Effects of different fungicides on the severity of phoma stem canker

Abstract

Phoma stem canker, a disease of oilseed rape (Brassica napus) caused by sibling pathogens Leptosphaeria maculans and L. biglobosa. Both pathogens follow a monocylic disease cycle that causes leaf spotting in autumn/winter and stem cankers in spring/summer. Most severe cankers decrease transportation of water and nutrients. Fungicides are important for phoma stem canker control. Triazole fungicides currently dominate the market, although reduced sensitivity in some plant pathogen species is a concern. Moreover, L. maculans and L. biglobosa have shown differing level of sensitivity to triazole fungicides. Therefore, increased knowledge on controlling phoma stem canker using non-triazole based fungicides is essential. Field trials were established for 2013/2014 cropping season. Four fungicides were applied: penthiopyrad, picoxystrobin, prothioconazole and a novel fungicide. Spray timings were divided into three sprays T1 (phoma leaf spotting incidence ≥10%), T2 (3/4 weeks post T1) and T3 (Sclerotinia). Phoma leaf spotting incidence and stem canker severity were recorded. In vitro sensitivity testing was done on one L. maculans (ME24) and one L. biglobosa (68) isolate. Prothioconazole showed no noteworthy advantage over the novel fungicide in canker severity scoring. No significant difference in growth inhibition was observed between L. biglobosa and L. maculans (P ≤ 0.05) when treated with novel fungicide. Canker severity indicates that the novel fungicide has a similar efficacy to triazole fungicides. Fungicide sensitivity testing shows that L. biglobosa does not have an increased sensitivity to non-triazole fungicides. This interaction suggests that the novel fungicide could be used to control both L. maculans and L. biglobosa