Effect of Coniothyrium minitans on sclerotial survival and apothecial production of Sclerotinia sclerotiorum in field-grown oilseed rape

In two field trials with oilseed rape, Coniothyrium minitans was applied to soil as a maizemeal-perlite preparation in order to determine its effect on sclerotial survival and apothecial production of Sclerotinia selerotiorum. The mycoparasite infected sclerotia and decreased sclerotial survival, carpogenic germination and production of apothecia. Effects were greatest when inoculum of C. minitans was applied in autumn, at the time of sowing, rather than when it was applied in spring. C. minitans survived in soil for 2 years and spread to adjacent control plots and infected sclerotia within those plots. However, despite the fact that the inoculum potential of S. selerotiorum was reduced by C. minitans treatment, no disease control was obtained either in trial 1, where disease levels were low (0-20% of plant stems affected), or in trial 2, where disease levels were high (up to 70% of plant stems affected). Possible reasons for this failure of C minitans to control sclerotinia disease in oilseed rape, and strategies to improve its efficacy in the field, are discussed.