Abstract
AbstractPhoma stem canker is a damaging disease of oilseed rape (Brassica napus) that causes annual 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/September and harvested in the following July. The disease epidemics are initiated by ascospores released from Leptosphaeria spp. pseudothecia (ascocarps) on stem stubble in the autumn/winter. Control of this disease is reliant on the use of cultivars with “field resistance” and azole fungicides. This study investigated the effects of cultivar resistance and application of the fungicide prothioconazole on the severity of stem canker before harvest and the subsequent production of pseudothecia on the infected stubble under natural conditions in the 2017/2018, 2018/2019, and 2019/2020 cropping seasons. The application of prothioconazole and cultivar resistance decreased the severity of phoma stem canker before harvest, and the subsequent production of Leptosphaeria spp. pseudothecia on stubble in terms of pseudothecial density. Results showed that stems with less severe stem cankers produced fewer mature pseudothecia of Leptosphaeria spp. on the infected stubble. This investigation suggests that the most sustainable and effective integrated control strategy for phoma stem canker in seasons with low quantities of inoculum is to use cultivars with medium or good field resistance and apply only one spray of prothioconazole when required.
Highlights
Phoma stem canker is caused by two closely related coexisting ascomycete fungal pathogens, Leptosphaeria maculans and L. biglobosa (Fitt et al, 2006a; Shoemaker & Brun, 2001)
The application of the fungicide prothioconazole and use of cultivar resistance directly affected the severity of phoma stem canker before harvest, which indirectly affected the subsequent pseudothecial density of Leptosphaeria spp. on the stubble after harvest
Quartz may be predominantly L. biglobosa because resistance genes like Rlm7 are effective in controlling phoma stem canker only when caused by L. maculans (Huang et al, 2018; Mitrousia et al, 2018)
Summary
Phoma stem canker ( known as blackleg) is caused by two closely related coexisting ascomycete fungal pathogens, Leptosphaeria maculans and L. biglobosa (Fitt et al, 2006a; Shoemaker & Brun, 2001). In the UK, phoma stem canker is a monocyclic disease that is initiated by ascospores as primary inoculum in autumn or winter These ascospores are released from pseudothecia (sexual fruiting bodies) that mature after harvest on infected stem debris, such as stubble left in fields from the previous cropping season. Growers are encouraged to grow cultivars with good “field resistance” as an economical and environmentally friendly method for phoma stem canker control These methods are supplemented by cultural practices such as oilseed rape rotations and sowing UK crops by late August (Huang et al, 2018; Marcroft et al, 2004). There is limited knowledge on the effect of integrated management approaches on the interseasonal transmission of pathogen inoculum It is not known how cultivar resistance and application of fungicides affect the production of primary inoculum in the UK. The aim of this investigation was to study the effects of fungicide application and cultivar resistance on the severity of phoma stem canker before harvest and on the subsequent development of pseudothecia on the crop stubble after harvest
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