Differential susceptibility to the mycoparasite Paraphaeosphaeria minitans among Sclerotinia sclerotiorum isolates

The mycoparasitic fungus Paraphaeosphaeria minitans (formerly Coniothyrium minitans), is increasingly used by farmers to reduce soilborne inoculum of Sclerotinia sclerotiorum. In France, its field efficacy tends to be higher in the North than in the South, leading to the hypothesis of possible regional differences in the susceptibility of the pathogen to the biocontrol agent. A standardized assay was developed and four quantitative criteria were used to assess the susceptibility to P. minitans of 75S. sclerotiorum isolates collected from different regions of France. There was no significant difference between the group of isolates from the North and that from the South, but wide differences were observed among isolates, with consistent responses for all quantitative criteria. This study suggests that biocontrol efficacy might vary locally depending on the frequency of less susceptible isolates and it brings to attention the possibility that selection pressure could lead to a gradual increase in the frequency of less susceptible isolates, as this biocontrol method becomes widely adopted by farmers. To our knowledge, this is the first report of the variable susceptibility of S. sclerotiorum to a commercialized strain of P. minitans and the first detailed characterization of a large group of isolates from France for traits related to fitness, such as mycelial growth and production of sclerotia.