New fungicides for oilseed rape: Defining dose-response activity

Abstract

Fungicides are used on over 90% of winter oilseed rape crops at a total annual cost to farmers of around £12 million. Diseases cause estimated losses of up to £80 million per annum and effective management requires integration of resistant cultivars with agronomic factors and fungicides. Cultivar resistance alone has not provided adequate control of the three most important diseases: stem canker (Leptosphaeria maculans), light leaf spot (Pyrenopeziza brassicae) and sclerotinia stem rot (Sclerotinia sclerotiorum). The overall aim of the project was to determine the dose-response activity of new and standard fungicides against the major diseases of oilseed rape.

Replicated field experiments with disease and yield assessments were done in farm crops of susceptible cultivars in harvest years 2006 2007 and 2008 against phoma stem canker (6 sites), light leaf spot (4 sites) and sclerotinia stem rot (6 sites). The seven or eight test fungicides were applied at 0.25. 0.50, 0.75 and 1.00 (full label) doses were applied in 200 litres water/ha by OPS knapsack sprayer to plots (40-60m²) when phoma experiments had 10-20% plants affected and 6-8 weeks later, in November and March/April for light leaf spot and at early to mid-flowering for sclerotinia stem rot control.

Disease data was obtained from all experiments and disease severity was moderate to high in 14 experiments. Yield data was obtained from 15 experiments and significant responses to the fungicide treatments were recorded at 10 sites. The response to fungicide treatments was generally related to disease severity, with largest responses (c. 2 t/ha) being obtained where sclerotinia stem rot was very severe (80-82% plants affected). When data for three years was combined, disease control and yield responses to fungicide products and dose were all highly significant for each disease.

Prothioconazole (Proline) performed well against light leaf spot and in mixture with tebuconazole (Prosaro) gave the highest yield at phoma stem canker sites. Filan, Compass and Proline were most effective against sclerotinia, though differences between products were most evident under high disease pressure. Whilst products can perform very well at 0.5 full label dose, higher doses are required to optimise economic performance under high disease pressure. Plant growth regulatory effects contributed to yield responses and both positive and negative effects were detected. Careful interpretation of the yield data is required when using it to guide fungicide inputs on farms.