Susceptibility of Carduus, Cirsium,And Cynaraspecies Artificially Inoculated with Puccinia Carduorumfrom Musk Thistle

Home , Cirsium andersonii, Cirsium arizonicum, Cynareae

BIOLOGICAL CONTROL 6, 215–221 (1996) ARTICLE NO. 0026

Susceptibility of Carduus, Cirsium, and Cynara Species Artiﬁcially Inoculated with Puccinia carduorum from Musk Thistle1

W. L. BRUCKART,* D. J. POLITIS,*,2 G. DEFAGO,† S. S. ROSENTHAL,‡ AND D. M. SUPKOFF§ *USDA-ARS-FDWSRU, Frederick, Maryland 21702; †Eidgeno¨ssische Technische Hochschule, Zu¨ rich, Switzerland; ‡USDA-ARS-BCWR, Bozeman, Montana 59717; and §California Environmental Protection Agency, Environmental Monitoring Branch, Sacramento, California 95814

Received December 28, 1994; accepted August 22, 1995

proposal to the USDA, Animal and Plant Health Inspec- Puccinia carduorum from Turkey was evaluated for tion Service (APHIS), for use of P. carduorum in the biological control of Carduus thoermeri under con- United States for biological control. Permission was trolled greenhouse and field conditions. We measured granted by APHIS and the Virginia Department of the susceptibility of 16 accessions of C. thoermeri (the Agriculture in 1987 to conduct a field study of P. target weed), 10 accessions of related weedy Carduus carduorum near Blacksburg, Virginia. spp., 22 native and 2 weedy Cirsium species, and KEY WORDS: Uredinales; rust fungus; musk thistle; Cynara scolymus (artichoke). All collections (including foreign pathogen; nontarget risk; artichoke; classical 99.2% of the individuals) of C. thoermeri were very biological control. susceptible, as were collections of putative hybrids between C. thoermeri and other large-flowered Card- uus spp. Disease incidence on Carduus nutans or Carduus macrocephalus was between 0 to 5% of plants INTRODUCTION inoculated. Of the 454 individual Cirsium thistles inoculated, 83% did not develop symptoms (5 immune) at ‘‘Musk thistle,’’ a complex of related Carduus species, any stage of growth, including all individuals in 11 has been the object of biological control research for species. For the Cirsium species in which symptoms many years. Puccinia carduorum Jacky is considered a were observed, 75% were immune when inoculated 4–5 good candidate for biological control of musk thistle in weeks after planting, and no susceptible reactions North America (Politis et al., 1984). We learned in an developed after reinoculation of these individuals 2 earlier study that under optimal greenhouse conditions weeks later.A similar effect of plant age and susceptibil- P. carduorum from musk thistle could infect a number ity was noted for Cy. scolymus. Results similar to those of other plant species, but only those in the subtribe in the greenhouse occurred also from field inocula- Carduinae of the tribe Cynareae (5 Cardueae) in the tions of eight selected Cirsium spp., two Cynara spp., and three Carduus spp. in Switzerland. All individuals Asteraceae; no infections were noted on 10 species of C. thoermeri were susceptible; ratings for disease representing the subtribe Centaureinae (tribe Cynar- severity were between ‘‘2’’ and ‘‘4’’ (the maximum rat- eae), on 22 species from 12 other tribes in the Astera- ing). For the remainder of the species, the highest ceae (Politis et al., 1984), or on 17 species from 8 rating was ‘‘1’’ for Cirsium and Cynara spp. and ‘‘2’’ for families outside of the Asteraceae (Politis and Bruck- the other Carduus spp. Despite the symptoms on spe- art, unpublished results). In that study, we also noted cies other than C. thoermeri, P. carduorum was consid- that: (1) distinct differences in susceptibility occurred ered low risk because infections on these species were among accessions of large-flowered Carduus inoculated weak and the pathogen could not be maintained on with P. carduorum from Carduus thoermeri Weinm. any nontarget species under optimal greenhouse