USING ASCOSPORE INFESTATION OF PETALS TO FORECAST SCLEROTINIA STEM ROT OF RAPESEED
Turkington, Thomas Kelly
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The potential of using the percentage of ascospore infested rapeseed petals to provide a quantitative forecast of the incidence of sclerotinia stem rot in rapeseed was studied in several areas of Saskatchewan in 1985 and 1986. A pilot project in which farmers collected petal samples was set up to study the commercial feasibility of such an approach. Farmers normally collected rapeseed petals during early bloom and were then given a disease risk forecast based on the percentage of infested petals determined by a laboratory agar plate test. In crops that had been sampled disease incidence was determined later in the growing season. Intensive field studies were also conducted to investigate the effect of sample size on estimates of petal infestation and disease incidence. Finally, laboratory studies were conducted to find a growth medium for the agar plate test which did not restrict the growth of Sclerotinia sclerotiorum, allowed rapid identification of the fungus and assured good protection from bacterial contamination. The results indicated that disease forecasting based on petal infestation would be feasible on a commercial scale, with farmers collecting petal samples. Forecasts were relatively accurate when the disease risk was low. However, problems occurred in identifying crops at moderate and high disease risk. This was attributed to moisture conditions and inoculum levels changing during flowering. It is suggested that repeated petal sampling during flowering might alleviate this problem. Crop canopy density appeared to be an important factor affecting the relationship of petal infestation with disease incidence. Disease incidence was higher per unit of petal infestation in crops with dense canopies. The intensive field studies indicated that a sample size of five to six sites per crop, with 40 petals tested per site could be used to estimate the mean percentage of petal infestation in the crop with a standard error of 5%. Based on results from laboratory experiments the growth medium for plating petals was changed slightly from 1985 to 1986. This change allowed the identification of S. sclerotiorum after 3-4 days In 1986 compared to 5 days in 1985. The possibility of incorporating prevailing moisture conditions, canopy density and changing inoculum levels in a future forecasting system is discussed.