity and is highly effective for control ofyellow starthistle and other members of the Asteraceae. However,it may have some negative impacts on some native plantswithin the Asteraceae, Fabaceae, Polygonaceae, and Apiaceaeand short-term impacts on Violaceae (Reever Morghan et al.2003). Chlorsulfuron is an amino acid inhibitor and is alsovery effective on most broadleaf species, particularly membersof the mustard (Brassicaceae) and figwort (Scrophulariaceae)families. It has both preemergence and postemergence activityon most species, but only preemergence activity on yellowstarthistle. Like the growth regulator compounds, chlorsulfuronis fairly safe on most grasses. Glyphosate is a nonselectivearomatic amino acid inhibitor and provides excellentcontrol of annual and perennial grasses and broadleaf weeds,but it will also damage desirable plants.Aminopyralid is a new growth regulator herbicide registeredin California in 2006. It has about three times the activity ofclopyralid on yellow starthistle. In addition to its activityagainst yellow starthistle, aminopyralid has a broader spectrumof selectivity and has also been shown to be very effectiveon knapweeds, many other thistles, and fiddlenecks(Amsinckia spp.) (Kyser et al. 2007). Both products will injurenative legumes during the growing season, but some can beused safely when treatments are made after senescence or duringthe dormant phase of perennial legumes. Another newlyregistered herbicide (not yet in California), imazapic, hasproven to be very effective on medusahead, downy brome,ripgut brome, barb goatgrass, and other annual grasses, withoutsignificantly injuring seedlings of many native perennialgrass or broadleaf species (Kyser et al. 2007).Timing of herbicide applications can determine the effectivenessof the treatment. For yellow starthistle control, thebest timing for application of clopyralid and aminopyralidseems to be between December and the end of March,depending on the location (DiTomaso et al. 1999b). Fall orspring applications of imazapic can be effective for the controlof annual grasses, depending on the location. In areaswith snowpack, spring applications may be more desirable,but in typical Central Valley or foothill conditions, fall applicationshave proven successful. For perennials, timing ofapplication can depend on the herbicide. Chlorsulfuron cancontrol perennial species with spring, summer, or fall treatments(Drake and Whitson 1989; Whitson et al. 1989; Younget al. 1998), whereas glyphosate is best applied in spring,when invasive plants are at the late bud to early floweringstages (Waterhouse and Mahoney 1983; Young et al. 1998).Herbicides are generally applied as broadcast treatmentsover the entire field or directed (spot) applications to controlearly weed invasions or to prevent the spread of small infestations.However, it is also possible to achieve selective controlof a particular weed with otherwise nonselective or relativelynonselective postemergence herbicides by employing a ropewick or wick applicator. These can be either hand-held orvehicle-mounted boom wipers. As a benefit, this applicationmethod reduces the potential for herbicide drift and290 POLICY AND MANAGEMENT