con- (5 Carduus nutans L. ssp. leiophyllus [Petrovic] Stoj. & ditions. Data from these experiments were part of a Stef.), and (2) infections of species other than C. thoermeri were limited and occurred only on 6 (of 10) other species of Carduus, 8 (of 16) species of Cirsium, 1 Mention of a trademark or proprietary product does not consti- and 2 species of Cynara (Politis et al., 1984). Special tute a guarantee or warranty of the product by the U. S. Department concerns were raised because the Cirsium species are of Agriculture and does not imply its approval to the exclusion of native to North America and 1 Cynara species, Cynara other products that also may be suitable. 2 Present address: Benaki Phytopathological Institute, 8 Odos scolymus L. (artichoke), is of economic importance. Delta, Kifissa-Athens, 145 61, Greece. Experiments reported in this paper concern differences

215 1049-9644/96 $18.00 216 BRUCKART ET AL. in susceptibility among the large-flowered Carduus inoculated by spraying a suspension containing 0.1 mg accessions and the results serve to clarify the extent to urediniospores per plant in Freon 113, providing 12–16 which Cirsium and Cynara species are susceptible to P. h dew at 20°C immediately after inoculation, and carduorum from C. thoermeri. This report includes transferring inoculated plants to a greenhouse (20– results that relate to our current understanding of 25°C) for 3 weeks before rating for disease develop- large-flowered Carduus taxonomy in North America, ment. Disease reaction was based on ratings for pus- results from tests of 8 additional Cirsium spp., and tule size. Ratings for pustule size (5 infection type) specific data on susceptibility of artichokes to P. carduo- have been described (Politis et al., 1984) and range from rum. ‘‘0’’ (5 no macroscopic symptoms) to ‘‘4’’ (5 very large, Results reported herein were an important part of a possibly satellite pustules). The rating assigned to an prerelease risk assessment for P. carduorum and were individual plant was based upon the largest pustule used in a proposal to the USDA, APHIS, to include P. observed, thus providing a very conservative measure carduorum among the biological control options of C. of susceptibility. For perspective in reporting and dis- thoermeri in North America. APHIS approval is neces- cussing results, ratings for disease reactions were sary for the introduction of a foreign plant pathogen categorized either as ‘‘Immune’’ (rating of ‘‘0’’), ‘‘Resis- into the United States (Klingman and Coulson, 1982). tant’’ (ratings of ‘‘1’’ or ‘‘2’’), or ‘‘Susceptible’’ (ratings of ‘‘3’’ or ‘‘4’’). These categories also reflect the ease of maintenance of P. carduorum on each host under MATERIALS AND METHODS optimal greenhouse conditions; P. carduorum may be The data reported here are from experiments con- maintained on plants considered ‘‘resistant’’ only with ducted either in a disease containment greenhouse great effort and difficulty. facility in Frederick, Maryland, or at a field location in Pustule density was determined by removing inocu- Switzerland. All plants were inoculated with field isolated leaves, counting all the pustules, and measuring lates of P. carduorum collected in 1978 from C. tho- leaf area with a Lamda Area Meter (LI-COR, Inc., ermeri in Bulgaria, Romania, or Turkey by R. G. Emge. Lincoln, NE). Disease incidence was based on the Samples of P. carduorum shipped to the USDA-ARS- proportion of plants infected. FDWSRU (formerly the Plant Disease Research Labo- Large-flowered Carduus species were inoculated as ratory, PDRL), were processed and studied in contain- described, and reactions were compared on the basis of ment as previously described (Politis et al., 1984). disease severity. Evaluations of Cirsium spp. included Urediniospores of selected isolates were shipped to the inoculation of 22 native North American species