Lack of Association between Cerato-ulmin Production and Virulence in Ophiostoma novo-ulmi. Christine G Bowden. Department of Botany, University of Toronto, Erindale Campus, Mississauga, Ontario, L5L 1C6, Canada. Eugene Smalley(2), R. P. Guries(3), Martin Hubbes(4), Bradley Temple1, and Paul A. Horgen(1). (1)Department of Botany, University of Toronto, Erindale Campus, Mississauga, Ontario, L5L 1C6, Canada; (2)Department of Plant Pathology, University of Wisconsin, Madison 53706 U.S.A. (3)Department of Forestry, University of Wisconsin, Madison 53706 U.S.A. and (4) Faculty of Forestry, University of Toronto, Toronto, Ontario, M5S 3B3, Canada. MPMI 9:556-564. Accepted 20 June 1996. Copy right 1996 The American Phytopathological Society.

Cerato-ulmin (CU), a hydrophobin produced by Ophiostoma novo-ulmi, has been implicated in the pathogenicity of this fungus on elm. We have generated a CU^- mutant by transformation-mediated gene disruption of a highly virulent (aggressive) strain of O. novo-ulmi. The inability of the mutant to synthesize CU was confirmed by transcript analysis as well as turbidity and immunological measurements. Bioassay of the CU^- strain in highly susceptible elm trees indicated no difference in the virulence parameters, percent vascular discoloration, and percent foliar wilting, when compared with the wild type. Our results indicate that the inability to produce CU had no measurable effect on the ability of O. novo-ulmi to produce symptoms of Dutch elm disease on inoculated elms.