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Centaurea solstitialis L. (yellow starthistle), family Asteraceae, an invasive weed in California and the western United States, is targeted for biological control. In the summer of 2003, an epidemic of unknown etiology on dying C. solstitialis plants was observed near Kozani, Greece (40°22′07″N, 21°52′35″E, elevation, 634 m). Plants had necrotic light brown leaf spots on the lower leaves and the decurrent leaf bases along the stems. Often, necrotic lesions extended along the stems to the capitula. Virtually all plants in a solid stand of C. solstitialis (approximately 0.5 ha) showed disease symptoms. Diseased plants were collected, air dried, and sent to the quarantine facility of the Foreign Disease-Weed Science Research Unit (FDWSRU), USDA/ARS, Fort Detrick, MD. On the basis of culture growth (45-cm diameter after 2 weeks at 25°C on malt extract agar), fungal morphology (1), and comparison with 21 internal transcribed spacer sequences in GenBank, the putative causal organism was identified as Cladosporium herbarum (Pers.:Fr.) Link. (teleomorph = Davidiella tassiana (De Not.) Crous & U. Braun). Sixteen C. solstitialis plants in the rosette stage and 16 plants in the bolted stage were inoculated with an aqueous suspension of spores (106 conidia ml-1) and placed in an environmentally controlled chamber at 25°C with 8 h of dew and 12 h of light daily. Plants in the rosette stage were resistant, but the fungus was very aggressive on bolted plants. Within 4 to 6 days of inoculation, necrosis developed on leaves and stems and then spread up the stems to the capitula, often resulting in plant death. The fungus also infected developing flowers. Cladosporium herbarum was reisolated from each of the 16 bolted C. solstitialis plants in two separate tests at the FDWSRU and from all bolted inoculated plants at the European Biological Control Laboratory (EBCL) in Greece. In the greenhouse at the EBCL, the pathogen readily spread to (and was isolated from) another 10 noninoculated C. solstitialis plants in close vicinity to the inoculated C. solstitialis plants. Results of host range tests will establish if this isolate of Cladosporium herbarum has the potential as a biological control agent of C. solstitialis in the United States and does not pose a threat to other Centaurea spp. used in horticulture. A voucher specimen has been deposited with the U.S. National Fungus Collections (BPI 863446). Live cultures are being maintained at the FDWSRU and EBCL, Greece. To our knowledge, this is the first report of a disease caused by Cladosporium herbarum on C. solstitialis. Reference: (1) M. H. M. Ho et al. Mycotaxon 72:115, 1999.

Slender wheatgrass (Elymus trachycaulus (Link) Gould ex Shinners subsp. trachycaulus), family Poaceae, tribe Triticeae, is a native North American grass that is used as a livestock forage. Ustilago phrygica, a systemic ovary-smut fungus, is native to Turkey and West Asia and is pathogenic on Aegilops spp. and Taeniatherum caput-medusae (L.) Nevski subsp. asperum (Simonk.) Melderis (medusahead), an invasive weed in the western United States that is targeted for biological control. An isolate of the fungus (U.S. National Fungus Collections, BPI 871725; GenBank Accession No. DQ139961) was collected from medusahead in Turkey and screened for possible use in classical biological control of this weed. Screening was done in quarantine in a BSL-3 facility of the Foreign Disease-Weed Science Research Unit, USDA, ARS, Ft. Detrick, MD. The focus of screening was determination of host range of the fungus among related native and agriculturally important grasses in North America. A procedure was developed to consistently and quickly produce disease on medusahead and other grasses. Without vernalization, plants inoculated with U. phrygica will not produce smutted spikes (seedheads). Teliospores of the fungus were vacuum inoculated (1) onto caryopses (seeds) of medusahead and slender wheatgrass, which were then placed on moist germination paper in a petri dish or on moist vermiculite in plastic boxes. The dishes, sealed with Parafilm, and the boxes, covered with lids, were placed in a dark refrigerator at 3°C. After 8 weeks, all seedlings were transplanted into pots on a greenhouse bench at 22 to 25°C and 14 h light (photosynthetic photon flux density [PPFD] 620 μmol·s−1·m−2). The plants began to flower and produce smutted spikes 40 days later. These tests were repeated once. Fourteen of sixty medusahead plants from inoculated caryopses incubated on germination paper and nine of twenty-four plants from caryopses incubated on vermiculite became smutted and produced numerous smutted spikes per plant. Partial systemic infection was the norm, and all diseased plants had some spikes that were not diseased. One slender wheatgrass plant of nine plants grown from inoculated caryopses incubated on germination paper was also smutted and produced three diseased spikes. Nielsen (2) indicated susceptibility of slender wheatgrass to U. phrygica, but only as a single entry in a table under the synonym Agropyron trachycaulum (Link) Malte ex H. F. Lewis in a report on susceptibility of Aegilops spp. to U. phrygica. Because this is an obscure mention of the susceptibility of slender wheatgrass to U. phrygica, the fungus-host association does not explicitly appear in literature and is absent from relevant databases. Our tests with the fungus confirm that slender wheatgrass is susceptible to U. phrygica and lead us to conclude that the fungus would not be a good candidate for classical biological control of medusahead in North America. This formal report should establish this fungus-host association in literature and ensure reference in plant disease databases. References: (1) C. C. Allison. Univ. Minn. Agric. Exp. Stn. Tech. Bull. August:1, 1936. (2) J. Nielsen. Can. J. Bot. 70:581, 1992.