and Eidgeno¨ssische Technische Hochschule (ETH) in Zu¨ rich, two introduced weedy species. Each individual in the Switzerland, for use in field evaluations. All green- Cirsium study was inoculated twice using inoculum house and field research was conducted under permit from C. thoermeri; the second inoculation was 2 weeks from regulatory authorities. after the first. After each inoculation, plants were rated Plant species for detailed greenhouse and field evalu- for pustule size as described, and data were then placed ations in Switzerland were selected on the basis of in the categories describing relative susceptibility. Indi- results from previous studies in the greenhouse in viduals of 4-week-old C. thoermeri also were included in Frederick. Seeds of the target (C. thoermeri) and nontar- each inoculation as positive controls. Results of tests on get species were supplied by collectors familiar with Cirsium spp. were accepted only in the event of positive these plants; specific epithets they provided were used reactions on C. thoermeri controls. in this paper. Most accessions of large-flowered Car- Greenhouse studies to evaluate the susceptibility of duus species were provided by M. K. McCarty (USDA, artichoke involved two experiments. The first involved University of Nebraska, retired), which were identified seed-grown artichokes from 26 breeding lines inocu- using the key presented in ‘‘Flora Europaea’’ (Tutin et lated at the cotyledonary, first, third, and fifth true-leaf al., 1976). Other accessions of C. thoermeri and C. stages of growth to determine the effects of phenology nutans were collected by personnel at FDWSRU from on symptom development. Musk thistle plants at com- Frederick, and Sparks, Maryland, respectively. These parable stages of development were inoculated for were identified using the key of Moore and Frankton comparison. All pustules on each plant were counted (1974). Seeds of certain other collections of Carduus, and leaf areas were measured as described. Data from also supplied by M. K. McCarty, were either from the cotyledonary and fifth true-leaf stages are reported, putative hybrids or from an unidentified large-flowered and pooled means from the breeding lines are com- Carduus species growing in British Columbia (Mc- pared with counts on musk thistle controls. Carty, 1985). For the second artichoke experiment, the perennial Greenhouse studies. All plants for the greenhouse propagative units (crowns) were split and grown in tests were grown from seed for 4–5 weeks before 15-cm-diameter plastic pots for 3–6 months prior to inoculation unless otherwise described. Plants were inoculation. The tops of six plants were then removed P. Carduorum ON Carduus, Cirsium, AND Cynara SPP. 217 every 2 weeks for 8 weeks, and plants were allowed to TABLE 1 regenerate. Two weeks after the last tops were re- Susceptibility of Large-Flowered Carduus Species from moved, all plants were inoculated, thus testing suscep- M. K. McCarty Inoculated with Three Isolatesa of Puccinia tibility of artichokes after 2, 4, 6, and 8 weeks of carduorum regrowth. Musk thistle inoculated 6 weeks after planting was the positive control. The experiment was Carduus Disease b repeated, and data in Table 4 are combined from the species Origin Designation incidence two inoculations. C. macrocephalus Montana M-50-5 1/19 Data were subjected to analysis of variance and least Montana M-27-5 18/20 significant difference (LSD, P 5 0.05) for mean separa- Montana M-53-2 13/20 tion using PROC GLM (SAS Institute, 1990). C. nutansc Maryland M-25-1 0/15 Maryland M-25-2 1/15 Field studies in Switzerland. Field inoculations of France M-26-9 0/15 C. thoermeri and related species were made in each of C. thoermeri California Mt. Shasta 14/14 2 years. The first year, transplants of C. thoermeri (two Illinois M-11-1 23/24 collections), two species of Cynara, and Carthamus