Verticillium dahliae and other pathogenic fungi in Cirsium arvense from New Zealand pastures: occurrence, pathogenicity and biological control potential

Cirsium arvense (Californian thistle) populations in pastures throughout New Zealand were surveyed in NovemberDecember 2005 (30 sites) and JanuaryMarch 2006 (94 sites) to identify potential biological control agents for this weed. Fungi were isolated from healthy shoots and shoots showing leaf yellowing/browning, stunting or localized lesions. Verticillium dahliae was isolated most frequently, being detected at 30% of the sites in 2005 and at 51% in 2006. Other pathogenic/saprophytic fungi isolated included Sclerotinia sclerotiorum, Plectosphaerella cucumerina and species of Cylindrocarpon, Rhizoctonia and Phoma. Inoculating cut shoots of C. arvense with conidia of V. dahliae, or cutting shoots with a wetted blade previously used to cut infected shoots resulted in yellowing of leaves and shoot death. Spread of V. dahliae infection among plants facilitated by cutting when wet may explain the reported demise of C. arvense populations mown during rainfall and the fungus may have potential as a bioherbicide.