A COMPARISON OF AUXOTROPHIC AND WILD STRAINS OF SCLEROTINIA-SCLEROTIORUM USED AS A MYCOHERBICIDE AGAINST CALIFORNIAN THISTLE (CIRSIUM-ARVENSE)

Two auxotrophic mutant strains of Schlerotinia sclerotiorum were teste d in the greenhouse for pathogenicity on Cirsium arvense (Californian thistle) with and without amino acid amendments. An arginine auxotroph ic mutant, with an amendment of the amino acid, followed an identical disease progress curve to that of the wild strain of the pathogen from which it was derived. However, when deprived of the amino acid amendm ent it was still highly pathogenic. A leucine auxotrophic mutant demon strated poor pathogenicity without a leucine amendment, but improved p athogenicity with the addition of the amino acid. However, both of the se treatments were inferior to the two wild strains tested and the arg inine auxotroph with and without amendments. A field experiment was co nducted on C. arvense stems in permanent pasture to compare the pathog enicity of amended auxotrophic strains and wild strains of S. scleroti orum applied as a granule in a wheat-based carrier. The two wild strai ns gave significant reductions in thistle cover within 3 months of tre atment, and subsequent reductions in thistle stem height and density d uring the following season. There was no evidence that the auxotrophic strains reduced thistle cover in the season the treatments were appli ed, but they did reduce subsequent stem density in the following sprin g. To determine disease carry-over associated with the wild and auxotr ophic strains of the pathogen, rape was planted into subplots over the next three consecutive seasons. Despite substantial populations of sc lerotia being present in the soil, especially in the first season afte r treatment of the thistles, no disease of rape caused by S. sclerotio rum was detected over the three seasons in any of the plots. Sclerotiu m populations of S. sclerotiorum in the soil declined by over 50% betw een 20 and 32 months after treatment, but there was no decline over th e subsequent 12 months. The trial demonstrated that the auxotrophic st rains were less field fit compared with the wild strains and that the presence of inoculum and a susceptible host to S. sclerotiorum were no t the only prerequisites for disease development. It was concluded tha t use of a trap crop following treatment is not a suitable method for determining the risk of using this pathogen as a mycoherbicide in past ure.