Why do we see disease in varieties that are rated “R” or “MR”?

Prepared by Holly Derksen, MAFRI Field Crop Pathologist

It is important to remember that resistance does not equal immunity.

In fact, plants don’t have immune systems so you can’t expect them to be immune to any disease. Depending on the level of disease pressure varieties that are rated as resistant will be infected to some degree. If disease pressure is high (ie. high inoculum levels, conducive environmental conditions for a long period of time) then you can still see yield loss due to disease in R-rated varieties.

 The infection that we see on the R-rated varieties is caused by the part of the pathogen population on which that type of resistance is not effective. If we grow varieties with the same type of resistance over a widespread area and in a tight rotation we select for that part of the pathogen population (sometime referred to as a different race). This is when we really begin to see issues, now the majority of the pathogen population cannot be controlled with resistant varieties. This is why it is important to use more than one management technique – do not just depend on variety resistance. It varies from disease to disease, but other management options include crop rotation, rotation of variety, fungicide application(s), tillage, etc.

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