Iowa M-23-5 28/28 lanatus L. were set out in mid-May 1983 in Pfaffhausen Kentucky M-14-2 24/24 Maryland Frederick 36/36 and Eschikon (near Zu¨ rich) and in Cadenazzo (canton Maryland Sparks 36/36 of Tessen in southern Switzerland). Plants were inocu- Mississippi M-17-1 15/15 lated in the first half of June. Each plant was sprayed Missouri M-12-2 33/33 with 105 urediniospores of P. carduorum/ml water plus Montana M-27-11d 15/15 d a small amount of 0.125% (v/v) Tween 20 ([20 POE] Montana M-53-6 19/20 Nebraska M-19-1 27/28 polyoxyethylene sorbitan monolaureate, ICI Americas, Nebraska Nebr. 14/14 Wilmington, DE) wetting agent until upper and lower Wisconsin Wisc. 14/14 leaf surfaces were wet. Urediniospore germination was Virginia M-21-5 28/28 10% or less for each of the four isolates in the study. Canada, Sask. M-16-1 24/24 Canada, Sask. Sask. 14/14 Plants were rated for disease severity, from ‘‘0’’ (5 no sp.e Canada, BC M-24-5 25/26 macroscopic infection) to ‘‘3’’ (5 very susceptible). Plants Canada, BC Alexis Creek 14/14 were observed for 5 months. Canada, BC M-24-10-0 f 13/13 The second year, plots were set up at the locations Canada, BC M-24-10-S f 15/15 used in 1983 and at the ETH in Zu¨ rich. Representatives a Data from inoculations with two isolates (III and IV) from Turkey of three Carduus spp., eight Cirsium spp., two collec- and one isolate (VII) from Romania were combined for this table. tions of Cynara cardunculus L., and eight breeding b No. plants infected/No. plants inoculated. lines of C. scolymus were transplanted in at least one of c After ‘‘Flora Europaea’’ (Tutin et al., 1976) or ‘‘The Thistles of the four locations. Cynara and Cirsium species, except Canada’’ (Moore and Frankton, 1974). d Cirsium arizonicum (Gray) Petr., were selected because Putative hybrid of C. macrocephalus with a parental phenotype of C. thoermeri (McCarty, personal communication). of their reactions in greenhouse studies. Transplants e A large-flowered species, much like C. thoermeri, with white were set as rosettes (two to seven leaves) in late April flowers and some differences in the bract-leaf morphology (McCarty, and inoculated in mid-May, as described previously, personal communication). with 3 3 104 to 8 3 104 urediniospores of P. carduorum/ f Seeds from a putative F-1 hybrid between the British Columbian ml. Average urediniospore germination was 35% for the Carduus species (described in footnote e) and C. thoermeri (McCarty, personal communication). four isolates. Ratings for disease severity, from ‘‘0’’ (5 no macroscopic symptoms) to ‘‘4’’ (5 very susceptible), are described in Table 5. Plants were observed equally susceptible as C. thoermeri were the putative either until September in Eschikon and Cadenazzo or hybrids between either C. macrocephalus or the British until November in Pfaffhausen and Zu¨ rich. Data from Columbian Carduus sp. and C. thoermeri (Table 1). the four sites were pooled since there were no differ- Response of the Cirsium spp. to infection by P. ences in plant reactions among the sites. carduorum depended upon both the species and the plant age. Eleven species (130 individual plants) were RESULTS immune throughout the experiment. Of the remaining species, almost three-quarters of the individuals Collections of C. thoermeri were very susceptible to (242/324 5 74.7%) also were immune throughout the infection by P. carduorum in every experiment (Tables experiment (Table 2, totals from inoculations at 4–5 1 to 5). The incidence of disease was very low or zero for weeks after planting). In every species where individu- one collection of C. macrocephalus and three collections als were rated either resistant or susceptible, the of C. nutans (Table 1). The two accessions from British greatest proportion was immune throughout the study Columbia were as susceptible as the C. thoermeri. Also (Table 2). 218 BRUCKART ET AL.

