Improving stem canker control in winter oilseed rape by accurate timing of fungicide applications based on disease forecasts

Abstract

This three year project focused on improving strategies for control of canker (Leptosphaeria maculans) in winter oilseed rape. There were three main objectives, which were addressed using historical data and field experiments: a) to quantify yield losses in relation to the development of phoma leaf spot and the severity of stem canker epidemics, b) to identify factors affecting ascospore discharge and hence phoma leaf spot attacks and c) to optimise fungicide timing for stem canker control and yield response.

Canker incidence at harvest was related to the maximum incidence of phoma leaf spot up to 100 days from sowing and maximum incidence of phoma leaf spot up to stem extension, on four cultivars. A second relationship between phoma leaf spot pre-flowering and phoma stem lesions at harvest was also demonstrated, but this was not applicable across cultivars. The development of canker in spring was related to the development of phoma leaf spot in the previous autumn/winter. The thermal time (degree-days) from the first appearance of phoma leaf spot (autumn) to the first appearance of crown canker (spring) varied between cultivars and averaged 1220-1240 degree-days for cvs Lipton and Capitol and 1120-1140 for cv. Apex. The relationship between % yield loss and % plants with different stem canker severity scores and were alos associated with the early development of stem lesions. Air-borne ascospores of Leptosphaeria maculans were present from autumn (September/October) to spring (April/May). Ascospores were first detected after 16-23 days with rain from 1 August and this varied by three weeks between years in this project. Phoma leaf spotting appeared in untreated field plots 14-25 days after ascospores were first detected in autumn.

Control of canker and light leaf spot was investigated using sprays of difenoconazole + carbendazim applied to cv. Apex at four main timings with all combinations of one to four applications. Secondary treatments examined the impact of delaying the first or last spray by two or four weeks. Canker was more severe in 2000 than 1999, reflecting the earlier phoma leaf spot epidemic in autumn and was controlled by various multiple spray programmes. Significant yield responses (16-31%) were recorded in four out of six experiments, the larger responses being associated with control of light leaf spot and canker. Optimum timings for fungicides differed between sites and years and a two spray programme provided more consistent benefits than single sprays. Early phoma leaf spot epidemics are a major cause of yield loss but can be managed economically with fungicides applied in autumn and winter. Conversely, phoma epidemics developing from December onwards may have little effect on yield. The project has improved understanding of when canker causes yield loss and this will lead to better assessment of the need to apply fungicides.