Effect of volume and concentration of conidial suspensions of Coniothyrium minitans on infection of sclerotinia Sclerotiorum sclerotia

White mould, caused by Sclerotinia sclerotiorum, is a serious disease affecting a wide range of agricultural and horticultural crops. Biological control is one option available to limit its damage. Field experiments to evaluate various concentrations and volumes of Coniothyrium minitans spore suspensions applied to S. sclerotiorum-infected bean crops were conducted in 1997 and 1998. Percentage sclerotia infected by C. minitans were scored. Three replicate experiments were performed in time in 1997 with 21 combinations of isolates, volumes and concentrations, including two controls. In 1998, 22 combinations of isolates, volumes and concentrations plus two controls were used, combined with the absence or presence of a maize buffer, with two replicates for each. Isolates as well as concentration and volume had no effect on infection by C. minitans, but there was a significant effect of total dose (volumeconcentration) of inoculum applied over the full range from 100 L ha-1 at 104 conidia mL-1 to 1000 L ha-1 at 107 conidia mL-1. Percentage infected sclerotia increased linearly with log (dose) as well as from 1 to 4 weeks after application of C. minitans, and reached a level of about 100% at high doses under the humid conditions of 1998. Apothecia of S. sclerotiorum developing from sclerotia in collected soil samples from the 1997 experiment showed no significant effect of C. minitans inoculum dose, but there was a significant effect of the replicate experiments. The influence of weather conditions is highlighted, and the implications of the results for cost-effective biocontrol of S. sclerotiorum are discussed.