First Report of Smut Caused by Microbotryum silybum on Ivory Thistle

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.