Effects of cultivar resistance and fungicide application on stem canker of oilseed rape (Brassica napus) and potential interseasonal transmission of Leptosphaeria spp. inoculum

Phoma stem canker is a damaging disease of oilseed rape (Brassica napus) that causesannual yield losses to UK oilseed rape growers worth approximately £100 million,despite the use of fungicides. In the UK, oilseed rape is sown in August/Septemberand harvested in the following July. The disease epidemics are initiated by ascosporesreleased from Leptosphaeria spp. pseudothecia (ascocarps) on stem stubble in theautumn/winter. Control of this disease is reliant on the use of cultivars with “fieldresistance” and azole fungicides. This study investigated the effects of cultivar resistanceand application of the fungicide prothioconazole on the severity of stem cankerbefore harvest and the subsequent production of pseudothecia on the infected stubbleunder natural conditions in the 2017/2018, 2018/2019, and 2019/2020 croppingseasons. The application of prothioconazole and cultivar resistance decreased theseverity of phoma stem canker before harvest, and the subsequent production ofLeptosphaeria spp. pseudothecia on stubble in terms of pseudothecial density. Resultsshowed that stems with less severe stem cankers produced fewer mature pseudotheciaof Leptosphaeria spp. on the infected stubble. This investigation suggests that themost sustainable and effective integrated control strategy for phoma stem canker inseasons with low quantities of inoculum is to use cultivars with medium or good fieldresistance and apply only one spray of prothioconazole when required.