Pathogenicity of Sclerotinia sclerotiorum on Ranunculus acris is in dairy pasture

Fifty-four isolates of Sclerotinia sclerotiorum from Ranunculus acris and o ther natural hosts were applied as mycelial infested kibbled wheat onto 6 m onth-old R. acris plants in two glasshouse screening experiments. Most isol ates (90%) did not differ in their pathogenicity towards R. acris. One isol ate S. sclerotiorum G45, was selected based on its ability to cause severe disease and suppress regeneration of R. acris. A field experiment was condu cted to determine the efficacy of S. sclerotiorum (G45) against R. acris in infested dairy pastures in the Takaka Valley, Golden Bay New Zealand. Isol ate G45 was formulated as a wettable powder and was applied as a slurry at 20 and 40 ml/plant in December 1995. After 10 weeks, regeneration from the crown of treated plants was apparent and a second application of S. sclerot iorum was made in February 1996. Best control of R. acris was obtained when the plants were inoculated in full flower in December: At the first time o f treatment, the 40 mi application of S. sclerotiorum slurry reduced the to tal dry weight of R. acris by an average of 57%. The second application had no effect on total dry weight, possibly because moisture levels were not s ufficient for S. sclerotiorum infection. This study confirmed S. sclerotior um to be an aggressive pathogen of R. acris under both glasshouse and field conditions As a result, this pathogen has potential as a mycoherbicide for R. acris. Further experiments are required to explore ways of enhancing th e efficacy of S. sclerotiorum against R. acris by manipulation of the host, pathogen and environment.