Following a second inoculation 2 weeks later of the TABLE 3 same individual Cirsium plants described above, none Number of Pustules per Seed-Propagated Cynara scolymus was rated susceptible, and only five species had a low (Artichoke) and Carduus thoermeri after Inoculation with percentage of resistant individuals (Table 2). Similar Puccinia carduorum from C. thoermeri results occurred for Saussurea americana DC Eat. and Silybum marianum (L.) Gaertn. (data not shown). Stage of plant growth Susceptible reactions also were recorded in 12% of the Cy. cardunculus L. (cardoon) sample 4–5 weeks after Species Cotyledon Fifth true leaf planting, but these plants were not reinoculated (data C. thoermeri 4.28 7.61 not shown). C. scolymus 1.81a 0.15b Artichoke plants always were significantly less sus- LSDc 0.92 1.47 ceptible than C. thoermeri (Tables 3 and 4). Artichokes a Data are pooled from inoculations of 22 breeding lines. were most susceptible as seedlings, developing an b Data are pooled from inoculations of 26 breeding lines. 2 average of 1.8 pustules per cm leaf area (22 artichoke c LSD, least significant difference, P 5 0.05.

2 TABLE 2 lines). This compared to 4.3 pustules per cm for seedling C. thoermeri plants (Table 3). Disease severity Number of Individuals of 24 Cirsium Species and Their in artichokes declined to an average of 0.2 pustules per Reactions to Puccinia carduorum Isolate III from Carduus cm2 leaf area (mean of 26 artichoke lines) by the fifth thoermeria,b true-leaf stage of growth, but C. thoermeri remained 2 Plant age (weeks) at inoculation very susceptible (7.6 pustules per cm ) at that stage (Table 3). Similar differences were noted for pustule 4–5 6–7 counts at the one and three true-leaf stages. Actual Cirsium differences in leaf area infected (LAI) between C. species Imm. Res. Sus. Imm. Res. Sus. thoermeri and artichoke were more extreme, consider- ‘‘Immune reactions’’ ing that individual pustules on artichokes averaged 0.06 mm2, compared to 0.34 mm2 for pustules on musk C. altissimum (L.) Spreng. 18 0 0 18 0 0 C. arizonicum (Gray) Petr. 15 0 0 15 0 0 thistle (data not shown). Based on these figures, esti- C. arvense (L.) Scop. 22 0 0 22 0 0 mates of LAI were 2.58 mm2/cm2 leaf area for C. C. brevistylum Cronq. 4 0 0 4 0 0 thoermeri and 0.01 mm2/cm2 leaf area for artichokes. C. campylon H. K. Sharsm. 18 0 0 18 0 0 Disease severity on artichokes grown from crowns C. crassicaule (Greene) Jepson 12 0 0 12 0 0 C. discolor (Muhl.) Spreng. 5 0 0 5 0 0 also was significantly less than in the C. thoermeri C. douglasii DC. 7 0 0 7 0 0 controls, based on proportion of leaves infected, pustule C. flodmanii (Rybd.) Arthur 12 0 0 12 0 0 counts, and pustule density (Table 4). Artichokes grown C. muticum Michx. 5 0 0 5 0 0 from crowns were very resistant to infection, develop- C. ochrocentrum Gray 12 0 0 12 0 0 ing an average of 0.022 pustules/cm2 on infected leaves Subtotal, ‘‘Immune’’ 130 0 0 130 0 0 (Table 4). The time for regrowth of leaves from crowns ‘‘Resistant and susceptible reactions’’ (5 regeneration time) did not make any difference in disease severity. Comparative data for C. thoermeri 6 C. andrewsii (Gray) Jepson 38 1 0 39 0 0 2 C. hillii (Canby) Fern. 16 1 0 17 0 0 weeks after planting was 1.14 pustules/cm . C. pitcheri (Torr.) T. & G. 5 4 0 9 0 0 Field tests in Switzerland also indicated that C. C. pumilum Spreng. 9 1 0 9 1 0 thoermeri was much more susceptible than the other C. undulatum (Nutt.) Spreng. 