Limitations of In Vitro Strain Screening Methods for the Selection of Sclerotinia spp. as Potential Mycoherbicides against the Perennial Weed Ranunculus acris

Nineteen isolates of Sclerotinia sclerotiorum and three isolates of S. minor were inoculated on to excised tissues and intact plants of Ranunculus acris (giant buttercup), to evaluate their pathogenicity. All isolates proved pathogenic, with S. sclerotiorum being more pathogenic than S. minor on both excised tissues and intact plants. Seven of the S. sclerotiorum isolates were more pathogenic than the others on excised tissues, but no significant differences in pathogenicity were found between any of the isolates when they were inoculated on to intact plants. The results of this study indicate that the excised tissue method cannot be used to predict whole plant mortality, nor, therefore, the mycoherbicide potential of strains of S. sclerotiorum for this perennial weed.