The pine needle blight pathogen Dothistroma septosporum produces a polyketide toxin, dothistromin. This paper reports that loss of the ability to produce dothistromin did not affect the pathogenicity of D. septosporum to Pinus radiata in a laboratory-based pathogenicity test. However, dothistromin synthesis provided an advantage to the D. septosporum wild-type, compared to dothistromin-deficient mutants, in growth competition with other fungi in vitro. Other pine-needle inhabitants, such as the latent pathogen Cyclaneusma minus and the endophyte Lophodermium conigenum, were inhibited by dothistromin-producing D. septosporum. Therefore, it was concluded that dothistromin is not a pathogenicity factor, but that it may play a role in competition of D. septosporum with other fungi in its ecological niche.