Effects of host plant resistance and fungicide application on phoma stem canker, growth parameters and yield of winter oilseed rape

Multifactorial field trials were conducted over three cropping seasons (2012–2015) to assess the effects of selection of triazole fungicides when applied either alone or in combination on growth of winter oilseed rape, development of phoma stem canker, crop wintering and crop yield. The plots consisted of four winter oilseed rape cultivars differing in their levels of resistance to phoma stem canker and subplots of five fungicides with growth regulatory activity (prochloraz/tebuconazole, mepiquat/metconazole, tebuconazole, tebuconazole/prothioconazole and a mixture of fluxapyroxad/tebuconazole) which were applied twice; in autumn (BBCH 14–18) and in spring (BBCH 30–39). Visual scoring such as growth parameters like plants per m2 and plant height, yield parameters like number of side branches, pods and seeds per plant, thousand grain weights and seed yield as well as evaluation of phoma leaf spots and basal stem canker were performed using standard procedures. Plant height was significantly affected by cultivars and fungicide treatments. In contrast, winter killing (%) was not reduced significantly. Application of mepiquat/metconazole reduced the winter killing below 10% compared to the untreated control. Development of phoma stem canker was affected by fungicides and cultivars. At the final rating date (BBCH 81), the moderately resistant cultivars Genie and Vitara exhibited less disease severity than the susceptible cultivars Elektra and PR 46W20 in untreated plots. Fungicide treatments were more effective in reducing the disease in moderately resistant cultivars than in susceptible ones. The lowest percentage of disease severity was achieved when fluxapyroxad/tebuconazole combination was applied to plants. To our knowledge, this study is the first to report multi-year efficacy of fluxapyroxad/tebuconazole against phoma stem canker in oilseed rape cultivations. Cultivars showed differences in all yield parameters. However, with the exception of seed yield the application of fungicides did not significantly impact the different yield parameters. Application of fluxapyroxad/tebuconazole was the most effective treatment in reducing yield losses.