ARS | Publication request: MICROBOTRYUM CARDUI, A POTENTIAL BIOLOGICAL CONTROL AGENT FOR CARDUUS THISTLES IN THE U.S.

Submitted to: Proceedings of Northeastern Weed Science Society
Publication Type: Abstract Only
Publication Acceptance Date: November 15, 2003
Publication Date: January 12, 2004
Citation: Berner, D.K., Paxson, L.K., Luster, D.G. 2004. Microbotryum cardui, a potential biological control agent for carduus thistles in the U.S. Proceedings of Northeastern Weed Science Society. 57:6-9.

Interpretive Summary: Milk thistle is a problematic invasive weed in the USA and a target of classical biological control efforts. During a pathogen collection trip in June, 2001 numerous diseased flower heads of milk thistle were found in a weedy field in southeastern Greece. Infected plants were mixed with apparently healthy milk thistle plants in the field. When flower heads were split open, powdery masses of spores were found in all of the individual flowers. All of the flower heads on infected plants were similarly infected, and no seeds were found from infected plants. Spores from diseased flowers conformed to the description for a smut fungus (Microbotryum cardui). Eight other reported hosts of the fungus are all weedy species. This narrow host range and the effectiveness of the smut in eliminating weed reproduction make the fungus a good candidate agent for biological control of milk thistle.

Technical Abstract: Carduus species and the related thistles Silybum marianum (L.) Gaertn. and Onopordum acanthium L. are problematic invasive weeds in the USA and targets of classical biological control efforts. During a pathogen collection trip in June 2001, numerous smutted capitula of Silybum marianum (L.) Gaertn. syn. Carduus marianus L. (milk thistle) were found in a weedy field in southeastern Greece. Infected plants were mixed with apparently healthy plants in the field. When smutted capitula were split open, powdery masses of teliospores were found in the ovaries of all florets. All of the capitula on infected plants were similarly infected, and no seeds were found from infected plants. Teliospores from smutted ovaries conformed to the description for Microbotryum cardui (A. Fischer v. Waldh.) Vanky (syn. ustilago cardui A. Fischer v. Waldh.): globose, pale brown, 14-18 pm diameter, coarsely reticulate with coarse wings on the margins extending out 2-3 pm. Florets of S. marianum and Onopordum arabica were inoculated with sporidial cultures of M. cardui, and the organism was recovered from embryos of the resulting seeds of both species. Eight other reported hosts of M. cardui are all weedy species of Carduus. Both the narrow host range and the effectiveness of the smut in eliminating weed reproduction make M. cardui a good candidate agent for biological control of Carduus and related thistles in the USA.