Effects of fungicide on growth of Leptosphaeria maculans and L. biglobosa (phoma stem canker) in oilseed rape

Controlled environment (CE) and field experiments were done to investigate effects of fungicide on growth of L. maculans and L. biglobosa in relation to development of phoma leaf spots and phoma stem canker on oilseed rape. In CE experiments, for plants inoculated with L. maculans, fungicide treatment decreased lesion size and amount of L. maculans DNA in leaves; for plants inoculated with L. biglobosa, fungicide did not affect lesion size or amount of pathogen DNA. In field experiments in 2006/07 and 2007/08, fungicide treatment decreased phoma leaf spot incidence in autumn and stem canker severity at harvest, and increased yield. Fungicide treatment decreased stem canker severity more on cv. Courage, with a good yield response, than on cv. Canberra. In 2006/07, fungicide decreased the amount of L. maculans DNA more than that of L. biglobosa in stem tissues (measured by quantitative PCR). There was a linear relationship between the amount of L. maculans DNA in stems and the stem canker severity score at harvest, but there was no clear relationship between the amount of L. biglobosa DNA in stems and the stem canker severity score. These results suggest that effects of fungicides on interactions between L. maculans and L. biglobosa might affect severity of phoma stem canker and yield response.