Phoma stem canker may become more difficult to control due to target site resistance to azole chemistry (azoles include protioconazole and tebuconazole). Phoma stem canker, a major disease of oilseed rape, is caused by two related pathogens, Plenodomus lingamand Plenodomus biglobosus.

Reduced sensitivity to azoles in western European populations of P. lingam was first reported last year, but researchers from Rothamsted Research have discovered that there is now decreased azole sensitivity in worldwide P. biglobosus populations.

The reduction in sensitivity is associated with a single mutation within the CYP51 gene.

This gene is the target of azole fungicides, which have been widely used to control Phoma for decades. However, Rothamsted says the seven-fold decrease in sensitivity is unlikely to result in immediate resistance or management issues for farmers, especially with no resistance issues discovered for SDHIs or strobilurins so far.

Ongoing monitoring is now required to track any further changes in Phoma populations as it could lead to resistance building up against azole chemistry in the future.

Rothamsted highlighted the importance of implementing a fungicide resistance management strategy on-farm, including the use of appropriate dose rates, tank mixes of at least two different active ingredients, and the alternation of different modes of action.