16 1 0 17 0 0 species. There was no infection on the nontarget species C. quercetorum (Gray) Jepson 4 1 0 5 0 0 in the first year of study, but disease incidence (100%) C. andersonii (Gray) Petr. 23 7 1 30 1 0 C. callilepis (Greene) J. T. Howell 4 4 6 14 0 0 and disease severity ratings (from ‘‘2’’ to a maximum of C. cymosum (Greene) J. T. Howell 22 7 1 30 0 0 ‘‘4’’) were high on C. thoermeri, despite an apparently C. occidentale (Nutt.) Jepson 16 9 2 24 3 0 low rate of urediniospore germination (data not shown). C. pastoris J. T. Howell 14 5 10 28 1 0 During the second year, 70% (120 of 168) individual C. proteanum Howell 11 7 1 19 0 0 C. vulgare (Savi) Tenore 64 12 1 72 5 0 plants in the genera Cirsium and Cynara were not infected (rating of ‘‘0’’), and the maximum rating as- Subtotal, infected 242 60 22 313 11 0 signed to the remaining plants in two (of eight) Cirsium Grand totals 372 60 22 443 11 0 spp. and both Cynara spp. was ‘‘1’’ (Table 5). Of the two nontarget Carduus species, C. pycnocephalus and C. a Includes data from Politis et al., 1984. b See Table 1 and Bruckart and Peterson (1991) for comparison tenuiflorus, less than half the individuals (17 of 38) with reactions on Carduus thoermeri. All individuals of C. thoermeri were infected, and the maximum rating for the remain- used as positive control were severely infected in this study. der was ‘‘2’’ (Table 5). Incidence of disease for C. P. Carduorum ON Carduus, Cirsium, AND Cynara SPP. 219 thoermeri was 100% (90 of 90 plants) and severity TABLE 5 ratings were from ‘‘2,’’ for a few individual plants, to the Susceptibility of Carduus, Cirsium, and Cynara spp. to maximum of ‘‘4’’ (Table 5). Four Isolates of Puccinia carduorum from Carduus thoermeri in Swiss Field Testsa DISCUSSION Disease

The rust fungus P. carduorum was of interest for b c biological control of C. thoermeri because it is autoe- Test plant Incidence Reaction cious (Gaümann, 1959), and as a rust fungus, it would Carduus thoermeri 90/90 2d–4 likely be host-specific and have the capability to spread C. pycnocephalus 6/18 0–2 very rapidly with no or very little artificial manipula- C. tenuiflorus 11/18 0–2 tion (see Chaps. 15 and 16 in Van der Plank, 1963). Cirsium andersonii 0/3 0 C. arizonicum 0/5 0 These are desirable attributes for any foreign pathogen C. callilepis 0/4 0 intended for use in classical biological weed control. C. cymosum 0/2 0 Indeed, evidence in the literature suggests that P. C. occidentale 3/5 0–1 carduorum is host-specific. Gaümann (1959) described C. pastoris 7/13 0–1 a new forma specialis of P. carduorum (f. sp. nutantis) C. proteanum 0/2 0 C. vulgaree 0/36 0 and discussed two others created by Jacky and Probst Cynara cardunculuse 6/23 0–(1) (f. sp. deflorati Jacky and f. sp. crispi Probst). We have C. scolymuse 26/75 0–1 evidence that P. carduorum isolated from different Carduus species is distinctly different, both biologically a Tests were conducted with one isolate per location near Zu¨ rich and biochemically (Bruckart and Peterson, 1991). (three sites) or southern Switzerland (one site). Isolates of P. carduorum were from Turkey (two tested), Bulgaria, and Romania. Despite this evidence, we were able to infect some b No. plants infected/no. plants inoculated. closely related species in the subtribe Carduinae of the c Rating scale for disease reactions: 0 5 no symptoms and nonsporu- tribe Cynareae with P. carduorum from C. thoermeri lating pustules; (1) 5 sporadic sori, 5% of the rosette leaves; 1 5 (Politis et al., 1984; results in this paper). Such phenom- 0.1–0.5 sori/cm2 on 5–10% plant mass; 2 5 0.5–1.0 sori/cm2 on 10–20% plant mass; 3 5 0.5–1.0 sori/cm2 on 20–40% plant mass; 4 5 1 ena are not uncommon in plant pathology, but such sorus/cm2 on .40% plant mass. nontarget reactions make risk assessment difficult in d Only a few plants were given this rating. cases where host specificity is highly desirable (Bruck- e More than one collection tested. art and Shishkoff, 1993; Watson, 1986a). The subject of nontarget effects in the laboratory, greenhouse, and field has been the cause