Efficacy of Fungicides on Coexisting Leptophaeria spp. Causing Phoma Stem Canker on Winter Oilseed Rape
Abstract
Phoma stem canker is a disease of oilseed rape (Brassica napus) caused by closely
related plant pathogens Leptosphaeria maculans and L. biglobosa. It is an economically
important disease, causing annual yield losses of approximately £770 million worldwide.
When colonising oilseed rape, L. maculans and L. biglobosa exist in close proximity on
the leaf, competing for resources as they move through the main leaf vein and into the
stem. Fungicides are commonly used to decrease severity of phoma stem canker on
oilseed rape. However, the efficacy and longevity of active chemicals is under threat from
evolution of resistance in pathogen populations and government legalisation. Moreover, it
has been suggested that both L. maculans and L. biglobosa differ in their sensitivity to
azoles, and important class of fungicides that are used to control the disease through the
inhibition of lanosterol 14-α demethylase (erg11, CYP51). This project aims to further
understand the role that fungicides have in controlling phoma stem canker by
investigating their efficacy against L. maculans and L. biglobosa in crops, in vitro and in
planta. In field experiments, established in Cambridgeshire across four cropping seasons,
the fungicide mixture penthiopyrad (SDHI) plus picoxystrobin (QoI) was as effective at
controlling phoma leaf spotting and phoma stem canker in winter oilseed rape as
prothioconazole (DMI), suggesting that both fungicides could be used to reduce phoma
stem canker symptoms. The two pathogens differed in their growth rates in vitro, with L.
biglobosa growing faster than L. maculans when untreated or treated with lower fungicide
concentrations. Fungicide sensitivity assays suggest that L. maculans and L. biglobosa
are both sensitive to DMI, SDHI and QoI fungicides and that differences between the
species are minor. Prothioconazole and penthiopyrad + picoxystrobin had a similar
efficacy on oilseed rape cotyledons colonised with either L. maculans or L. biglobosa.
There was no difference between species on prothioconazole treated plants, although
there was a difference between L. maculans and L. biglobosa when treated with 20 μg/ml
penthiopyrad + picoxystrobin. Heterologous yeast expression of LmCYP51B and
LbCYP51B with fungicide sensitivity testing of the yeast transformants suggests that
LmCYP51B and LbCYP51B are similarly sensitive to azole fungicides flusilazole,
prothioconazole-desthio and tebuconazole. These findings are supported by homology
protein modelling, which predicts that LmCYP51B and LbCYP51B are structurally very
similar, specifically at the azole-binding site. In conclusion, fungicides are still an effective
control method for reducing phoma stem canker symptoms caused by Leptosphaeria
species in the UK, and a useful tool to in the sustainable production of oilseed rape.
Publication date
2017-07-19Published version
https://doi.org/10.18745/th.18990https://doi.org/10.18745/th.18990