Risk Analysis of Sclerotinia sclerotiorum for Biological Control of Cirsium arvense in Pasture: Sclerotium Survival

Abstract

The plurivorous ascomycete, Sclerotinia sclerotiorum , is being evaluated as a mycoherbicide for the biocontrol of Cirsium arvense in pastures. Increased inoculum levels potentially add to the risk of disease in susceptible crops sown at or near the biocontrol site. This paper summarises: (1) two series of experiments that quantify the survival of soilborne sclerotia in sheep-grazed pastures in the Canterbury province of New Zealand; (2) a survey of the natural sclerotium population densities in four contrasting agricultural activity strata; and (3) a model for predicting the time period for which susceptible crops should be withheld from a site following the cessation of biocontrol. In the first series of two experiments, mycoherbicide-induced populations of sclerotia were sampled annually for four years. Half-lives were estimated to be 395 and 218 days. In the second series of eight experiments, sclerotia were buried in mesh bags in four different soils for three to four years, and exhibited half-lives of 206-506 days. In the survey, the four strata were permanent pasture, arable crop/pasture rotation, continuous cropping and market garden. The estimated sclerotium density in the market garden stratum was 9m -2 ; few sclerotia were found in the other three strata. Using the withholding period model, the average time for a biocontrol-induced sclerotium population to decay exponentially to a natural 'market garden' density of 9 sclerotia m -2 was estimated to be four years, assuming an average initial density of 125 sclerotia m -2 and an average half-life of 351 days. A conservative upper limit for the withholding period was estimated to be ten years.