Effects of a penthiopyrad and picoxystrobin fungicide mixture on phoma stem canker (Leptosphaeria spp.) on UK winter oilseed rape

In the UK, fungicides are often used to control phoma stem canker on winter oilseed rape. Field trials were established near Boxworth, Cambridgeshire for four cropping seasons (2011/2012, 2012/2013, 2013/2014 and 2014/15) to test the efficacy of a new fungicide mixture Refinzar® (penthiopyrad + picoxystrobin) by comparison to an existing fungicide Proline 275® (prothioconazole) against phoma stem canker (Leptosphaeria spp.) and the effect on winter oilseed rape (cv. Catana) yield. In each season, weather data were collected from a weather station at Boxworth and the release of ascospores was monitored using a nearby Burkard spore sampler. The patterns of ascospore release differed between seasons and related to weather conditions. Fungicides penthiopyrad + picoxystrobin and prothioconazole were applied in October/November when 10 % of plants had phoma leaf spotting (T1, early), 4/8 weeks after T1 (T2, late) or at both T1 and T2 (combined). When phoma leaf spot symptoms were assessed in autumn/winter, penthiopyrad + picoxystrobin and prothioconazole both decreased numbers of phoma leaf spots caused by L. maculans; there were few leaf spots caused by L. biglobosa. Penthiopyrad + picoxystrobin and prothioconazole both reduced phoma stem canker severity before harvest compared to the untreated control but did not increase yield in these seasons when epidemics were not severe. In 2013/2014, the presence of L. maculans and L. biglobosa in upper stem lesions or stem base cankers was determined by species-specific PCR. The proportions of stems with L. maculans DNA were much greater than those with L. biglobosa DNA for both upper stem lesions and basal stem cankers. These results suggest that both penthiopyrad + picoxystrobin and prothioconazole can decrease phoma stem canker severity on winter oilseed rape in severe disease seasons.