Susceptibility of Carduus, Cirsium,And Cynaraspecies Artificially Inoculated with Puccinia Carduorumfrom Musk Thistle
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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 inocu- lated, 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 iso- lated 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.