EVALUATION OF FUNGICIDE APPLICATION AT LATE BLOOM FOR THE CONTROL OF SCLEROTINIA STEM ROT IN RAPESEED (CANOLA)

According to current recommendations, foliar fungicides for the control of sclerotinia stem rot in rapeseed should be applied at early or full bloom but not at late bloom. Field experiments were conducted in several areas of Saskatchewan in 1987 and 1988 to determine if spraying with benomyl at late bloom could adequately control the disease in at least some situations. The experiments were designed to determine if the growth stage at which inoculum of 12. sclerotiorum first becomes widespread in the crop affects the efficacy of late bloom application. Rate of application and nozzle type were varied in a few tests. Disease levels were high enough to warrant collection and analysis of disease incidence data at 4 of 5 locations in 1987 and 5 of 10 locations in 1988. Late bloom application of benomyl at the recommended rate (0.6 kg a.i./ha) significantly reduced disease incidence compared to the check in all tests except one. In four tests, disease reductions of >/= 80% were achieved. Generally, the highest levels of control were achieved when high levels of inoculum were not present in the crop until at least full bloom. In these situations benomyl was applied at approximately the same time that most infections were initiated. Application at 0.3 kg a.i./ha caused significant reductions in disease compared to the check at 3 of 4 locations, indicating that it may be possible to control stem rot adequately by spraying at less than the recommended rate. Significant control was also achieved at 2 locations by spraying at 0.15 kg a.i./ha. There was limited evidence that small spray droplets generated by 11001 nozzles may improve control with benomyl. The effect of time of infection (ie. plant growth stage) on yield loss was studied in growth chamber and field experiments. It was found that plants infected at late bloom suffer a significantly lower yield reduction than those infected earlier in the flowering period. In a field experiment, the yield reduction per plant was only 8% when high inoculum levels were not present until late bloom. More research is required in this area before fungicide application at late bloom can be considered an economically feasible option for rapeseed growers. Finally, the disease incidence/disease severity (1-S) relationship for sclerotinia stem rot was studied using regression analysis. The slopes of regression equations were similar between years and, with the exception of one field, were similar among fields. This suggests that factors such as rainfall, stand density and inoculum levels have little effect on the I-S relationship. Fungicide application also did not appear to have an effect. The equation disease severity= 0 + 0.692 x disease incidence described the overall relationship (R2=95.5%). This could be used to predict severity from incidence data, at least within Brassica napus cv. westar, thereby simplifying disease assessment