USE OF PETAL INFESTATION TO FORECAST SCLEROTINIA STEM ROT OF CANOLA - EVALUATION OF EARLY BLOOM SAMPLING, 1985-90

A pilot project for forecasting sclerotinia stem rot of canola was conducted in Saskatchewan from 1985 to 1990. Petals were collected from commercial canola crops at early bloom by farmers and research personnel. Using an agar plate test, percentage petal infestation with inoculum of Sclerotinia sclerotiorum was assessed; disease risk was then forecast based on a relationship previously demonstrated between petal infestation and disease incidence. Disease incidence was determined in each crop shortly before harvest. Regression analysis demonstrated a positive relationship between petal infestation at early bloom and disease incidence in some years; however, the strength of the relationship varied. Forecasts based on petal infestation during the early bloom period were relatively accurate when disease risk and incidence were low, but less so when they were moderate or high. Crop canopy density also influenced the relationship of disease incidence to petal infestation: in dense crops disease incidence was higher per unit of petal infestation. The petal testing system had an overall success rate of 73%. The results demonstrated that forecasts of sclerotinia stem rot based on petal infestation and collection of samples by farmers may be commercially feasible. However, factors such as canopy density and changing inoculum levels during flowering need to be considered in developing a commercially acceptable system.