Maris Peer was infected on stems as shown above. Quite often the node becomes infected as rain tends to lodge in the leaf axil. But you can also see internode infection. Multiple stem infections suggest that the mother seed tuber was carrying a latent infection from last year. The mycelium grows up the inside of the stem and erupts in places. There was also quite a bit of stem apex infection as below.
Sometimes plots of Maris Peer resist infection as the variety carries the R1 and R2 resistance genes and if the gene products recognise the strain of blight then the big resistance guns are rolled out. Maris Peer obviously does not recognise this current strain of P. infestans.