Silybum eburneum Coss. & Durieu. (ivory thistle) and S. marianum (L.) Gaertn. (milk thistle) are dominant, invasive weeds in northern Tunisia (1). S. marianum is also invasive in the United States and targeted for biological control. The smut fungus Microbotryum silybum Vánky & Berner is a naturally occurring pathogen of S. marianum in Greece (2) but not in Tunisia or the United States. To assess the safety of the fungus for biological control in the United States, plants related to S. marianum were evaluated for susceptibility to M. silybum in the quarantine facility of the Foreign Disease-Weed Science Research Unit (FDWSRU), USDA/ARS, Fort Detrick, MD. Because of the close genetic relationship of S. eburneum to S. marianum, both were tested for susceptibility under greenhouse conditions at the FDWSRU. All inoculations were done by placing 5 mg of teliospores of M. silybum in the central whorl of rosettes with three to five true leaves. Individual plants in soil-filled pots were placed in a controlled chamber at 16°C with 10 h of light daily. Photon flux density in the chamber was 34 μmol·m-2·s-1 supplied by three 1.8-m long 115W fluorescent tubes and three 52W incandescent bulbs. The central whorl was misted with distilled water twice daily for 2 weeks and the temperature was then lowered to 8°C for 6 weeks. The plants were transferred to a greenhouse bench at 22 to 25°C with 14 h of light daily. Photon flux density on the bench was 620 μmol·m-2·s-1 provided by two 500W sodium vapor lamps, one 1,000W metal halide lamp, and incidental sunlight. After approximately 7 weeks, plants of each species had fully developed capitula that flowered normally, produced no flowers, or formed abnormal flowers. Abnormal capitula contained powdery masses of teliospores in the ovaries of the florets. In contrast to systemic infections that were observed in the field (2), different branches of bolted plants bore both diseased and normal capitula. In turn, diseased capitula of both species were either completely diseased (all florets filled with teliospores) or partially diseased. Four of ten S. marianum plants and six of nine S. eburneum plants were diseased. Pathogenicity tests were repeated four times with similar results. In Greece, field inoculation of S. marianum with 5 mg of teliospores produced an average of 89% diseased plants with an average of 250 g of teliospores produced per plant. A similar level of disease is possible for S. eburneum under field conditions. Teliospores from smutted ovaries of both plant species conformed to the description for M. silybum (2). Both species are annual plants that reproduce solely by seeds. Since M. silybum prevents seed production, this fungus has great potential as a biological control agent in the United States and Tunisia. A voucher specimen has been deposited with the U.S. National Fungus Collections (BPI 863477). Nucleotide sequences for the internal transcribed spacer region are available in GenBank (Accession No. AY285774). To our knowledge, this is the first report of M. silybum parasitizing S. eburneum. References: (1) G. Pottier-AlaPetite. Flore de la Tunisie: Angiospermes-Dicotylédones, Gamopétales, Tunis, 1981. (2) K. Vánky and D. Berner. Mycotaxon 85:307, 2003.

Severe leaf blight of Japanese stiltgrass (JSG) fromBipolarisdisease, causing significant decline in population density at some locations, has been reported sporadically in the field. Even so, much of the JSG in the mid-Atlantic is not diseased. Six populations of JSG from the field, one that was severely diseased byB. microstegiiand the others “healthy,” were tested by artificial inoculation for susceptibility to bothB. microstegii(five isolates) andB. drechsleri(three isolates). Populations of JSG in this study differed in their response to the twoBipolarisspecies, but within species ofBipolaristhe plant responses were consistent. Plants from the diseased population of JSG from Frederick, MD, were very susceptible toB. microstegii, and plants from other populations from Maryland (three locations), Delaware, and Indiana were not. In contrast,B. drechslericaused moderate disease on plants from all accessions but one, and it was significantly less aggressive than wasB. microstegiion the susceptible accession of JSG. Results of a limited host range determination only withB. microstegiirevealed hypersensitive responses, and therefore high levels of resistance, in corn (four cultivars) and sorghum (three accessions). The native, sympatric grass deertongue was not diseased in these tests. Results reveal a distinct differential response among populations of JSG to disease fromB. microstegii, while in contrast,B. drechsleriis capable of causing disease on a broader range of JSG populations.