INOCULUM-DISEASE RELATIONSHIP IN SCLEROTINIA STEM ROT OF RAPESEED IN SASKATCHEWAN

Several factors relating to inoculum density and disease incidence of sclerotinia stem rot were monitored in commercial rapeseed fields in east and west central Saskatchewan in 1983 and 1984 when the crops were in bloom. The density of carpogenically germinated sclerotia was determined. The frequencies of live and dead petals, leaf axils and leaf bases infested with Sclerotinia sclerotiorum were determined by plating on potato dextrose agar containing rose bengal and streptomycin. Disease incidence was determined shortly before the crops were swathed. The results demonstrated significant relationships between petal infestation at early bloom and disease incidence. Significant relationships between germinated sclerotia and disease incidence were obtained only with large-scale sampling. Infested petals and disease were regularly found when apothecia were absent, thereby demonstrating the role of extrinsically produced ascospores in the infestation of crops. The possibility of using petal infestation to forecast stem rot and improve the economics of chemical control is discussed.