of discussion among entomolo- risk assessment (Watson, 1986b; 1986c) that resulted gists as well (Cullen, 1990). Precedence was set for the in approval by Agriculture Canada of a foreign nema- use of scientific information about nontarget effects in a tode, Subanguina picridis (Kirjanova, 1944) Brzeski, 1981, for use in Canada against Russian knapweed, Acroptilon repens (L) DC. (Watson, 1991). TABLE 4 The intent of the research in this paper was to Susceptibility of Artichoke (Cynara scolymus) from Crowns confirm and solidify interpretation of results from an Inoculated with Puccinia carduorum Isolate III from Car- earlier study that documented nontarget infections duus thoermeri caused by P. carduorum (Politis et al., 1984). Green- house data from the present study support the fact that Leaves/ Number of P. carduorum is clearly virulent on C. thoermeri and is plant pustules not a good pathogen on other species, including those in Host genus Regen. No. In- Per Per the genus Carduus. Direct inoculation of plants in the species timea plantsb Total fected plant cm2 field gave similar results. It was not possible to use these or other plants for maintenance of P. carduorum, Carduus thoermeri 6 12 7.5 7.5 40.4 1.141 even under optimal conditions for infection and disease Cynara scolymus 8 12 4.9 0.0 0.0 0.000 6 12 4.3 1.0 3.6 0.048 in the greenhouse. Results in this study support find- 4 12 4.5 1.4 2.3 0.022 ings in other studies concerning the specificity of P. 2 12 2.4 0.4 1.1 0.018 carduorum from C. thoermeri (Baudoin et al., 1993; LSDc 0.6 4.9 0.115 Bruckart and Peterson, 1991). There has been a general application of ‘‘C. nutans’’ to a Regen. time, regeneration time (in weeks) for regrowth of C. ‘‘musk thistle’’ in the United States literature, includ- scolymus from crowns; C. thoermeri grown from seed and inoculated ing an important survey by Dunn (1976), a general key six weeks after planting. b Data are combined from two inoculations of six plants per by Gleason and Cronquist (1963), and in general, lists inoculation and treatment. of plant species by the Weed Science Society of America c LSD, least significant difference, P 5 0.05. (Weed Science Society of America, 1984), and the 220 BRUCKART ET AL.

USDA-ARS (Terrell et al., 1986). Musk thistle also has species, and severe infections (plants considered suscep- been identified as C. nutans in much of the entomologi- tible) were infrequent (,6% of the total individuals of cal literature on biological control. Cirsium). Older plants were not susceptible. McCarty (1985) suggests there are as many as four Considering these facts and the very susceptible species of large-flowered Carduus thistles (musk thistle) nature of C. thoermeri, a judgment was made that in North America, based on his identifications using the species other than C. thoermeri would not be at risk by ‘‘Flora Europaea’’ (Tutin et al., 1976). These are C. introduction of P. carduorum from C. thoermeri for thoermeri, C. nutans, C. macrocephalus Desf., and an biological weed control into North America. To date we unnamed species from British Columbia, Canada (Mc- know of no rust disease on artichoke, including those Carty, 1985). Moore and Frankton (1974) and Kartesz grown in California where another strain of P. carduo- and Kartesz (1980) each list three subspecies of C. rum occurs commonly on Carduus tenuiflorus Curtis nutans that are synonymous with names applied by (Watson and Brunetti, 1984). The C. tenuiflorus isolate McCarty: ssp. leiophyllus (5 C. thoermeri), ssp. macro- from California also caused limited infection of arti- lepis (Peterm.) Kazmi, [either C. nutans ssp. nutans choke under our greenhouse conditions (Bruckart and (Kartesz and Kartesz, 1980) or C. thoermeri (U.S. Peterson, 1991). Department of