Roles of varieties and fungicides in managing light leaf spot and canker in winter oilseed rape

Abstract

The contribution of varietal resistance and fungicides to disease control and yield in winter oilseed rape was investigated at sites in the south west, east and north of England and in the north of Scotland in each of three harvest years, 1995-1997. The varieties, Bristol, susceptible to light leaf spot and resistant to canker, and Nickel or Rocket, resistant to light leaf spot and susceptible to canker, were treated with a range of fungicides at different timings and doses at four sites.

The active ingredients used were flusilazole plus carbendazim (as Punch C), carbendazim (as Bavistin or Carbate), tebuconazole (as Folicur) and difenconazole (as Plover). These were applied once at full dose in the autumn and as split half-dose treatments in the autumn and spring. Two full dose sprays of flusilazole plus carbendazim were used as a standard against which to compare the effectiveness of half-dose treatments.

Overall, light leaf spot was the most important disease in these experiments. Some phoma leaf spotting and canker developed at sites in eastern and northern England and white leaf spot was present at sites in Devon.

Nickel or Rocket were consistently higher yielding than Bristol at all sites except for two years in Suffolk. The mean cross-site yields were calculated from 11 sites (excluding one experiment in Cornwall where no autumn sprays were applied because of small plant size) and averaged 3.44 t/ha for cv.Bristol and 3.97 t/ha for cvs Nickel or Rocket. The mean response to all fungicide treatments was 0.28 t/ha on cv. Bristol and 0.17 t/ha on cvs Nickel or Rocket.

Significant treatment differences for yield were recorded in 9 out of 12 experiments, all eight showed differences between varieties and 7 showed differences between fungicides. A number of treatments yielded less than the untreated control and whilst not statistically significant effects, these represented 22% of site x variety x fungicide combinations.

Two sprays of flusilazole plus carbendazim at full dose gave the largest yield response of 0.52 t/ha on cv. Bristol and 0.35 t/ha on cvs Nickel or Rocket and this was just profitable. The most profitable treatments were a full dose of flusilazole plus carbendazim in autumn for Bristol (£32/ha) and a full dose of difenoconazole in autumn on cvs Nickel or Rocket (£12.5/ha). Caution is needed in extrapolating from these results as phoma disease pressure was generally low and light leaf spot infection was severe in the autumn at some sites.

Light leaf spot infection was most severe in Scotland and in the south west, resulting in an estimated yield loss of 3 t/ha in Aberdeen in 1995/96. In contrast, there was no yield response to fungicide treatment at Aberdeen in 1997, despite light leaf spot affecting 13% leaf area in April. These experiments demonstrated that light leaf spot control in the spring can be worthwhile (0.74 t/ha response on cv. Bristol in Cornwall in 1995) but the most damaging attacks occur when symptoms are apparent in the autumn and severe frosts lead to substantial loss of plants.

The project provided comparative data on fungicide efficacy. Flusilazole plus carbendazim and tebuconazole were highly effective against light leaf spot. Difenoconazole and flusilazole plus carbendazim showed good activity against white leaf spot and phoma leaf spot, though epidemics of the latter were limited. Convincing control of canker was not achieved. Carbendazim treatments were inconsistent and the presence of MBC resistant strains of light leaf spot in Scotland is thought to have contributed to this.

Routine treatment of winter oilseed rape with fungicides is unlikely to be cost-effective. At individual sites losses approaching 1 t/ha have been recovered with fungicides, so careful targeting of sprays is required. There are significant advantages in using resistant varieties and, in the case of light leaf spot resistance, this equated to 0.53 t/ha (£79/ha).

The untreated yield of the resistant variety (Nickel/Rocket) was higher than the fungicide treated yield of the susceptible variety (Bristol). Single spray treatments may be adequate for control of light leaf spot, provided disease pressure is not high. Further work to optimise fungicide inputs and to forecast disease epidemics is recommended.