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Why can pathogens overcome disease resistance in crop plants?

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Blight — Plant disease Potato leaf infected with a fungal blight.

Pathogens can overcome disease resistance in crop plants primarily through a process of gene-for-gene coevolution.

In this dynamic relationship, plants often develop resistance genes that allow them to detect and defend against specific substances emitted by a pathogen. However, pathogens are genetically flexible and can circumvent these defenses through mutation. If a mutation occurs in the gene responsible for the product the host recognizes, the pathogen’s molecular structure is altered, rendering the host's detection mechanism ineffective.

Because the spread of such mutant genes is favored by natural selection, a continuous "genetic Ping-Pong match" often ensues. The host may develop new mutations to detect the pathogen, while the pathogen responds with further genetic alterations to avoid detection. Consequently, while resistant varieties are a vital tool in plant breeding, the inherent adaptability of pathogens means that resistance can be overcome as both species accumulate matching genes over time.1