Agriculture, Soil Conservation Service, These data were used in a proposal to the APHIS for 1982)], and ssp. nutans (5 C. nutans). In ‘‘Flora Euro- permission to introduce P. carduorum into the United paea,’’ the large-flowered Carduus section has its own States for biological control of C. thoermeri. Permit was specific key within Carduus and the disclaimer, ‘‘this is issued in 1987 for a field study in Virginia by APHIS a difficult group in need of further study’’ (Tutin et al., and the Virginia Department of Agriculture. Results 1976). from the field study, which included tests for susceptibil- Results in this study support the concept of clearly ity of nontarget Cirsium and Cynara spp., support distinct Carduus species. All individuals in the collec- conclusions made from results in this study involving tions identified as C. thoermeri were uniformly suscep- artificial inoculations of nontarget species (Baudoin et tible. Those clearly identified as C. nutans or C. macro- al., 1993). cephalus had very low incidence of disease. The fact that accessions suspected to be hybrids of C. thoermeri ACKNOWLEDGMENTS were also as susceptible as C. thoermeri was not anticipated and the meaning of this is unclear. Also, the Research described herein was funded partially through a coopera- tive agreement with the California Department of Food and Agricul- high incidence of susceptibility among the unidentified ture (58-32U4-3-569, CDFA No. 5565). The technical support of Dave Carduus species from British Columbia indicates it Koogle and Dennis Johnson (USDA-FDWSRU) and Dr. Ludmilla may be a variant or hybrid of C. thoermeri. This has not Sedlar and Urs Lochner (ETH) is acknowledged. Artichoke seeds been confirmed to date. were provided by Dr. Neil DeVos, Artichoke Research Association and Host specificity is an important part of risk assess- USDA-ARS, Vegetable Production Research Unit, Salinas, Califor- nia. Seeds of Carduus spp. were provided by Dr. M. K. McCarty, ments for foreign plant pathogens in weed control. USDA-ARS (retired), Lincoln, Nebraska. Seeds of the various Cir- Difficulty in identification and naming of the large- sium species were provided by the California Department of Food and flowered Carduus spp. is one issue. In this study, we Agriculture (Cathlene Casanave), the Indiana Dunes National Lake- wanted also to determine the ability of P. carduorum to shore, Porter, Indiana (Christy Fox, Noel Pavlovic); Lafayette Home Nursery, Lafayette, Illinois (Jock Ingels); Michigan Department infect other, closely related species. Carduus spp. are Natural Resources, Mio, Michigan (Sylvia Taylor); J. K. Morton, most closely related to species of Cirsium, separated in University of Waterloo, Guelph, Ontario, Canada; Ohio Department Gleason and Cronquist (1963) on the morphology of the of Natural Resources, Columbus, Ohio (Susan Munch); and S. S. pappus hairs. There are 96 species of Cirsium recog- Rosenthal, USDA-ARS, Biological Control of Weeds Research, Boze- nized in the United States (U.S. Department of Agricul- man, Montana. We are sincerely grateful for the interest and support of these individuals. ture, 1982), including two that are important introduced weeds. The most closely related species of commercial importance is Cy. scolymus (artichoke). REFERENCES Despite the close relationship between Carduus, Baudoin, A. B. A. M., Abad, R. G., Kok, L. T., and Bruckart, W. L. Cirsium, and Cynara spp., it is evident from the data in 1993. Field evaluation of Puccinia carduorum for biological control this study that species outside the genus Carduus are of musk thistle. Biol. Control 3, 53–60. unsuitable hosts of P. carduorum. This includes 25% Bruckart, W. L., and Peterson, G. L. 1991. 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