Reduced Virulence of Trichothecene-Nonproducing Mutants ofGibberella zeaein Wheat Field Tests

We have analyzed the role of trichothecene toxins in the virulence of the fungus Gibberella zeae (anamorph, Fusarium graminearum) on wheat (Triticum aestivum). Trichothecene-nonproducing mutants of G. zeae were obtained by disrupting Tri5, the gene encoding trichodiene synthase, which catalyzes the first committed step in the trichothecene biosynthetic pathway. Trichothecene-nonproducing mutants appear to be normal in growth and development under laboratory conditions. One such mutant was selfed to generate a meiotic revertant that lost the disruption vector and recovered trichothecene production. In the present study, virulence was assessed in 1994 and 1995 by controlled field inoculation of G. zeae spore suspensions into flowering wheat heads. Trichothecene-nonproducing (Tri5-) mutants were less virulent than the trichothecene-producing (Tri5+) parental and revertant strains in their ability to cause head scab on field-grown wheat. Although trichothecene-nonproducing strains colonized wheat heads, the infected heads showed less disease by several parameters we tested, including head bleaching symptoms, seed weight, seed viability, and trichothecene contamination. This evidence indicates that trichothecenes are virulence factors in wheat head scab.