Blight is spread in 2 ways, by the spores being blown on the wind and more often by aphids that feed on infected plants and then fly/are blown onto other plants and infect them.

The spores and the aphids do not differentiate between the growers of the crops so if you raise infected spuds by letting volunteers grow or deliberately plant non-certified seed you could infect other amateurs and farmers and vice versa.

So please root out volunteers and don't save your own seed when certified seed is so cheap