Seed World

First Phoma Leaf Spot Resistance To Azole Fungicide Found in Western Europe

The fungal species that cause Phoma leaf spot and stem canker in oilseed rape are showing decreased sensitivity to some chemical controls, sparking concerns for growers in Western Europe, according to a new study and in a news release by Rothamsted Research.

The diseases damage brassicas and are caused by two fungal species Plenodomus lingam (Leptosphaeria maculans) and P. biglobosus (L. biglobosa). In Europe, a range of fungicides are used for control, but azoles (known as DMIs) which act as inhibitors for a fungal enzyme, are fast becoming ineffective.

“Decreased DMI sensitivity has already emerged in Australian and eastern European P. lingam populations,” says Dr. Kevin King, who led the research. “However, we are now seeing it in Western Europe, which is very worrying.”

The study conducted in vitro sensitivity testing. It revealed reduced DMI sensitivity in contemporary P. lingamisolates from Western Europe (collected between 2022 and 2023) compared to historical baseline isolates (from 1992 to 2005).

The genetic sequence linked to this change was correlated with a 3-10 fold decrease in sensitivity to the tested DMIs.

In contrast, modern Western European P. biglobosus isolates (collected between 2021 and 2023) did not exhibit the same genetic sequences associated with reduced sensitivity.

“To date, there is no evidence that sensitivity to other control agents is changing for either species,” said King. “So other fungicides such as QoI (pyraclostrobin) or SDHI (boscalid) should remain effective for now – but relying too heavily on a smaller arsenal of chemicals may well generate more instances of resistance developing in future. More integrated pest control options that incorporate biological agents and different approaches to cultivation and cropping should be explored.”

The research was funded by the BBSRC under the Growing Health and Resilient Farming Futures strategic research programmes.